A MICROTEXTURE STUDY OF PALYGORSKITE-RICH SEDIMENTS FROM THEHAWTHORNE FORMATION SOUTHERN GEORGIA BY TRANSMISSION

ELECTRON MICROSCOPY AND ATOMIC FORCE MICROSCOPY

MARK P S KREKELER1 STEPHEN GUGGENHEIM

1AND JOHN RAKOVAN

2

1 Department of Earth and Environmental Sciences University of Illinois at Chicago Chicago Illinois 60607 USA2 Department of Geology Miami University Oxford Ohio 45056 USA

AbstractmdashA microtexture analysis by TEM and AFM of palygorskite deposits from the HawthorneFormation southern Georgia is given Palygorskite is the dominant mineral comprising an average of65 ndash70 of the sample volume with smaller volumes of smectite illite and kaolinite Morphologicobservations indicate that the palygorskite formed in an unconfined environment such as in the watercolumn or in open-pore space Some palygorskite textures appear to be secondary growths filling voids Anunusual texture is observed where smectite or illite-smectite (Reichweite R = 0) form epitaxially ondetrital illite and kaolinite particles Oxides of Fe and Ti are common and authigenic cassiterite is presentbut rare Apatite is a common trace mineral in these sediments and occurs in a variety of textures Apatiteoccurs as clusters which are believed to be small fecal pellets These clusters have been partially dissolvedand recrystallized and the crystals in the clusters are 50 ndash100 mm in diameter Other apatite crystals occureither as single crystals or in clusters that are not associated with fecal pellets

The textural data of this study suggest that there was an evolving and complex mineralogical andgeochemical system during and after deposition of the palygorskite deposits in the Hawthorne Theepitaxial overgrowths of smectite on detrital illite and kaolinite particles indicate an intermittent stratifiedwater column occurring in the system Freshwater was introduced into the system from the northeast of theApalachicola embayment and overrode more saline water in the southwest portion of the embayment Theresults of this study are consistent with previous environmental interpretations and provide additionaldetailsKey WordsmdashAFM Georgia Hawthorne Formation Palygorskite TEM

INTRODUCTION

Palygorskite deposits of the Hawthorne Formation areof special geological interest such deposits are rare inthe rock record and no modern analogue exists Oneparticular problem in the study of palygorskite depositsis the cause of the variation in physical propertiesCommonly within a single palygorskite deposit proper-ties such as commercial grade color and bulk texturevary widely Understanding why there are such varia-tions of physical properties may provide importantinsight into the processes involved in their formationIn addition such an understanding may improveutilization of these clays in commercial applications

Previous studies of this deposit involved bulkmineralogy and sedimentology (Merkl 1989 Weaver1984 Weaver and Beck 1977 Patterson 1974) Thebulk mineralogy of these deposits was studied by powderX-ray diffraction (XRD) techniques (Patterson 1974Weaver and Beck 1977) or a combination of powderXRD and scanning electron microscopy (SEM) techni-

ques (Merkl 1989) These studies described the majormineral content at a macroscopic scale To date therehas not been an analysis of minor and trace minerals oran analysis of the undisturbed micro-texture of thesedeposits Thus the in situ textural relationships betweenpalygorskite fibers and other minerals remain unclear

The palygorskite-rich sediments of the HawthorneFormation have been interpreted as representing alsquoperimarinersquo environment by Patterson (1974) Weaverand Beck (1977) and Merkl (1989) The depositionalenvironment of this sedimentary system is an estuarywhich is believed to vary from saline to nearly fresh-water in a shallow low-energy environment A geo-chemical gradient in this model is implied with generallyhigher salinities in the southwestern regions to near-fresh water conditions in the northeastern regions in theApalachicola Embayment (Merkl 1989)

To provide a better understanding of the nature of thepalygorskite sediments of the Hawthorne Formation wehave examined a representative section of the depositusing transmission electron microscopy (TEM) andatomic force microscopy (AFM) Unlike previous TEMstudies of palygorskite-rich sediments we have pre-served the original texture of the palygorskite specimensusing a modification of the technique of Kim et al(1995)

Clays and Clay Minerals Vol 52 No 3 263ndash274 2004

Copyright 2004 The Clay Minerals Society 263

E-mail address of corresponding authorrhodochrositeemailm sncomDOI 101346CCMN20040520302

EXPERIMENTAL

Study area

The study area is located in the Apalachicolaembayment in the central portion of the Meigs-Attapulgus-Quincy district of southeast Georgia andnorthern Florida (Figure 1 in Krekeler (2004) thisissue) At the study site a sharp contact defines analteration horizon consisting of a tan clay unit thatoverlies a blue clay unit in the Hawthorne Formation(Figure 1) at the Pittman Quarry near the town ofOcklocknee Georgia Although not formally defined asstratigraphic units these units are hereafter referred to aslsquotan clayrsquo and lsquoblue clayrsquo respectively The origin ofthese units is described by Merkl (1989) as reflecting``roll front type weatheringrsquorsquo and the color differenceis believed to be related to a paleohydrologic zone Thetan clay probably results from the spatial extent of avadose zone permitting oxidation of minerals andformation of Fe oxides whereas the blue clay reflectsthe spatial extent of a phreatic zone The tan clay ischaracterized by numerous 1 ndash5 cm ped faces oxidestaining and lack of well-defined sedimentary struc-tures The blue clay is weakly and finely laminated Ithas occasional ped faces and contains primary sedimen-tary structures such as worm burrows Bulk mineralogyof the clay fraction in both the tan and blue clay isessentially uniform Minerals in the clay fraction ofthese deposits include palygorskite traces of sepiolitesmectite illite kaolinite and quartz (Merkl 1989) Thetan clay and its stratigraphic equivalents are of lowercommercial grade than the blue clay (Merkl 1989) andmost applications of these clay units are related to theirsorption properties

Samples

Approximately 15 m of sample were obtained from afresh exposure above and below the contact of the tanand blue clay horizons Samples ~5 cm in length wereplaced in sealed rigid containers to preserve textureAfter returning to the laboratory small chips with anapproximate diameter of 05 to 10 cm were carefullybroken from selected samples spanning the stratigraphicsection The sample-preparation process as describedbelow was modified from Kim et al (1995) and thismethod appears to preserve the original clay texture

Sample preparation and TEM analysis

Samples were prepared for TEM study by placingchips in small glass beakers for immersion and exchangeunder refrigeration Each sample chip was immersed inmethyl alcohol for 1 week with the methyl alcohol beingreplaced twice Chips were then immersed in a solutionof 3334 London Resin White (LRW) medium and6666 methyl alcohol for 1 week refreshing thesolution on day 4 The chips were then immersed in asolution of 50 LRW and methyl alcohol for a period of

1 week again refreshing the solution on day fourImmersion solutions of 75 LRW and 25 methylalcohol were used for a period of 1 week refreshing thesolution on the fourth day Finally chips were immersedin 100 LRW and the resin was refreshed once a weekfor a period of 4 weeks Sample chips were thenremoved from the beakers and placed in plasticpolyethylene bottles Fresh LRW was added to theseand then the samples were refrigerated for one day Twoto three drops of LRW accelerator were then added tothe bottle at room temperature in a water bath todissipate heat Polymerization occurred within5 ndash10 min Samples were then allowed to cure for24 h Thin-sections were produced and 3 mm Cu gridswere prepared Grids were ion milled to produceperforations at a glancing angle of 22ordm using a Gatanion-mill The TEM analyses were obtained using a300 kV JEOL JEM-3010 transmission electron micro-scope equipped with an ultra high-resolution pole pieceresulting in a point resolution of 017 nm Images werecaptured electronically using a CCD camera

Sample preparation and AFM analysis

For examination using the AFM clay samples wereprepared using gentle sonification in deionized water tocreate a colloidal suspension Approximately 001 g ofeach sample was added to approximately 15 mL ofdeionized (DI) water and then sonicated for 15 minThis sonication process is sufficiently vigorous to break

Figure 1 Stratigraphic column from the study site showinglithological differences between the tan clay and blue clay

264 Krekeler Guggenheim and Rakovan Clays and Clay Minerals

loosely bound clay aggregates into individual grains orintergrowths that are suitably fine grained that they canbe imaged by the AFM but sonication was sufficientlygentle to preserve morphological features of the claysUsing a calibrated pipette (005 mL) these colloidalsuspensions created with sonication were placed on afreshly cleaved mica substrate that was glued to astainless steel Digital Instruments AFM puck Thesamples were then covered to prevent excessivecontamination by adventitious material and dust andwere allowed to dry thoroughly for 24 h Dried sampleswere then imaged in TappingMode in open-air condi-tions with a Digital Instruments Nanoscope IIIaMultiMode SFM using a sharpened silicon probe (typeTESP) This microscope arrangement has routinelyproduced nanometer resolution of clay minerals (Nagyand Blum 1994 Bickmore et al 2001) and is an idealtechnique for measuring micromorphological features ofclay minerals

RESULTS

Phyllosilicates

Palygorskite is the dominant phyllosilicate presentwith lesser amounts of smectite illite kaolinite andinterstratified illite-smectite (I-S) phases Based on TEMpetrography observations the relative proportions ofphyllosilicate minerals between samples show onlyminor variation Estimated proportions by volume ofthe phyllosilicates for the section are 65 ndash70 paly-gorskite 25 ndash30 smectite minerals and 1 ndash10 illiteand kaolinite Although Merkl (1989) reported minoramounts of sepiolite in the study area using SEM nosepiolite only palygorskite was observed by TEM in thesample set of our study The identification of palygors-kite in sample material is based on the fibrousmorphology and the high Al content of the particleswhich distinguishes palygorskite from sepiolite

A ``birdrsquos-nestrsquorsquo fabric which consists of interwovenpalygorskite fibers that cross each other in a three-dimensional array is dominant and comprises ~90 ndash95of the palygorskite fabric (Figure 2a) in both the tan andblue clays The remaining fabric (5 ndash10) involvesclose compact parallel palygorskite fibers that arefound locally These parallel fibers appear to be fillingvoid space between fibers comprising the birdrsquos-nesttexture (Figure 2b) The blue clay generally has thinfibers of ~20 nm in diameter whereas the tan clay hasfibers that vary in width between 20 to 40 nmSonication of samples in water used in preparation forAFM analyses destroys the birdrsquos-nest fabric but doesnot disturb single-crystal morphologies or cohesiveintergrowths of palygorskite fibers Figure 2c showsthe typical morphology for palygorskite in most of thesamples Fibers exhibit the euhedral elongate morphol-ogy that is characteristic of this mineral Most crystalsare found as singles or groups of two or three tightly

intergrown parallel individuals The compact lsquoraft-likersquoparallel intergrowths of palygorskite were also observedby AFM with abundance similar to that found in TEMobservations (Figure 2d)

Semiquantitative energy dispersive spectra (EDS)analyses of selected palygorskite fibers show limitedvariation in chemistry The chemical composition(Table 1) of palygorskite is consistent with analyses ofpalygorskite within the Hawthorne Formation (Merkl1989) and from other locations (Jones and Galan 1988)Palygorskite in these samples does have an appreciable Fecontent and thus deviates from ideal compositions but itis within limits established by Galan and Carretero (1999)

Smectite and interstratified I-S (Reichweite R = 0)constitutes 25 to 30 of the sample volume in thesection studied These minerals exhibit two predominanttextures The least-common texture consists of largediscrete particles of I-S often 005 to 5 mm in length(Figure 3) These particles are platy in morphology andEDS analyses show some K (lt33 wt K2O) presentthat is consistent with a montmorillonite compositionImages of these particles show occasional ~10 AEcirc layersindicating that these are I-S (R = 0) Another texture isobserved and consists of smaller crystallites that are~20 ndash60 nm thick and range in average diameter from~200 nm to 01 mm Analyses by EDS of these smectitesare consistent with montmorillonite composition Latticeimages occasionally show ~10 AEcirc layers that are inter-preted as illite fringes and these fringes are randomlyinterstratified with this smectite texture

In addition to the above textures consisting ofdiscrete particles of smectite and R = 0 I-S anothertexture occurs where detrital illite and kaolinite particleshave epitaxial growths of smectite or I-S to formcomposite grains (Figure 4) These composite grainsare common in both the tan clay and the blue clayEpitaxial growths occur on illite and kaolinite particlesand these growths are typically lt005 mm in diameterAlso small particles of illite are nearly alwaysenveloped in smectite or I-S whereas larger particlesusually 005 ndash10 mm are often incompletely envelopedin smectite Rarely do large particles gt10 mm have anyepitaxial growth Finally the occurrence of illite layersamong regions of I-S in composite grains is mostcommon near the interface with the detrital particleOccasionally regions 5ndash8 nm thick of I-S are regularlyinterstratified (R = 1) on composite grains There is noapparent correlation between the occurrence of theepitaxial growth of illite layers in smectite and whetherthe substrate is illite or kaolinite

Illite is more common in the tan clay (5 ndash10) thanin the blue clay (5) of the sample volumeApproximately 35 of the illite occurs as discreteparticles and 65 is overgrown with smectite Discreteparticles are commonly ~01 to 05 mm thick and 1 to5 mm in diameter (Figure 5) whereas overgrowthparticles are smaller usually lt05 mm in diameter

Vol 52 No 3 2004 Microtextures of palygorskite-rich sediments 265

Kaolinite only rarely occurs as discrete particles inthese samples and is more common in composite grainsKaolinite that is overgrown by smectite is typicallylt005 mm in diameter The relative abundance ofkaolinite in the tan clay and blue clay is approximatelyequal and most commonly occurs in the compositegrains and commonly comprises ~5 of the sediment Inrare instances where kaolinite occurs as a discreteparticle it is typically 05 to 005 mm in diameterPlaty phyllosilicate minerals were also observed byAFM In Figure 2c several platy crystals are visible inthe upper left The stretched hexagonal morphology ofthese is similar to illite (Nagy 1994) however several

well resolved step heights were measured at ~7 AEcirc

suggesting that these may be kaolinite

Phosphates

Apatite is present throughout the Pittman Quarrysection Apatite is most common in the tan clay andcomprises ~3 ndash5 of the bulk rock in the section at 150to 125 cm above the blue clay-tan clay contact Theapatite content decreases to ~05 in the top of the blueclay Apatite occurs in three dominant textures(Figure 6) The mineral occurs as single isolatedeuhedral hexagonal blocky crystals that are usually 30to 100 nm long and 20 ndash50 nm wide These apatite

Figure 2 TEM and AFM images showing two palygorskite textures TEM imaging indicates that the predominant texture is thebirdrsquos-nest texture (a) with pore-filling bundles of palygorskite (b) being much less common (c) AFM image (deflection data) oflsquoraft-likersquo parallel intergrowths of palygorskite crystals dispersed on a single-crystal mica substrate (d) AFM image (height data) ofsingle elongate palygorskite crystals and a platy layer silicate (probably kaolinite or illite) dispersed on a single crystal micasubstrate

266 Krekeler Guggenheim and Rakovan Clays and Clay Minerals

crystals are well crystallized without any apparentdomain structures Apatite crystals also occur as smallclusters commonly of 5 to 10 crystals with individualcrystals being similar in size to the single euhedralcrystals A third apatite texture involves aggregates thatare typically 05 to 2 ndash3 mm in diameter but theseaggregates may be as large as 20 mm The EDS data

indicate that all apatite is hydroxylapatite as any Cl or Fpresent was below detectable limits Often all threetextural types are intimately associated Large aggre-gates are the least common texture observed whereassingle euhedral crystals of apatite are very commonand clusters of apatite crystals are intermittentlycommon

Table 1 Representative EDS analyses of palygorskite and smectite normalized to 100 wt Samples are from thetan clay 125 cm above the contact and from the blue clay 24 cm below the contact Some elements were belowdetection (bd) in some samples

Palygorskite T125-1 T125-2 T125-3 T125-4 T125-5 T125-6 T125-7 T125-8

SiO2 6252 6409 6405 6129 6545 6517 6610 6654TiO2 009 011 355 009 017 010 009 012Al2O3 1825 1580 848 2204 1628 1616 1420 1358Fe2O3 554 514 570 557 603 611 649 619MgO 946 1148 1380 656 925 962 1038 1088MnO 002 003 007 003 009 006 007 004CaO 169 073 378 197 151 173 158 146K2O 177 107 015 168 066 056 062 055Na2O 066 155 042 077 056 049 047 064

Total 10000 10000 10000 10000 10000 10000 10000 10000

Palygorskite B24-1 B24-2 B24-3 B24-4 B24-5 B24-6 B24-7 B24-8

SiO2 6803 6871 6786 6704 6712 6732 6782 6601TiO2 014 011 011 016 018 011 007 010Al2O3 1381 1159 1348 1532 1351 1338 1019 1543Fe2O3 607 601 587 565 582 556 510 635MgO 1064 1204 1064 1019 1160 1178 1528 990MnO bd bd bd bd bd bd bd bdCaO 064 043 065 071 055 047 027 060K2O 024 028 046 068 035 038 007 054Na2O 043 083 093 025 087 100 120 107

Total 10000 10000 10000 10000 10000 10000 10000 10000

Smectite T125-1 T125-2 T125-3 T125-4 T125-5 T125-6 T125-7 T125-8

SiO2 6012 6601 6705 6364 6401 6609 6440 6052TiO2 027 007 012 000 513 002 016 049Al2O3 2300 1912 1723 2140 1308 1754 2013 1852Fe2O3 565 532 531 622 696 652 671 664MgO 651 755 851 559 912 757 538 747MnO 004 001 010 007 007 010 003 006CaO 077 117 085 125 067 102 119 360K2O 226 021 020 128 056 065 162 151Na2O 138 054 063 055 040 049 038 119

Total 10000 10000 10000 10000 10000 10000 10000 10000

Smectite B24-1 B24-2 B24-3 B24-4 B24-5 B24-6 B24-7 B24-8

SiO2 6466 6585 6690 6676 5943 6061 6494 6460TiO2 011 032 020 013 009 018 011 010Al2O3 2141 1633 1552 1580 2909 2466 1662 1770Fe2O3 720 618 604 462 358 525 605 593MgO 465 954 1011 1065 398 574 1010 944MnO bd bd bd bd bd bd bd bdCaO 111 107 078 043 030 053 045 041K2O 049 029 021 058 333 207 065 073Na2O 037 042 024 103 020 096 108 109

Total 10000 10000 10000 10000 10000 10000 10000 10000

Vol 52 No 3 2004 Microtextures of palygorskite-rich sediments 267

Oxides

Oxide minerals are common at the Pittman QuarryOxide minerals in the tan clay comprise an estimated3 ndash5 whereas these minerals comprise ~1 ndash3 in theblue clay The oxides are Ti-rich or Fe-Ti phases withthree textures being recognized (Figure 7) in smectite-dominated regions and in palygorskite-dominatedregions Nearly all of the grains are lt01 mm in diameterThe most common Ti oxide texture is one of smallaggregates usually lt200 nm in average diameter Theseaggregates consist of small crystals typically 5 ndash15 nmin width Where these aggregates occur they arecommonly separated by distances of 200 to 3000 nmAnother Fe-Ti oxide texture involves grains that are

typically 01 to 003 mm in diameter The grains aretypically equant and consist of crystal domains rangingin size from 5 nm to 700 nm or more The least-commonTi oxides observed are crystals or groups of crystals thatare usually square in profile These grains are~100 ndash200 nm on edge The EDS data indicate thatthese phases commonly have a limited (lt15) Fecontent These crystals may be zoned nearly perfector almost free of defects

Aggregates of cassiterite occur also in trace quan-tities in the clays studied (Figure 8) Cassiterite wasobserved primarily in the tan clay near the contact withthe blue clay Grains of ~200 nm in diameter arecommon and they may consist of several small crystalstypically 10 ndash20 nm in diameter Chemical analysisindicates that the cassiterite is pure Sn and O

DISCUSSION

Phyllosilicates

The proportion of birdrsquos-nest texture and bundles ofpalygorskite filling apparent voids is generally uniformin the tan and blue clay strata These textures coexist andare intimately related However they are interpreted asreflecting two separate phases of growth The birdrsquos-nesttexture is interpreted as an accumulation of palygorskitefibers precipitating from the water column In contrastthe bundles of palygorskite fibers are interpreted as alater-stage pore filling that occurred early in diagenesisperhaps at the sediment-water interface The textures donot suggest extensive compression or compaction ofsediment Thus the birdrsquos-nest texture of palygorskiteindicates that the hydraulic regime near the sediment-water interface was very stable and sedimentation rateswere low This supports the general environmentalinterpretations of Patterson (1974) Weaver and Beck(1977) Weaver (1984) and Merkl (1989)

The fine-grained smectite in these samples isauthigenic Regions rich in fine-grained smectite haveparticle orientations that are semi-random to anastomos-ing and do not have a preferred orientation as expectedfor smectite settling from suspension Fine-grainedsmectite textures are discordant with larger smectiteparticles that are clearly detrital eg the detrital smectitein Figure 3b Furthermore in no instance were texturesobserved indicating the preservation of detrital smectitefloccules For example small smectite particles do notradiate or are parallel to the (001) face of other smectiteparticles

Analysis by EDS of palygorskite and smectite showthat the chemical composition of these phases is similar(Table 1) This similarity suggests that changes in watercomposition are responsible for the conditions ofprecipitation for smectite or palygorskite Water chem-istry variations involving changes in Eh and pH forexample may be readily modified with an influx of freshwater or an inundation of seawater The Apalachicola

Figure 3 (a) TEM image showing fine-grained authigenicsmectite particles (b) TEM image showing large detritalsmectite particle

268 Krekeler Guggenheim and Rakovan Clays and Clay Minerals

embayment which was a narrow body of water wouldbe expected to have undergone extensive and rapidchanges in water chemistry owing to its narrowgeometry

An important textural relationship observed in thePittman Quarry samples is the epitaxial overgrowth ofsmectite and I-S on detrital illite and kaolinite particlesSimilar epitaxial relationships for illite and illite-smectite have been found in mudrocks by Huggett andShaw (1997) and Huggett (1996)

Merkl (1989) interpreted SEM images as evidence ofa transition between smectite palygorskite and kaolinite

Although these transformations are plausible and mayexist (eg palygorskite and smectite) in these sedimentsthe TEM textural data of this study indicate that theassociations of smectite with kaolinite and illite arerelated by epitaxy

The composite grains of epitaxial growth of smectiteon detrital kaolinite and illite may be related to astratified water column within the ApalachicolaEmbayment We define stratified water column as aregion of freshwater derived from continental dischargefrom rivers that overlies a water mass that is marine orhas elevated salinity Epitaxial growth of smectite is

Figure 4 TEM images of composite grains of detrital kaolinite and illite with smecite and I-S epitaxy (a) Kaolinite particlesurrounded by I-S (b) Small illite particle with I-S on one side of particle (c) Illite particle with I-S growth on both sides of grain(d) Kaolinite particle showing contact between smectite rich-illite smectite and kaolinite Kaolinite particles in these images arebeam damaged owing to the sensitivity of kaolinite to the electron beam

Vol 52 No 3 2004 Microtextures of palygorskite-rich sediments 269

most extensively developed on small detrital particlesThese particles were probably transported by freshwaterand deposited as individual crystals and not as flocculesFreshwater transport impedes flocculation whereassaline water enhances flocculation especially of smec-tite (Chamley 1989 Porrenga 1966 Grim and Johns1954 Shepard and Moore 1954) No textures wereobserved to indicate that composite grains are sedimen-tary aggregates

Detrital particles of kaolinite and illite are suffi-ciently small to be suspended in the water column for anextended period During this period smectite begins toprecipitate epitaxially on the detrital particles Assmectite crystallizes and adds mass to the particleeventually the particle becomes sufficiently large tosettle from the suspension producing the compositegrain textures observed

The possible formation of smectite or I-S oncomposite grains only after illite or kaolinite particleshave settled from suspension is not likely because thetextures of surrounding matrix of smectite and palygors-kite are discordant with the composite grain Smectiteparticles and palygorskite fibers surrounding compositegrains commonly have random or nearly random textureComposite grains are also much larger than thesurrounding matrix of smectite and palygorskite

Figure 5 TEM image of a detrital particle of illite in matrix ofsmectite

Figure 6 (right) TEM images of textures of hydroxylapatite(a) This rounded oval aggregate of hydroxylapatite is inter-preted as a fecal pellet (b) Cluster of apatite crystals in matrix ofpalygorskite fibers (c) Near atomic resolution structural imageof apatite along the [001] direction The lsquodiamondrsquo patternspacing is ~95 AEcirc and corresponds to the cell dimensions ofhydroxylapatite

270 Krekeler Guggenheim and Rakovan Clays and Clay Minerals

Smectite overgrowths in composite grains are oftenthicker and more coherent with fewer stacking defectsthan the smaller less ordered matrix of smectite Thistexture suggests that smectite and I-S in composite grainsmust have a different origin than the matrix smectite

This model of the water column is consistent with theresults of Patterson (1974) Weaver (1984) and Merkl(1989) Merkl (1989) suggested a gradient in salinitybased on the prevalence of smectite in the northeasternportion of the Apalachicola embayment and the morepalygorskite-rich sediments in the southwestern regionsof the embayment Such a distribution suggests astratified water column Patterson (1974) observedexamples of very organic-rich strata within the paly-gorskite clays of the Hawthorne Formation and thesestrata also suggest a stratified water column Stratifiedwater columns are known to be an important mechanismto accumulate organic matter in the sedimentary record(eg Murphy et al 2000 Canfield 1994 Arthur andSagemon 1994 Tyson and Pearson 1991 Pendersonand Calvert 1990 Demaison and Moore 1980)

Phosphates

Hydroxylapatite is the only recognized phosphatemineral occurring in the Pittman Quarry samples Thetextures of the hydroxylapatite suggest that much of thephosphate is derived from fecal pellets The largeraggregates of hydroxylapatite have well defined bound-aries are ovoid in shape and are generally consistent insize with that expected of fecal pellets of smallorganisms Fecal pellets from zooplankton and otherorganisms are believed to be an important mechanismfor concentrating and depositing phosphate in othersedimentary environments (Lamboy 1982 Porter andRobbins 1981 Robbins and Porter 1980 Bremmer1975) The sedimentary source of phosphate is inter-preted as being from fecal material from zooplankton orpossibly small arthropods

The three textures for hydroxylapatite suggest adissolution and precipitation process involving phos-phate originally from the fecal pellets The intimateassociations of single crystals and small clusters ofapatite crystals and the relative decrease in theprevalence of these textures away from the largeaggregate phosphate grains is interpreted as reflectinga process involving a concentration mechanism Adissolution and re-precipitation mechanism would beexpected to mobilize the phosphate to produce localapatite concentrations

Figure 7 (left) Representative TEM images of oxide minerals(a) This mineral is intepreted as a detrital Ti oxide owing to itsrounded nature SAED shows that this grain is composed ofmultiple crystals (b) Aggregates of Ti oxides with grains10 ndash20 nm in diameter The aggregates are interpreted asauthigenic (c) Euhedral outline and zoning indicate that thisFe-Ti oxide grain is authigenic

Vol 52 No 3 2004 Microtextures of palygorskite-rich sediments 271

Thus caution should be used in interpreting whole-rock geochemical data from these sediments for prove-nance and other studies because the distribution of Ca SrP and rare earth elements (REE) are strongly controlled bythe distribution of apatite Therefore whole-rock studies

using REE and Sr isotopes may be of limited use tointerpret provenance of these deposits in future studiesbut may possibly be of some use in understandingphosphate mobilization The ``roll-front weatheringrsquorsquo asdescribed by Merkl (1989) is an oxidation process whichimplies a fluctuation in a paleo-water table and may be thecause of higher concentrations of hydroxylapatite in thetan clay Study of REE and Sr isotopes may provideinsight into this mobilization process

Phosphate complexes are well known to interferewith sorption of organic molecules on clay mineralsurfaces (Theng 1974 Solomon et al 1971 Solomon1968 Solomon et al 1968) particularly with respect topalygorskite Aluminum in the octahedral sheet of 21ribbons at the edges of palygorskite fibers normally actas electron acceptors (Theng 1974 Solomon 1968Solomon and Rosser 1965) In the presence ofphosphate however these surface sites become deacti-vated by phosphate and polyphosphate (Theng 1974Solomon 1968) More recent work has shown thatphosphate complexes decrease sorption of herbicides onpillared clay and crystal violet montmorillonite(Polubesova et al 2002) Gimsing and Borggaard(2002a 2002b) have shown that phosphate interfereswith sorption of glyphosate in clay minerals

The identification of hydroxylapatite in these sedi-ments possibly explains the differences in commercialgrade between the blue clay and the tan clay Palygorskitefibers may be naturally coated with phosphate as a resultof diagenesis during the roll-front weathering processAlternatively because of the small size of crystals (30 to100 nm) and moderate solubility of hydroxylapatite (logKSP of 849 Brown 1960) hydroxylapatite may readilydissolve when these clays are used in applicationsinvolving water or mixed organic-water fluids Thereforepalygorskite-rich sediments such as the tan clay with 3 to5 hydroxylapatite are more likely to produce phosphatecomplexes when interacting with aqueous or mixedorganic-aqueous fluids As a result the tan clays have alower sorption capacity than palygorskite-rich sedimentswith less or no hydroxylapatite

Oxides

Although the oxide minerals generally comprise lt5of the bulk mineralogy of the clay units studied theyshow a range of diversity that suggests most are ofauthigenic origin Most of the Ti oxides are interpretedas authigenic phases based on textures which involvesmectite and palygorskite The phyllosilicates are gen-erally intimately associated with the oxide grains andthe oxides often have delicate features that mechanicaltransport would probably disrupt or destroy Detritaloxides are common and all of the Fe-Ti oxides that areinterpreted as detrital grains are probably derived fromthe Appalachian Piedmont although they may be eolian

Oxides occur in the tan and blue clays but they aresomewhat more abundant and diverse in the tan clay

Figure 8 (a) TEM image of aggregate of cassiteri te withindividual crystals ~15 nm in diameter (b) Cassiterite aggre-gate showing multiple single crystals in the aggregate (c) EDSspectra from a cassiterite aggregate showing Sn and O peaks theC and Cu are from C coating and Cu substrate

272 Krekeler Guggenheim and Rakovan Clays and Clay Minerals

The tan clay is believed to involve the ``roll-front typeweatheringrsquorsquo process (Merkl 1989) and this implies thatwater has percolated through the unit to producealteration and oxidation of the tan clay We interpretthe bluetan contact and adjacent tan and blue clay unitsas reflecting a paleo-water table where the tan clayexisted in the vadose zone and was subject to drying andre-wetting as the water table fluctuated This enabled theelemental components for the oxide minerals to bemobilized and oxidized in the tan clay as would beexpected in a vadose zone The higher concentrations ofoxide minerals in the tan clay are consistent with thisinterpretation This may be the cause of the colordifference between the tan and blue clays Alternativelythe oxidation state of cations in the palygorskite orsmectite may be the cause of color variation and may belinked to a fluctuating paleo-base level

Cassiterite which appears restricted to the lowermosttan clay unit is believed to be an authigenic phase owingto the small intergrown crystals and euhedral cross-sections The restricted stratigraphic occurrence at thebase of the paleo-vadose zone in the tan clay alsosuggests that it is authigenic Tin was probablymobilized throughout the section and was re-precipitatedduring fluctuations of the water table

CONCLUSIONS

The textures of the minerals at the Pittman quarrysection indicate that there was an evolving and complexmineralogical and geochemical system during and afterdeposi tion of the palygorski te deposits of theHawthorne Textures indicate that several differentreactions occurred to form these sediments such asmultiple stages of growth of palygorskite overgrowth ofsmectite and I-S on detrital clay particles dissolutionand re-precipitation of hydroxylapatite and precipitationof oxide minerals These reactions occurred in geologi-cal environments on scales ranging from the depth of thewater column to nanometers

This study documents smectite overgrowths ondetrital particles in a palygorskite deposit and thisunusual texture is probably a result of a stratifiedwater column that existed in the Apalachicola embay-ment The findings of our study are consistent withenvironmental interpretations of Patterson (1974)Weaver and Beck (1977) Weaver (1984) and Merkl(1989) Our model may be tested by the analysis ofmicrofossils that may indicate mixed freshwater andmarine assemblages and organic geochemistry whichmay yield molecular evidence of environmentallyspecific bacteria

ACKNOWLEDGMENTS

We thank W F Moll for valuable discussions and JRoth and A Nichols of the Research Resources Center atthe University of Illinois at Chicago for assistance with

electron microscopy We thank the US National ScienceFoundation for support under grants EAR0001122 andEAR0001251

REFERENCES

Arthur MA and Sagemon BB (1994) Marine black shalesA review of depositional mechanisms and significance ofancient deposits Annual Review Earth and PlanetarySciences 22 499 ndash551

Bickmore B Bosbach D Hochella MF Jr Charlet L andRufe E (2001) In situ atomic force microscopy study ofhectorite and nontronite dissolution Implicat ions forphyllosilicate edge surface structures and dissolutionmechanisms American Mineralogist 86 411ndash423

Bremner JM (1975) Faecal pellets glauconite phosphoriteand bedrock from the Kunene-Walvis continental marginTechnical Report ndash South African National Committee forOceanographic Research Marine Geology Programme 7Progress Report for the Year pp 59 ndash67

Brown WE (1960) Behaviour of slightly soluble calciumphosphate as revealed by phase equilibrium calculationsSoil Science 90 51 ndash57

Canfield DE (1994) Factors influencing organic carbonpreservation in marine sediments Chemical Geology 114315ndash329

Chamley H (1989) Clay Sedimentology Springer-VerlagNew York 623 pp

Demaison GJ and Moore GT (1980) Anoxic environmentsand oil source bed genesis American Association ofPetroleum Geologists Bulletin 64 1179 ndash1209

Galan E and Carretero MI (1999) A new approach tocompositional limits for sepiolite and palygorskite Claysand Clay Minerals 47 399 ndash409

Gimsing AL and Borggaard OK (2002a) Competitiveadsorption and desorption of glyphosate and phosphate onclay silicates and oxides Clay Minerals 37 509ndash515

Gimsing AL and Borggaard OK (2002b) Effect ofphosphate on the adsorption of glyphosate on soils claymi ne ra l s and ox id e s In t e rna t iona l J our na l o fEnvironmental Analytical Chemistry 82 545 ndash552

Grim RE and Johns WD (1954) Clay mineral investigationsof sediments in the northern Gulf of Mexico Clays and ClayMinerals 2nd National Conference Pergamon New Yorkpp 81 ndash103

Huggett JM (1996) Aluminosilicate diagenesis in a Tertiarysandstone-mudrock sequence from the central North SeaUK Clay Minerals 31 523 ndash536

Huggett JM and Shaw HF (1997) Field emission electronmicroscopy ndash a high resolution technique for the study ofclay minerals in sediments Clay Minerals 32 197 ndash203

Jones B and Galan E (1988) Sepiolite and palygorskite Pp631ndash674 in Hydrous Phyllosillicates (SW Bailey editor)Reviews in Mineralogy 19 Mineralogical Society ofAmerica Washington DC

Kim J Peacor DR Tessier D and Elass F (1995) Atechnique for maintaining texture and permanent expansionof smectite interlayers for TEM observations Clays andClay Minerals 43 51 ndash57

Krekeler MPS (2004) Improved constraints on sedimentaryenvironments of palygorskite deposits of the HawthorneFormation southern Georgia from a detailed study of acore Clays and Clay Minerals 52 253 ndash262

Lamboy M (1982) Importance des pelotes fecales commeorigine des grains de phosphate lrsquoexemple du gisement deGaf sa (Tunisie) Comptes-R endus des Seances delrsquo Academie des Sciences Serie 2 Mecanique-PhysiqueChimie Sciences de lrsquoUnivers Sciences de la Terre 295595ndash600

Vol 52 No 3 2004 Microtextures of palygorskite-rich sediments 273

loosely bound clay aggregates into individual grains orintergrowths that are suitably fine grained that they canbe imaged by the AFM but sonication was sufficientlygentle to preserve morphological features of the claysUsing a calibrated pipette (005 mL) these colloidalsuspensions created with sonication were placed on afreshly cleaved mica substrate that was glued to astainless steel Digital Instruments AFM puck Thesamples were then covered to prevent excessivecontamination by adventitious material and dust andwere allowed to dry thoroughly for 24 h Dried sampleswere then imaged in TappingMode in open-air condi-tions with a Digital Instruments Nanoscope IIIaMultiMode SFM using a sharpened silicon probe (typeTESP) This microscope arrangement has routinelyproduced nanometer resolution of clay minerals (Nagyand Blum 1994 Bickmore et al 2001) and is an idealtechnique for measuring micromorphological features ofclay minerals

RESULTS

Phyllosilicates

Palygorskite is the dominant phyllosilicate presentwith lesser amounts of smectite illite kaolinite andinterstratified illite-smectite (I-S) phases Based on TEMpetrography observations the relative proportions ofphyllosilicate minerals between samples show onlyminor variation Estimated proportions by volume ofthe phyllosilicates for the section are 65 ndash70 paly-gorskite 25 ndash30 smectite minerals and 1 ndash10 illiteand kaolinite Although Merkl (1989) reported minoramounts of sepiolite in the study area using SEM nosepiolite only palygorskite was observed by TEM in thesample set of our study The identification of palygors-kite in sample material is based on the fibrousmorphology and the high Al content of the particleswhich distinguishes palygorskite from sepiolite

A ``birdrsquos-nestrsquorsquo fabric which consists of interwovenpalygorskite fibers that cross each other in a three-dimensional array is dominant and comprises ~90 ndash95of the palygorskite fabric (Figure 2a) in both the tan andblue clays The remaining fabric (5 ndash10) involvesclose compact parallel palygorskite fibers that arefound locally These parallel fibers appear to be fillingvoid space between fibers comprising the birdrsquos-nesttexture (Figure 2b) The blue clay generally has thinfibers of ~20 nm in diameter whereas the tan clay hasfibers that vary in width between 20 to 40 nmSonication of samples in water used in preparation forAFM analyses destroys the birdrsquos-nest fabric but doesnot disturb single-crystal morphologies or cohesiveintergrowths of palygorskite fibers Figure 2c showsthe typical morphology for palygorskite in most of thesamples Fibers exhibit the euhedral elongate morphol-ogy that is characteristic of this mineral Most crystalsare found as singles or groups of two or three tightly

intergrown parallel individuals The compact lsquoraft-likersquoparallel intergrowths of palygorskite were also observedby AFM with abundance similar to that found in TEMobservations (Figure 2d)

Semiquantitative energy dispersive spectra (EDS)analyses of selected palygorskite fibers show limitedvariation in chemistry The chemical composition(Table 1) of palygorskite is consistent with analyses ofpalygorskite within the Hawthorne Formation (Merkl1989) and from other locations (Jones and Galan 1988)Palygorskite in these samples does have an appreciable Fecontent and thus deviates from ideal compositions but itis within limits established by Galan and Carretero (1999)

Smectite and interstratified I-S (Reichweite R = 0)constitutes 25 to 30 of the sample volume in thesection studied These minerals exhibit two predominanttextures The least-common texture consists of largediscrete particles of I-S often 005 to 5 mm in length(Figure 3) These particles are platy in morphology andEDS analyses show some K (lt33 wt K2O) presentthat is consistent with a montmorillonite compositionImages of these particles show occasional ~10 AEcirc layersindicating that these are I-S (R = 0) Another texture isobserved and consists of smaller crystallites that are~20 ndash60 nm thick and range in average diameter from~200 nm to 01 mm Analyses by EDS of these smectitesare consistent with montmorillonite composition Latticeimages occasionally show ~10 AEcirc layers that are inter-preted as illite fringes and these fringes are randomlyinterstratified with this smectite texture

In addition to the above textures consisting ofdiscrete particles of smectite and R = 0 I-S anothertexture occurs where detrital illite and kaolinite particleshave epitaxial growths of smectite or I-S to formcomposite grains (Figure 4) These composite grainsare common in both the tan clay and the blue clayEpitaxial growths occur on illite and kaolinite particlesand these growths are typically lt005 mm in diameterAlso small particles of illite are nearly alwaysenveloped in smectite or I-S whereas larger particlesusually 005 ndash10 mm are often incompletely envelopedin smectite Rarely do large particles gt10 mm have anyepitaxial growth Finally the occurrence of illite layersamong regions of I-S in composite grains is mostcommon near the interface with the detrital particleOccasionally regions 5ndash8 nm thick of I-S are regularlyinterstratified (R = 1) on composite grains There is noapparent correlation between the occurrence of theepitaxial growth of illite layers in smectite and whetherthe substrate is illite or kaolinite

Illite is more common in the tan clay (5 ndash10) thanin the blue clay (5) of the sample volumeApproximately 35 of the illite occurs as discreteparticles and 65 is overgrown with smectite Discreteparticles are commonly ~01 to 05 mm thick and 1 to5 mm in diameter (Figure 5) whereas overgrowthparticles are smaller usually lt05 mm in diameter

Vol 52 No 3 2004 Microtextures of palygorskite-rich sediments 265

Kaolinite only rarely occurs as discrete particles inthese samples and is more common in composite grainsKaolinite that is overgrown by smectite is typicallylt005 mm in diameter The relative abundance ofkaolinite in the tan clay and blue clay is approximatelyequal and most commonly occurs in the compositegrains and commonly comprises ~5 of the sediment Inrare instances where kaolinite occurs as a discreteparticle it is typically 05 to 005 mm in diameterPlaty phyllosilicate minerals were also observed byAFM In Figure 2c several platy crystals are visible inthe upper left The stretched hexagonal morphology ofthese is similar to illite (Nagy 1994) however several

well resolved step heights were measured at ~7 AEcirc

suggesting that these may be kaolinite

Phosphates

Apatite is present throughout the Pittman Quarrysection Apatite is most common in the tan clay andcomprises ~3 ndash5 of the bulk rock in the section at 150to 125 cm above the blue clay-tan clay contact Theapatite content decreases to ~05 in the top of the blueclay Apatite occurs in three dominant textures(Figure 6) The mineral occurs as single isolatedeuhedral hexagonal blocky crystals that are usually 30to 100 nm long and 20 ndash50 nm wide These apatite

Figure 2 TEM and AFM images showing two palygorskite textures TEM imaging indicates that the predominant texture is thebirdrsquos-nest texture (a) with pore-filling bundles of palygorskite (b) being much less common (c) AFM image (deflection data) oflsquoraft-likersquo parallel intergrowths of palygorskite crystals dispersed on a single-crystal mica substrate (d) AFM image (height data) ofsingle elongate palygorskite crystals and a platy layer silicate (probably kaolinite or illite) dispersed on a single crystal micasubstrate

266 Krekeler Guggenheim and Rakovan Clays and Clay Minerals

crystals are well crystallized without any apparentdomain structures Apatite crystals also occur as smallclusters commonly of 5 to 10 crystals with individualcrystals being similar in size to the single euhedralcrystals A third apatite texture involves aggregates thatare typically 05 to 2 ndash3 mm in diameter but theseaggregates may be as large as 20 mm The EDS data

indicate that all apatite is hydroxylapatite as any Cl or Fpresent was below detectable limits Often all threetextural types are intimately associated Large aggre-gates are the least common texture observed whereassingle euhedral crystals of apatite are very commonand clusters of apatite crystals are intermittentlycommon

Table 1 Representative EDS analyses of palygorskite and smectite normalized to 100 wt Samples are from thetan clay 125 cm above the contact and from the blue clay 24 cm below the contact Some elements were belowdetection (bd) in some samples

Palygorskite T125-1 T125-2 T125-3 T125-4 T125-5 T125-6 T125-7 T125-8

SiO2 6252 6409 6405 6129 6545 6517 6610 6654TiO2 009 011 355 009 017 010 009 012Al2O3 1825 1580 848 2204 1628 1616 1420 1358Fe2O3 554 514 570 557 603 611 649 619MgO 946 1148 1380 656 925 962 1038 1088MnO 002 003 007 003 009 006 007 004CaO 169 073 378 197 151 173 158 146K2O 177 107 015 168 066 056 062 055Na2O 066 155 042 077 056 049 047 064

Total 10000 10000 10000 10000 10000 10000 10000 10000

Palygorskite B24-1 B24-2 B24-3 B24-4 B24-5 B24-6 B24-7 B24-8

SiO2 6803 6871 6786 6704 6712 6732 6782 6601TiO2 014 011 011 016 018 011 007 010Al2O3 1381 1159 1348 1532 1351 1338 1019 1543Fe2O3 607 601 587 565 582 556 510 635MgO 1064 1204 1064 1019 1160 1178 1528 990MnO bd bd bd bd bd bd bd bdCaO 064 043 065 071 055 047 027 060K2O 024 028 046 068 035 038 007 054Na2O 043 083 093 025 087 100 120 107

Total 10000 10000 10000 10000 10000 10000 10000 10000

Smectite T125-1 T125-2 T125-3 T125-4 T125-5 T125-6 T125-7 T125-8

SiO2 6012 6601 6705 6364 6401 6609 6440 6052TiO2 027 007 012 000 513 002 016 049Al2O3 2300 1912 1723 2140 1308 1754 2013 1852Fe2O3 565 532 531 622 696 652 671 664MgO 651 755 851 559 912 757 538 747MnO 004 001 010 007 007 010 003 006CaO 077 117 085 125 067 102 119 360K2O 226 021 020 128 056 065 162 151Na2O 138 054 063 055 040 049 038 119

Total 10000 10000 10000 10000 10000 10000 10000 10000

Smectite B24-1 B24-2 B24-3 B24-4 B24-5 B24-6 B24-7 B24-8

SiO2 6466 6585 6690 6676 5943 6061 6494 6460TiO2 011 032 020 013 009 018 011 010Al2O3 2141 1633 1552 1580 2909 2466 1662 1770Fe2O3 720 618 604 462 358 525 605 593MgO 465 954 1011 1065 398 574 1010 944MnO bd bd bd bd bd bd bd bdCaO 111 107 078 043 030 053 045 041K2O 049 029 021 058 333 207 065 073Na2O 037 042 024 103 020 096 108 109

Total 10000 10000 10000 10000 10000 10000 10000 10000

Vol 52 No 3 2004 Microtextures of palygorskite-rich sediments 267

Oxides

Oxide minerals are common at the Pittman QuarryOxide minerals in the tan clay comprise an estimated3 ndash5 whereas these minerals comprise ~1 ndash3 in theblue clay The oxides are Ti-rich or Fe-Ti phases withthree textures being recognized (Figure 7) in smectite-dominated regions and in palygorskite-dominatedregions Nearly all of the grains are lt01 mm in diameterThe most common Ti oxide texture is one of smallaggregates usually lt200 nm in average diameter Theseaggregates consist of small crystals typically 5 ndash15 nmin width Where these aggregates occur they arecommonly separated by distances of 200 to 3000 nmAnother Fe-Ti oxide texture involves grains that are

typically 01 to 003 mm in diameter The grains aretypically equant and consist of crystal domains rangingin size from 5 nm to 700 nm or more The least-commonTi oxides observed are crystals or groups of crystals thatare usually square in profile These grains are~100 ndash200 nm on edge The EDS data indicate thatthese phases commonly have a limited (lt15) Fecontent These crystals may be zoned nearly perfector almost free of defects

Aggregates of cassiterite occur also in trace quan-tities in the clays studied (Figure 8) Cassiterite wasobserved primarily in the tan clay near the contact withthe blue clay Grains of ~200 nm in diameter arecommon and they may consist of several small crystalstypically 10 ndash20 nm in diameter Chemical analysisindicates that the cassiterite is pure Sn and O

DISCUSSION

Phyllosilicates

The proportion of birdrsquos-nest texture and bundles ofpalygorskite filling apparent voids is generally uniformin the tan and blue clay strata These textures coexist andare intimately related However they are interpreted asreflecting two separate phases of growth The birdrsquos-nesttexture is interpreted as an accumulation of palygorskitefibers precipitating from the water column In contrastthe bundles of palygorskite fibers are interpreted as alater-stage pore filling that occurred early in diagenesisperhaps at the sediment-water interface The textures donot suggest extensive compression or compaction ofsediment Thus the birdrsquos-nest texture of palygorskiteindicates that the hydraulic regime near the sediment-water interface was very stable and sedimentation rateswere low This supports the general environmentalinterpretations of Patterson (1974) Weaver and Beck(1977) Weaver (1984) and Merkl (1989)

The fine-grained smectite in these samples isauthigenic Regions rich in fine-grained smectite haveparticle orientations that are semi-random to anastomos-ing and do not have a preferred orientation as expectedfor smectite settling from suspension Fine-grainedsmectite textures are discordant with larger smectiteparticles that are clearly detrital eg the detrital smectitein Figure 3b Furthermore in no instance were texturesobserved indicating the preservation of detrital smectitefloccules For example small smectite particles do notradiate or are parallel to the (001) face of other smectiteparticles

Analysis by EDS of palygorskite and smectite showthat the chemical composition of these phases is similar(Table 1) This similarity suggests that changes in watercomposition are responsible for the conditions ofprecipitation for smectite or palygorskite Water chem-istry variations involving changes in Eh and pH forexample may be readily modified with an influx of freshwater or an inundation of seawater The Apalachicola

Figure 3 (a) TEM image showing fine-grained authigenicsmectite particles (b) TEM image showing large detritalsmectite particle

268 Krekeler Guggenheim and Rakovan Clays and Clay Minerals

embayment which was a narrow body of water wouldbe expected to have undergone extensive and rapidchanges in water chemistry owing to its narrowgeometry

An important textural relationship observed in thePittman Quarry samples is the epitaxial overgrowth ofsmectite and I-S on detrital illite and kaolinite particlesSimilar epitaxial relationships for illite and illite-smectite have been found in mudrocks by Huggett andShaw (1997) and Huggett (1996)

Merkl (1989) interpreted SEM images as evidence ofa transition between smectite palygorskite and kaolinite

Although these transformations are plausible and mayexist (eg palygorskite and smectite) in these sedimentsthe TEM textural data of this study indicate that theassociations of smectite with kaolinite and illite arerelated by epitaxy

The composite grains of epitaxial growth of smectiteon detrital kaolinite and illite may be related to astratified water column within the ApalachicolaEmbayment We define stratified water column as aregion of freshwater derived from continental dischargefrom rivers that overlies a water mass that is marine orhas elevated salinity Epitaxial growth of smectite is

Figure 4 TEM images of composite grains of detrital kaolinite and illite with smecite and I-S epitaxy (a) Kaolinite particlesurrounded by I-S (b) Small illite particle with I-S on one side of particle (c) Illite particle with I-S growth on both sides of grain(d) Kaolinite particle showing contact between smectite rich-illite smectite and kaolinite Kaolinite particles in these images arebeam damaged owing to the sensitivity of kaolinite to the electron beam

Vol 52 No 3 2004 Microtextures of palygorskite-rich sediments 269

most extensively developed on small detrital particlesThese particles were probably transported by freshwaterand deposited as individual crystals and not as flocculesFreshwater transport impedes flocculation whereassaline water enhances flocculation especially of smec-tite (Chamley 1989 Porrenga 1966 Grim and Johns1954 Shepard and Moore 1954) No textures wereobserved to indicate that composite grains are sedimen-tary aggregates

Detrital particles of kaolinite and illite are suffi-ciently small to be suspended in the water column for anextended period During this period smectite begins toprecipitate epitaxially on the detrital particles Assmectite crystallizes and adds mass to the particleeventually the particle becomes sufficiently large tosettle from the suspension producing the compositegrain textures observed

The possible formation of smectite or I-S oncomposite grains only after illite or kaolinite particleshave settled from suspension is not likely because thetextures of surrounding matrix of smectite and palygors-kite are discordant with the composite grain Smectiteparticles and palygorskite fibers surrounding compositegrains commonly have random or nearly random textureComposite grains are also much larger than thesurrounding matrix of smectite and palygorskite

Figure 5 TEM image of a detrital particle of illite in matrix ofsmectite

Figure 6 (right) TEM images of textures of hydroxylapatite(a) This rounded oval aggregate of hydroxylapatite is inter-preted as a fecal pellet (b) Cluster of apatite crystals in matrix ofpalygorskite fibers (c) Near atomic resolution structural imageof apatite along the [001] direction The lsquodiamondrsquo patternspacing is ~95 AEcirc and corresponds to the cell dimensions ofhydroxylapatite

270 Krekeler Guggenheim and Rakovan Clays and Clay Minerals

Smectite overgrowths in composite grains are oftenthicker and more coherent with fewer stacking defectsthan the smaller less ordered matrix of smectite Thistexture suggests that smectite and I-S in composite grainsmust have a different origin than the matrix smectite

This model of the water column is consistent with theresults of Patterson (1974) Weaver (1984) and Merkl(1989) Merkl (1989) suggested a gradient in salinitybased on the prevalence of smectite in the northeasternportion of the Apalachicola embayment and the morepalygorskite-rich sediments in the southwestern regionsof the embayment Such a distribution suggests astratified water column Patterson (1974) observedexamples of very organic-rich strata within the paly-gorskite clays of the Hawthorne Formation and thesestrata also suggest a stratified water column Stratifiedwater columns are known to be an important mechanismto accumulate organic matter in the sedimentary record(eg Murphy et al 2000 Canfield 1994 Arthur andSagemon 1994 Tyson and Pearson 1991 Pendersonand Calvert 1990 Demaison and Moore 1980)

Phosphates

Hydroxylapatite is the only recognized phosphatemineral occurring in the Pittman Quarry samples Thetextures of the hydroxylapatite suggest that much of thephosphate is derived from fecal pellets The largeraggregates of hydroxylapatite have well defined bound-aries are ovoid in shape and are generally consistent insize with that expected of fecal pellets of smallorganisms Fecal pellets from zooplankton and otherorganisms are believed to be an important mechanismfor concentrating and depositing phosphate in othersedimentary environments (Lamboy 1982 Porter andRobbins 1981 Robbins and Porter 1980 Bremmer1975) The sedimentary source of phosphate is inter-preted as being from fecal material from zooplankton orpossibly small arthropods

The three textures for hydroxylapatite suggest adissolution and precipitation process involving phos-phate originally from the fecal pellets The intimateassociations of single crystals and small clusters ofapatite crystals and the relative decrease in theprevalence of these textures away from the largeaggregate phosphate grains is interpreted as reflectinga process involving a concentration mechanism Adissolution and re-precipitation mechanism would beexpected to mobilize the phosphate to produce localapatite concentrations

Figure 7 (left) Representative TEM images of oxide minerals(a) This mineral is intepreted as a detrital Ti oxide owing to itsrounded nature SAED shows that this grain is composed ofmultiple crystals (b) Aggregates of Ti oxides with grains10 ndash20 nm in diameter The aggregates are interpreted asauthigenic (c) Euhedral outline and zoning indicate that thisFe-Ti oxide grain is authigenic

Vol 52 No 3 2004 Microtextures of palygorskite-rich sediments 271

Thus caution should be used in interpreting whole-rock geochemical data from these sediments for prove-nance and other studies because the distribution of Ca SrP and rare earth elements (REE) are strongly controlled bythe distribution of apatite Therefore whole-rock studies

using REE and Sr isotopes may be of limited use tointerpret provenance of these deposits in future studiesbut may possibly be of some use in understandingphosphate mobilization The ``roll-front weatheringrsquorsquo asdescribed by Merkl (1989) is an oxidation process whichimplies a fluctuation in a paleo-water table and may be thecause of higher concentrations of hydroxylapatite in thetan clay Study of REE and Sr isotopes may provideinsight into this mobilization process

Phosphate complexes are well known to interferewith sorption of organic molecules on clay mineralsurfaces (Theng 1974 Solomon et al 1971 Solomon1968 Solomon et al 1968) particularly with respect topalygorskite Aluminum in the octahedral sheet of 21ribbons at the edges of palygorskite fibers normally actas electron acceptors (Theng 1974 Solomon 1968Solomon and Rosser 1965) In the presence ofphosphate however these surface sites become deacti-vated by phosphate and polyphosphate (Theng 1974Solomon 1968) More recent work has shown thatphosphate complexes decrease sorption of herbicides onpillared clay and crystal violet montmorillonite(Polubesova et al 2002) Gimsing and Borggaard(2002a 2002b) have shown that phosphate interfereswith sorption of glyphosate in clay minerals

The identification of hydroxylapatite in these sedi-ments possibly explains the differences in commercialgrade between the blue clay and the tan clay Palygorskitefibers may be naturally coated with phosphate as a resultof diagenesis during the roll-front weathering processAlternatively because of the small size of crystals (30 to100 nm) and moderate solubility of hydroxylapatite (logKSP of 849 Brown 1960) hydroxylapatite may readilydissolve when these clays are used in applicationsinvolving water or mixed organic-water fluids Thereforepalygorskite-rich sediments such as the tan clay with 3 to5 hydroxylapatite are more likely to produce phosphatecomplexes when interacting with aqueous or mixedorganic-aqueous fluids As a result the tan clays have alower sorption capacity than palygorskite-rich sedimentswith less or no hydroxylapatite

Oxides

Although the oxide minerals generally comprise lt5of the bulk mineralogy of the clay units studied theyshow a range of diversity that suggests most are ofauthigenic origin Most of the Ti oxides are interpretedas authigenic phases based on textures which involvesmectite and palygorskite The phyllosilicates are gen-erally intimately associated with the oxide grains andthe oxides often have delicate features that mechanicaltransport would probably disrupt or destroy Detritaloxides are common and all of the Fe-Ti oxides that areinterpreted as detrital grains are probably derived fromthe Appalachian Piedmont although they may be eolian

Oxides occur in the tan and blue clays but they aresomewhat more abundant and diverse in the tan clay

Figure 8 (a) TEM image of aggregate of cassiteri te withindividual crystals ~15 nm in diameter (b) Cassiterite aggre-gate showing multiple single crystals in the aggregate (c) EDSspectra from a cassiterite aggregate showing Sn and O peaks theC and Cu are from C coating and Cu substrate

272 Krekeler Guggenheim and Rakovan Clays and Clay Minerals

The tan clay is believed to involve the ``roll-front typeweatheringrsquorsquo process (Merkl 1989) and this implies thatwater has percolated through the unit to producealteration and oxidation of the tan clay We interpretthe bluetan contact and adjacent tan and blue clay unitsas reflecting a paleo-water table where the tan clayexisted in the vadose zone and was subject to drying andre-wetting as the water table fluctuated This enabled theelemental components for the oxide minerals to bemobilized and oxidized in the tan clay as would beexpected in a vadose zone The higher concentrations ofoxide minerals in the tan clay are consistent with thisinterpretation This may be the cause of the colordifference between the tan and blue clays Alternativelythe oxidation state of cations in the palygorskite orsmectite may be the cause of color variation and may belinked to a fluctuating paleo-base level

Cassiterite which appears restricted to the lowermosttan clay unit is believed to be an authigenic phase owingto the small intergrown crystals and euhedral cross-sections The restricted stratigraphic occurrence at thebase of the paleo-vadose zone in the tan clay alsosuggests that it is authigenic Tin was probablymobilized throughout the section and was re-precipitatedduring fluctuations of the water table

CONCLUSIONS

The textures of the minerals at the Pittman quarrysection indicate that there was an evolving and complexmineralogical and geochemical system during and afterdeposi tion of the palygorski te deposits of theHawthorne Textures indicate that several differentreactions occurred to form these sediments such asmultiple stages of growth of palygorskite overgrowth ofsmectite and I-S on detrital clay particles dissolutionand re-precipitation of hydroxylapatite and precipitationof oxide minerals These reactions occurred in geologi-cal environments on scales ranging from the depth of thewater column to nanometers

This study documents smectite overgrowths ondetrital particles in a palygorskite deposit and thisunusual texture is probably a result of a stratifiedwater column that existed in the Apalachicola embay-ment The findings of our study are consistent withenvironmental interpretations of Patterson (1974)Weaver and Beck (1977) Weaver (1984) and Merkl(1989) Our model may be tested by the analysis ofmicrofossils that may indicate mixed freshwater andmarine assemblages and organic geochemistry whichmay yield molecular evidence of environmentallyspecific bacteria

ACKNOWLEDGMENTS

We thank W F Moll for valuable discussions and JRoth and A Nichols of the Research Resources Center atthe University of Illinois at Chicago for assistance with

electron microscopy We thank the US National ScienceFoundation for support under grants EAR0001122 andEAR0001251

REFERENCES

Arthur MA and Sagemon BB (1994) Marine black shalesA review of depositional mechanisms and significance ofancient deposits Annual Review Earth and PlanetarySciences 22 499 ndash551

Bickmore B Bosbach D Hochella MF Jr Charlet L andRufe E (2001) In situ atomic force microscopy study ofhectorite and nontronite dissolution Implicat ions forphyllosilicate edge surface structures and dissolutionmechanisms American Mineralogist 86 411ndash423

Bremner JM (1975) Faecal pellets glauconite phosphoriteand bedrock from the Kunene-Walvis continental marginTechnical Report ndash South African National Committee forOceanographic Research Marine Geology Programme 7Progress Report for the Year pp 59 ndash67

Brown WE (1960) Behaviour of slightly soluble calciumphosphate as revealed by phase equilibrium calculationsSoil Science 90 51 ndash57

Canfield DE (1994) Factors influencing organic carbonpreservation in marine sediments Chemical Geology 114315ndash329

Chamley H (1989) Clay Sedimentology Springer-VerlagNew York 623 pp

Demaison GJ and Moore GT (1980) Anoxic environmentsand oil source bed genesis American Association ofPetroleum Geologists Bulletin 64 1179 ndash1209

Galan E and Carretero MI (1999) A new approach tocompositional limits for sepiolite and palygorskite Claysand Clay Minerals 47 399 ndash409

Gimsing AL and Borggaard OK (2002a) Competitiveadsorption and desorption of glyphosate and phosphate onclay silicates and oxides Clay Minerals 37 509ndash515

Gimsing AL and Borggaard OK (2002b) Effect ofphosphate on the adsorption of glyphosate on soils claymi ne ra l s and ox id e s In t e rna t iona l J our na l o fEnvironmental Analytical Chemistry 82 545 ndash552

Grim RE and Johns WD (1954) Clay mineral investigationsof sediments in the northern Gulf of Mexico Clays and ClayMinerals 2nd National Conference Pergamon New Yorkpp 81 ndash103

Huggett JM (1996) Aluminosilicate diagenesis in a Tertiarysandstone-mudrock sequence from the central North SeaUK Clay Minerals 31 523 ndash536

Huggett JM and Shaw HF (1997) Field emission electronmicroscopy ndash a high resolution technique for the study ofclay minerals in sediments Clay Minerals 32 197 ndash203

Jones B and Galan E (1988) Sepiolite and palygorskite Pp631ndash674 in Hydrous Phyllosillicates (SW Bailey editor)Reviews in Mineralogy 19 Mineralogical Society ofAmerica Washington DC

Kim J Peacor DR Tessier D and Elass F (1995) Atechnique for maintaining texture and permanent expansionof smectite interlayers for TEM observations Clays andClay Minerals 43 51 ndash57

Krekeler MPS (2004) Improved constraints on sedimentaryenvironments of palygorskite deposits of the HawthorneFormation southern Georgia from a detailed study of acore Clays and Clay Minerals 52 253 ndash262

Lamboy M (1982) Importance des pelotes fecales commeorigine des grains de phosphate lrsquoexemple du gisement deGaf sa (Tunisie) Comptes-R endus des Seances delrsquo Academie des Sciences Serie 2 Mecanique-PhysiqueChimie Sciences de lrsquoUnivers Sciences de la Terre 295595ndash600

Vol 52 No 3 2004 Microtextures of palygorskite-rich sediments 273

Kaolinite only rarely occurs as discrete particles inthese samples and is more common in composite grainsKaolinite that is overgrown by smectite is typicallylt005 mm in diameter The relative abundance ofkaolinite in the tan clay and blue clay is approximatelyequal and most commonly occurs in the compositegrains and commonly comprises ~5 of the sediment Inrare instances where kaolinite occurs as a discreteparticle it is typically 05 to 005 mm in diameterPlaty phyllosilicate minerals were also observed byAFM In Figure 2c several platy crystals are visible inthe upper left The stretched hexagonal morphology ofthese is similar to illite (Nagy 1994) however several

well resolved step heights were measured at ~7 AEcirc

suggesting that these may be kaolinite

Phosphates

Apatite is present throughout the Pittman Quarrysection Apatite is most common in the tan clay andcomprises ~3 ndash5 of the bulk rock in the section at 150to 125 cm above the blue clay-tan clay contact Theapatite content decreases to ~05 in the top of the blueclay Apatite occurs in three dominant textures(Figure 6) The mineral occurs as single isolatedeuhedral hexagonal blocky crystals that are usually 30to 100 nm long and 20 ndash50 nm wide These apatite

Figure 2 TEM and AFM images showing two palygorskite textures TEM imaging indicates that the predominant texture is thebirdrsquos-nest texture (a) with pore-filling bundles of palygorskite (b) being much less common (c) AFM image (deflection data) oflsquoraft-likersquo parallel intergrowths of palygorskite crystals dispersed on a single-crystal mica substrate (d) AFM image (height data) ofsingle elongate palygorskite crystals and a platy layer silicate (probably kaolinite or illite) dispersed on a single crystal micasubstrate

266 Krekeler Guggenheim and Rakovan Clays and Clay Minerals

crystals are well crystallized without any apparentdomain structures Apatite crystals also occur as smallclusters commonly of 5 to 10 crystals with individualcrystals being similar in size to the single euhedralcrystals A third apatite texture involves aggregates thatare typically 05 to 2 ndash3 mm in diameter but theseaggregates may be as large as 20 mm The EDS data

indicate that all apatite is hydroxylapatite as any Cl or Fpresent was below detectable limits Often all threetextural types are intimately associated Large aggre-gates are the least common texture observed whereassingle euhedral crystals of apatite are very commonand clusters of apatite crystals are intermittentlycommon

Table 1 Representative EDS analyses of palygorskite and smectite normalized to 100 wt Samples are from thetan clay 125 cm above the contact and from the blue clay 24 cm below the contact Some elements were belowdetection (bd) in some samples

Palygorskite T125-1 T125-2 T125-3 T125-4 T125-5 T125-6 T125-7 T125-8

SiO2 6252 6409 6405 6129 6545 6517 6610 6654TiO2 009 011 355 009 017 010 009 012Al2O3 1825 1580 848 2204 1628 1616 1420 1358Fe2O3 554 514 570 557 603 611 649 619MgO 946 1148 1380 656 925 962 1038 1088MnO 002 003 007 003 009 006 007 004CaO 169 073 378 197 151 173 158 146K2O 177 107 015 168 066 056 062 055Na2O 066 155 042 077 056 049 047 064

Total 10000 10000 10000 10000 10000 10000 10000 10000

Palygorskite B24-1 B24-2 B24-3 B24-4 B24-5 B24-6 B24-7 B24-8

SiO2 6803 6871 6786 6704 6712 6732 6782 6601TiO2 014 011 011 016 018 011 007 010Al2O3 1381 1159 1348 1532 1351 1338 1019 1543Fe2O3 607 601 587 565 582 556 510 635MgO 1064 1204 1064 1019 1160 1178 1528 990MnO bd bd bd bd bd bd bd bdCaO 064 043 065 071 055 047 027 060K2O 024 028 046 068 035 038 007 054Na2O 043 083 093 025 087 100 120 107

Total 10000 10000 10000 10000 10000 10000 10000 10000

Smectite T125-1 T125-2 T125-3 T125-4 T125-5 T125-6 T125-7 T125-8

SiO2 6012 6601 6705 6364 6401 6609 6440 6052TiO2 027 007 012 000 513 002 016 049Al2O3 2300 1912 1723 2140 1308 1754 2013 1852Fe2O3 565 532 531 622 696 652 671 664MgO 651 755 851 559 912 757 538 747MnO 004 001 010 007 007 010 003 006CaO 077 117 085 125 067 102 119 360K2O 226 021 020 128 056 065 162 151Na2O 138 054 063 055 040 049 038 119

Total 10000 10000 10000 10000 10000 10000 10000 10000

Smectite B24-1 B24-2 B24-3 B24-4 B24-5 B24-6 B24-7 B24-8

SiO2 6466 6585 6690 6676 5943 6061 6494 6460TiO2 011 032 020 013 009 018 011 010Al2O3 2141 1633 1552 1580 2909 2466 1662 1770Fe2O3 720 618 604 462 358 525 605 593MgO 465 954 1011 1065 398 574 1010 944MnO bd bd bd bd bd bd bd bdCaO 111 107 078 043 030 053 045 041K2O 049 029 021 058 333 207 065 073Na2O 037 042 024 103 020 096 108 109

Total 10000 10000 10000 10000 10000 10000 10000 10000

Vol 52 No 3 2004 Microtextures of palygorskite-rich sediments 267

Oxides

Oxide minerals are common at the Pittman QuarryOxide minerals in the tan clay comprise an estimated3 ndash5 whereas these minerals comprise ~1 ndash3 in theblue clay The oxides are Ti-rich or Fe-Ti phases withthree textures being recognized (Figure 7) in smectite-dominated regions and in palygorskite-dominatedregions Nearly all of the grains are lt01 mm in diameterThe most common Ti oxide texture is one of smallaggregates usually lt200 nm in average diameter Theseaggregates consist of small crystals typically 5 ndash15 nmin width Where these aggregates occur they arecommonly separated by distances of 200 to 3000 nmAnother Fe-Ti oxide texture involves grains that are

typically 01 to 003 mm in diameter The grains aretypically equant and consist of crystal domains rangingin size from 5 nm to 700 nm or more The least-commonTi oxides observed are crystals or groups of crystals thatare usually square in profile These grains are~100 ndash200 nm on edge The EDS data indicate thatthese phases commonly have a limited (lt15) Fecontent These crystals may be zoned nearly perfector almost free of defects

Aggregates of cassiterite occur also in trace quan-tities in the clays studied (Figure 8) Cassiterite wasobserved primarily in the tan clay near the contact withthe blue clay Grains of ~200 nm in diameter arecommon and they may consist of several small crystalstypically 10 ndash20 nm in diameter Chemical analysisindicates that the cassiterite is pure Sn and O

DISCUSSION

Phyllosilicates

The proportion of birdrsquos-nest texture and bundles ofpalygorskite filling apparent voids is generally uniformin the tan and blue clay strata These textures coexist andare intimately related However they are interpreted asreflecting two separate phases of growth The birdrsquos-nesttexture is interpreted as an accumulation of palygorskitefibers precipitating from the water column In contrastthe bundles of palygorskite fibers are interpreted as alater-stage pore filling that occurred early in diagenesisperhaps at the sediment-water interface The textures donot suggest extensive compression or compaction ofsediment Thus the birdrsquos-nest texture of palygorskiteindicates that the hydraulic regime near the sediment-water interface was very stable and sedimentation rateswere low This supports the general environmentalinterpretations of Patterson (1974) Weaver and Beck(1977) Weaver (1984) and Merkl (1989)

The fine-grained smectite in these samples isauthigenic Regions rich in fine-grained smectite haveparticle orientations that are semi-random to anastomos-ing and do not have a preferred orientation as expectedfor smectite settling from suspension Fine-grainedsmectite textures are discordant with larger smectiteparticles that are clearly detrital eg the detrital smectitein Figure 3b Furthermore in no instance were texturesobserved indicating the preservation of detrital smectitefloccules For example small smectite particles do notradiate or are parallel to the (001) face of other smectiteparticles

Analysis by EDS of palygorskite and smectite showthat the chemical composition of these phases is similar(Table 1) This similarity suggests that changes in watercomposition are responsible for the conditions ofprecipitation for smectite or palygorskite Water chem-istry variations involving changes in Eh and pH forexample may be readily modified with an influx of freshwater or an inundation of seawater The Apalachicola

Figure 3 (a) TEM image showing fine-grained authigenicsmectite particles (b) TEM image showing large detritalsmectite particle

268 Krekeler Guggenheim and Rakovan Clays and Clay Minerals

embayment which was a narrow body of water wouldbe expected to have undergone extensive and rapidchanges in water chemistry owing to its narrowgeometry

An important textural relationship observed in thePittman Quarry samples is the epitaxial overgrowth ofsmectite and I-S on detrital illite and kaolinite particlesSimilar epitaxial relationships for illite and illite-smectite have been found in mudrocks by Huggett andShaw (1997) and Huggett (1996)

Merkl (1989) interpreted SEM images as evidence ofa transition between smectite palygorskite and kaolinite

Although these transformations are plausible and mayexist (eg palygorskite and smectite) in these sedimentsthe TEM textural data of this study indicate that theassociations of smectite with kaolinite and illite arerelated by epitaxy

The composite grains of epitaxial growth of smectiteon detrital kaolinite and illite may be related to astratified water column within the ApalachicolaEmbayment We define stratified water column as aregion of freshwater derived from continental dischargefrom rivers that overlies a water mass that is marine orhas elevated salinity Epitaxial growth of smectite is

Figure 4 TEM images of composite grains of detrital kaolinite and illite with smecite and I-S epitaxy (a) Kaolinite particlesurrounded by I-S (b) Small illite particle with I-S on one side of particle (c) Illite particle with I-S growth on both sides of grain(d) Kaolinite particle showing contact between smectite rich-illite smectite and kaolinite Kaolinite particles in these images arebeam damaged owing to the sensitivity of kaolinite to the electron beam

Vol 52 No 3 2004 Microtextures of palygorskite-rich sediments 269

most extensively developed on small detrital particlesThese particles were probably transported by freshwaterand deposited as individual crystals and not as flocculesFreshwater transport impedes flocculation whereassaline water enhances flocculation especially of smec-tite (Chamley 1989 Porrenga 1966 Grim and Johns1954 Shepard and Moore 1954) No textures wereobserved to indicate that composite grains are sedimen-tary aggregates

Detrital particles of kaolinite and illite are suffi-ciently small to be suspended in the water column for anextended period During this period smectite begins toprecipitate epitaxially on the detrital particles Assmectite crystallizes and adds mass to the particleeventually the particle becomes sufficiently large tosettle from the suspension producing the compositegrain textures observed

The possible formation of smectite or I-S oncomposite grains only after illite or kaolinite particleshave settled from suspension is not likely because thetextures of surrounding matrix of smectite and palygors-kite are discordant with the composite grain Smectiteparticles and palygorskite fibers surrounding compositegrains commonly have random or nearly random textureComposite grains are also much larger than thesurrounding matrix of smectite and palygorskite

Figure 5 TEM image of a detrital particle of illite in matrix ofsmectite

Figure 6 (right) TEM images of textures of hydroxylapatite(a) This rounded oval aggregate of hydroxylapatite is inter-preted as a fecal pellet (b) Cluster of apatite crystals in matrix ofpalygorskite fibers (c) Near atomic resolution structural imageof apatite along the [001] direction The lsquodiamondrsquo patternspacing is ~95 AEcirc and corresponds to the cell dimensions ofhydroxylapatite

270 Krekeler Guggenheim and Rakovan Clays and Clay Minerals

Smectite overgrowths in composite grains are oftenthicker and more coherent with fewer stacking defectsthan the smaller less ordered matrix of smectite Thistexture suggests that smectite and I-S in composite grainsmust have a different origin than the matrix smectite

This model of the water column is consistent with theresults of Patterson (1974) Weaver (1984) and Merkl(1989) Merkl (1989) suggested a gradient in salinitybased on the prevalence of smectite in the northeasternportion of the Apalachicola embayment and the morepalygorskite-rich sediments in the southwestern regionsof the embayment Such a distribution suggests astratified water column Patterson (1974) observedexamples of very organic-rich strata within the paly-gorskite clays of the Hawthorne Formation and thesestrata also suggest a stratified water column Stratifiedwater columns are known to be an important mechanismto accumulate organic matter in the sedimentary record(eg Murphy et al 2000 Canfield 1994 Arthur andSagemon 1994 Tyson and Pearson 1991 Pendersonand Calvert 1990 Demaison and Moore 1980)

Phosphates

Hydroxylapatite is the only recognized phosphatemineral occurring in the Pittman Quarry samples Thetextures of the hydroxylapatite suggest that much of thephosphate is derived from fecal pellets The largeraggregates of hydroxylapatite have well defined bound-aries are ovoid in shape and are generally consistent insize with that expected of fecal pellets of smallorganisms Fecal pellets from zooplankton and otherorganisms are believed to be an important mechanismfor concentrating and depositing phosphate in othersedimentary environments (Lamboy 1982 Porter andRobbins 1981 Robbins and Porter 1980 Bremmer1975) The sedimentary source of phosphate is inter-preted as being from fecal material from zooplankton orpossibly small arthropods

The three textures for hydroxylapatite suggest adissolution and precipitation process involving phos-phate originally from the fecal pellets The intimateassociations of single crystals and small clusters ofapatite crystals and the relative decrease in theprevalence of these textures away from the largeaggregate phosphate grains is interpreted as reflectinga process involving a concentration mechanism Adissolution and re-precipitation mechanism would beexpected to mobilize the phosphate to produce localapatite concentrations

Figure 7 (left) Representative TEM images of oxide minerals(a) This mineral is intepreted as a detrital Ti oxide owing to itsrounded nature SAED shows that this grain is composed ofmultiple crystals (b) Aggregates of Ti oxides with grains10 ndash20 nm in diameter The aggregates are interpreted asauthigenic (c) Euhedral outline and zoning indicate that thisFe-Ti oxide grain is authigenic

Vol 52 No 3 2004 Microtextures of palygorskite-rich sediments 271

Thus caution should be used in interpreting whole-rock geochemical data from these sediments for prove-nance and other studies because the distribution of Ca SrP and rare earth elements (REE) are strongly controlled bythe distribution of apatite Therefore whole-rock studies

using REE and Sr isotopes may be of limited use tointerpret provenance of these deposits in future studiesbut may possibly be of some use in understandingphosphate mobilization The ``roll-front weatheringrsquorsquo asdescribed by Merkl (1989) is an oxidation process whichimplies a fluctuation in a paleo-water table and may be thecause of higher concentrations of hydroxylapatite in thetan clay Study of REE and Sr isotopes may provideinsight into this mobilization process

Phosphate complexes are well known to interferewith sorption of organic molecules on clay mineralsurfaces (Theng 1974 Solomon et al 1971 Solomon1968 Solomon et al 1968) particularly with respect topalygorskite Aluminum in the octahedral sheet of 21ribbons at the edges of palygorskite fibers normally actas electron acceptors (Theng 1974 Solomon 1968Solomon and Rosser 1965) In the presence ofphosphate however these surface sites become deacti-vated by phosphate and polyphosphate (Theng 1974Solomon 1968) More recent work has shown thatphosphate complexes decrease sorption of herbicides onpillared clay and crystal violet montmorillonite(Polubesova et al 2002) Gimsing and Borggaard(2002a 2002b) have shown that phosphate interfereswith sorption of glyphosate in clay minerals

The identification of hydroxylapatite in these sedi-ments possibly explains the differences in commercialgrade between the blue clay and the tan clay Palygorskitefibers may be naturally coated with phosphate as a resultof diagenesis during the roll-front weathering processAlternatively because of the small size of crystals (30 to100 nm) and moderate solubility of hydroxylapatite (logKSP of 849 Brown 1960) hydroxylapatite may readilydissolve when these clays are used in applicationsinvolving water or mixed organic-water fluids Thereforepalygorskite-rich sediments such as the tan clay with 3 to5 hydroxylapatite are more likely to produce phosphatecomplexes when interacting with aqueous or mixedorganic-aqueous fluids As a result the tan clays have alower sorption capacity than palygorskite-rich sedimentswith less or no hydroxylapatite

Oxides

Although the oxide minerals generally comprise lt5of the bulk mineralogy of the clay units studied theyshow a range of diversity that suggests most are ofauthigenic origin Most of the Ti oxides are interpretedas authigenic phases based on textures which involvesmectite and palygorskite The phyllosilicates are gen-erally intimately associated with the oxide grains andthe oxides often have delicate features that mechanicaltransport would probably disrupt or destroy Detritaloxides are common and all of the Fe-Ti oxides that areinterpreted as detrital grains are probably derived fromthe Appalachian Piedmont although they may be eolian

Oxides occur in the tan and blue clays but they aresomewhat more abundant and diverse in the tan clay

Figure 8 (a) TEM image of aggregate of cassiteri te withindividual crystals ~15 nm in diameter (b) Cassiterite aggre-gate showing multiple single crystals in the aggregate (c) EDSspectra from a cassiterite aggregate showing Sn and O peaks theC and Cu are from C coating and Cu substrate

272 Krekeler Guggenheim and Rakovan Clays and Clay Minerals

The tan clay is believed to involve the ``roll-front typeweatheringrsquorsquo process (Merkl 1989) and this implies thatwater has percolated through the unit to producealteration and oxidation of the tan clay We interpretthe bluetan contact and adjacent tan and blue clay unitsas reflecting a paleo-water table where the tan clayexisted in the vadose zone and was subject to drying andre-wetting as the water table fluctuated This enabled theelemental components for the oxide minerals to bemobilized and oxidized in the tan clay as would beexpected in a vadose zone The higher concentrations ofoxide minerals in the tan clay are consistent with thisinterpretation This may be the cause of the colordifference between the tan and blue clays Alternativelythe oxidation state of cations in the palygorskite orsmectite may be the cause of color variation and may belinked to a fluctuating paleo-base level

Cassiterite which appears restricted to the lowermosttan clay unit is believed to be an authigenic phase owingto the small intergrown crystals and euhedral cross-sections The restricted stratigraphic occurrence at thebase of the paleo-vadose zone in the tan clay alsosuggests that it is authigenic Tin was probablymobilized throughout the section and was re-precipitatedduring fluctuations of the water table

CONCLUSIONS

The textures of the minerals at the Pittman quarrysection indicate that there was an evolving and complexmineralogical and geochemical system during and afterdeposi tion of the palygorski te deposits of theHawthorne Textures indicate that several differentreactions occurred to form these sediments such asmultiple stages of growth of palygorskite overgrowth ofsmectite and I-S on detrital clay particles dissolutionand re-precipitation of hydroxylapatite and precipitationof oxide minerals These reactions occurred in geologi-cal environments on scales ranging from the depth of thewater column to nanometers

This study documents smectite overgrowths ondetrital particles in a palygorskite deposit and thisunusual texture is probably a result of a stratifiedwater column that existed in the Apalachicola embay-ment The findings of our study are consistent withenvironmental interpretations of Patterson (1974)Weaver and Beck (1977) Weaver (1984) and Merkl(1989) Our model may be tested by the analysis ofmicrofossils that may indicate mixed freshwater andmarine assemblages and organic geochemistry whichmay yield molecular evidence of environmentallyspecific bacteria

ACKNOWLEDGMENTS

We thank W F Moll for valuable discussions and JRoth and A Nichols of the Research Resources Center atthe University of Illinois at Chicago for assistance with

electron microscopy We thank the US National ScienceFoundation for support under grants EAR0001122 andEAR0001251

REFERENCES

Arthur MA and Sagemon BB (1994) Marine black shalesA review of depositional mechanisms and significance ofancient deposits Annual Review Earth and PlanetarySciences 22 499 ndash551

Bickmore B Bosbach D Hochella MF Jr Charlet L andRufe E (2001) In situ atomic force microscopy study ofhectorite and nontronite dissolution Implicat ions forphyllosilicate edge surface structures and dissolutionmechanisms American Mineralogist 86 411ndash423

Bremner JM (1975) Faecal pellets glauconite phosphoriteand bedrock from the Kunene-Walvis continental marginTechnical Report ndash South African National Committee forOceanographic Research Marine Geology Programme 7Progress Report for the Year pp 59 ndash67

Brown WE (1960) Behaviour of slightly soluble calciumphosphate as revealed by phase equilibrium calculationsSoil Science 90 51 ndash57

Canfield DE (1994) Factors influencing organic carbonpreservation in marine sediments Chemical Geology 114315ndash329

Chamley H (1989) Clay Sedimentology Springer-VerlagNew York 623 pp

Demaison GJ and Moore GT (1980) Anoxic environmentsand oil source bed genesis American Association ofPetroleum Geologists Bulletin 64 1179 ndash1209

Galan E and Carretero MI (1999) A new approach tocompositional limits for sepiolite and palygorskite Claysand Clay Minerals 47 399 ndash409

Gimsing AL and Borggaard OK (2002a) Competitiveadsorption and desorption of glyphosate and phosphate onclay silicates and oxides Clay Minerals 37 509ndash515

Gimsing AL and Borggaard OK (2002b) Effect ofphosphate on the adsorption of glyphosate on soils claymi ne ra l s and ox id e s In t e rna t iona l J our na l o fEnvironmental Analytical Chemistry 82 545 ndash552

Grim RE and Johns WD (1954) Clay mineral investigationsof sediments in the northern Gulf of Mexico Clays and ClayMinerals 2nd National Conference Pergamon New Yorkpp 81 ndash103

Huggett JM (1996) Aluminosilicate diagenesis in a Tertiarysandstone-mudrock sequence from the central North SeaUK Clay Minerals 31 523 ndash536

Huggett JM and Shaw HF (1997) Field emission electronmicroscopy ndash a high resolution technique for the study ofclay minerals in sediments Clay Minerals 32 197 ndash203

Jones B and Galan E (1988) Sepiolite and palygorskite Pp631ndash674 in Hydrous Phyllosillicates (SW Bailey editor)Reviews in Mineralogy 19 Mineralogical Society ofAmerica Washington DC

Kim J Peacor DR Tessier D and Elass F (1995) Atechnique for maintaining texture and permanent expansionof smectite interlayers for TEM observations Clays andClay Minerals 43 51 ndash57

Krekeler MPS (2004) Improved constraints on sedimentaryenvironments of palygorskite deposits of the HawthorneFormation southern Georgia from a detailed study of acore Clays and Clay Minerals 52 253 ndash262

Lamboy M (1982) Importance des pelotes fecales commeorigine des grains de phosphate lrsquoexemple du gisement deGaf sa (Tunisie) Comptes-R endus des Seances delrsquo Academie des Sciences Serie 2 Mecanique-PhysiqueChimie Sciences de lrsquoUnivers Sciences de la Terre 295595ndash600

Vol 52 No 3 2004 Microtextures of palygorskite-rich sediments 273

crystals are well crystallized without any apparentdomain structures Apatite crystals also occur as smallclusters commonly of 5 to 10 crystals with individualcrystals being similar in size to the single euhedralcrystals A third apatite texture involves aggregates thatare typically 05 to 2 ndash3 mm in diameter but theseaggregates may be as large as 20 mm The EDS data

indicate that all apatite is hydroxylapatite as any Cl or Fpresent was below detectable limits Often all threetextural types are intimately associated Large aggre-gates are the least common texture observed whereassingle euhedral crystals of apatite are very commonand clusters of apatite crystals are intermittentlycommon

Table 1 Representative EDS analyses of palygorskite and smectite normalized to 100 wt Samples are from thetan clay 125 cm above the contact and from the blue clay 24 cm below the contact Some elements were belowdetection (bd) in some samples

Palygorskite T125-1 T125-2 T125-3 T125-4 T125-5 T125-6 T125-7 T125-8

SiO2 6252 6409 6405 6129 6545 6517 6610 6654TiO2 009 011 355 009 017 010 009 012Al2O3 1825 1580 848 2204 1628 1616 1420 1358Fe2O3 554 514 570 557 603 611 649 619MgO 946 1148 1380 656 925 962 1038 1088MnO 002 003 007 003 009 006 007 004CaO 169 073 378 197 151 173 158 146K2O 177 107 015 168 066 056 062 055Na2O 066 155 042 077 056 049 047 064

Total 10000 10000 10000 10000 10000 10000 10000 10000

Palygorskite B24-1 B24-2 B24-3 B24-4 B24-5 B24-6 B24-7 B24-8

SiO2 6803 6871 6786 6704 6712 6732 6782 6601TiO2 014 011 011 016 018 011 007 010Al2O3 1381 1159 1348 1532 1351 1338 1019 1543Fe2O3 607 601 587 565 582 556 510 635MgO 1064 1204 1064 1019 1160 1178 1528 990MnO bd bd bd bd bd bd bd bdCaO 064 043 065 071 055 047 027 060K2O 024 028 046 068 035 038 007 054Na2O 043 083 093 025 087 100 120 107

Total 10000 10000 10000 10000 10000 10000 10000 10000

Smectite T125-1 T125-2 T125-3 T125-4 T125-5 T125-6 T125-7 T125-8

SiO2 6012 6601 6705 6364 6401 6609 6440 6052TiO2 027 007 012 000 513 002 016 049Al2O3 2300 1912 1723 2140 1308 1754 2013 1852Fe2O3 565 532 531 622 696 652 671 664MgO 651 755 851 559 912 757 538 747MnO 004 001 010 007 007 010 003 006CaO 077 117 085 125 067 102 119 360K2O 226 021 020 128 056 065 162 151Na2O 138 054 063 055 040 049 038 119

Total 10000 10000 10000 10000 10000 10000 10000 10000

Smectite B24-1 B24-2 B24-3 B24-4 B24-5 B24-6 B24-7 B24-8

SiO2 6466 6585 6690 6676 5943 6061 6494 6460TiO2 011 032 020 013 009 018 011 010Al2O3 2141 1633 1552 1580 2909 2466 1662 1770Fe2O3 720 618 604 462 358 525 605 593MgO 465 954 1011 1065 398 574 1010 944MnO bd bd bd bd bd bd bd bdCaO 111 107 078 043 030 053 045 041K2O 049 029 021 058 333 207 065 073Na2O 037 042 024 103 020 096 108 109

Total 10000 10000 10000 10000 10000 10000 10000 10000

Vol 52 No 3 2004 Microtextures of palygorskite-rich sediments 267

Oxides

Oxide minerals are common at the Pittman QuarryOxide minerals in the tan clay comprise an estimated3 ndash5 whereas these minerals comprise ~1 ndash3 in theblue clay The oxides are Ti-rich or Fe-Ti phases withthree textures being recognized (Figure 7) in smectite-dominated regions and in palygorskite-dominatedregions Nearly all of the grains are lt01 mm in diameterThe most common Ti oxide texture is one of smallaggregates usually lt200 nm in average diameter Theseaggregates consist of small crystals typically 5 ndash15 nmin width Where these aggregates occur they arecommonly separated by distances of 200 to 3000 nmAnother Fe-Ti oxide texture involves grains that are

typically 01 to 003 mm in diameter The grains aretypically equant and consist of crystal domains rangingin size from 5 nm to 700 nm or more The least-commonTi oxides observed are crystals or groups of crystals thatare usually square in profile These grains are~100 ndash200 nm on edge The EDS data indicate thatthese phases commonly have a limited (lt15) Fecontent These crystals may be zoned nearly perfector almost free of defects

Aggregates of cassiterite occur also in trace quan-tities in the clays studied (Figure 8) Cassiterite wasobserved primarily in the tan clay near the contact withthe blue clay Grains of ~200 nm in diameter arecommon and they may consist of several small crystalstypically 10 ndash20 nm in diameter Chemical analysisindicates that the cassiterite is pure Sn and O

DISCUSSION

Phyllosilicates

The proportion of birdrsquos-nest texture and bundles ofpalygorskite filling apparent voids is generally uniformin the tan and blue clay strata These textures coexist andare intimately related However they are interpreted asreflecting two separate phases of growth The birdrsquos-nesttexture is interpreted as an accumulation of palygorskitefibers precipitating from the water column In contrastthe bundles of palygorskite fibers are interpreted as alater-stage pore filling that occurred early in diagenesisperhaps at the sediment-water interface The textures donot suggest extensive compression or compaction ofsediment Thus the birdrsquos-nest texture of palygorskiteindicates that the hydraulic regime near the sediment-water interface was very stable and sedimentation rateswere low This supports the general environmentalinterpretations of Patterson (1974) Weaver and Beck(1977) Weaver (1984) and Merkl (1989)

The fine-grained smectite in these samples isauthigenic Regions rich in fine-grained smectite haveparticle orientations that are semi-random to anastomos-ing and do not have a preferred orientation as expectedfor smectite settling from suspension Fine-grainedsmectite textures are discordant with larger smectiteparticles that are clearly detrital eg the detrital smectitein Figure 3b Furthermore in no instance were texturesobserved indicating the preservation of detrital smectitefloccules For example small smectite particles do notradiate or are parallel to the (001) face of other smectiteparticles

Analysis by EDS of palygorskite and smectite showthat the chemical composition of these phases is similar(Table 1) This similarity suggests that changes in watercomposition are responsible for the conditions ofprecipitation for smectite or palygorskite Water chem-istry variations involving changes in Eh and pH forexample may be readily modified with an influx of freshwater or an inundation of seawater The Apalachicola

Figure 3 (a) TEM image showing fine-grained authigenicsmectite particles (b) TEM image showing large detritalsmectite particle

268 Krekeler Guggenheim and Rakovan Clays and Clay Minerals

embayment which was a narrow body of water wouldbe expected to have undergone extensive and rapidchanges in water chemistry owing to its narrowgeometry

An important textural relationship observed in thePittman Quarry samples is the epitaxial overgrowth ofsmectite and I-S on detrital illite and kaolinite particlesSimilar epitaxial relationships for illite and illite-smectite have been found in mudrocks by Huggett andShaw (1997) and Huggett (1996)

Merkl (1989) interpreted SEM images as evidence ofa transition between smectite palygorskite and kaolinite

Although these transformations are plausible and mayexist (eg palygorskite and smectite) in these sedimentsthe TEM textural data of this study indicate that theassociations of smectite with kaolinite and illite arerelated by epitaxy

The composite grains of epitaxial growth of smectiteon detrital kaolinite and illite may be related to astratified water column within the ApalachicolaEmbayment We define stratified water column as aregion of freshwater derived from continental dischargefrom rivers that overlies a water mass that is marine orhas elevated salinity Epitaxial growth of smectite is

Figure 4 TEM images of composite grains of detrital kaolinite and illite with smecite and I-S epitaxy (a) Kaolinite particlesurrounded by I-S (b) Small illite particle with I-S on one side of particle (c) Illite particle with I-S growth on both sides of grain(d) Kaolinite particle showing contact between smectite rich-illite smectite and kaolinite Kaolinite particles in these images arebeam damaged owing to the sensitivity of kaolinite to the electron beam

Vol 52 No 3 2004 Microtextures of palygorskite-rich sediments 269

most extensively developed on small detrital particlesThese particles were probably transported by freshwaterand deposited as individual crystals and not as flocculesFreshwater transport impedes flocculation whereassaline water enhances flocculation especially of smec-tite (Chamley 1989 Porrenga 1966 Grim and Johns1954 Shepard and Moore 1954) No textures wereobserved to indicate that composite grains are sedimen-tary aggregates

Detrital particles of kaolinite and illite are suffi-ciently small to be suspended in the water column for anextended period During this period smectite begins toprecipitate epitaxially on the detrital particles Assmectite crystallizes and adds mass to the particleeventually the particle becomes sufficiently large tosettle from the suspension producing the compositegrain textures observed

The possible formation of smectite or I-S oncomposite grains only after illite or kaolinite particleshave settled from suspension is not likely because thetextures of surrounding matrix of smectite and palygors-kite are discordant with the composite grain Smectiteparticles and palygorskite fibers surrounding compositegrains commonly have random or nearly random textureComposite grains are also much larger than thesurrounding matrix of smectite and palygorskite

Figure 5 TEM image of a detrital particle of illite in matrix ofsmectite

Figure 6 (right) TEM images of textures of hydroxylapatite(a) This rounded oval aggregate of hydroxylapatite is inter-preted as a fecal pellet (b) Cluster of apatite crystals in matrix ofpalygorskite fibers (c) Near atomic resolution structural imageof apatite along the [001] direction The lsquodiamondrsquo patternspacing is ~95 AEcirc and corresponds to the cell dimensions ofhydroxylapatite

270 Krekeler Guggenheim and Rakovan Clays and Clay Minerals

Smectite overgrowths in composite grains are oftenthicker and more coherent with fewer stacking defectsthan the smaller less ordered matrix of smectite Thistexture suggests that smectite and I-S in composite grainsmust have a different origin than the matrix smectite

This model of the water column is consistent with theresults of Patterson (1974) Weaver (1984) and Merkl(1989) Merkl (1989) suggested a gradient in salinitybased on the prevalence of smectite in the northeasternportion of the Apalachicola embayment and the morepalygorskite-rich sediments in the southwestern regionsof the embayment Such a distribution suggests astratified water column Patterson (1974) observedexamples of very organic-rich strata within the paly-gorskite clays of the Hawthorne Formation and thesestrata also suggest a stratified water column Stratifiedwater columns are known to be an important mechanismto accumulate organic matter in the sedimentary record(eg Murphy et al 2000 Canfield 1994 Arthur andSagemon 1994 Tyson and Pearson 1991 Pendersonand Calvert 1990 Demaison and Moore 1980)

Phosphates

Hydroxylapatite is the only recognized phosphatemineral occurring in the Pittman Quarry samples Thetextures of the hydroxylapatite suggest that much of thephosphate is derived from fecal pellets The largeraggregates of hydroxylapatite have well defined bound-aries are ovoid in shape and are generally consistent insize with that expected of fecal pellets of smallorganisms Fecal pellets from zooplankton and otherorganisms are believed to be an important mechanismfor concentrating and depositing phosphate in othersedimentary environments (Lamboy 1982 Porter andRobbins 1981 Robbins and Porter 1980 Bremmer1975) The sedimentary source of phosphate is inter-preted as being from fecal material from zooplankton orpossibly small arthropods

The three textures for hydroxylapatite suggest adissolution and precipitation process involving phos-phate originally from the fecal pellets The intimateassociations of single crystals and small clusters ofapatite crystals and the relative decrease in theprevalence of these textures away from the largeaggregate phosphate grains is interpreted as reflectinga process involving a concentration mechanism Adissolution and re-precipitation mechanism would beexpected to mobilize the phosphate to produce localapatite concentrations

Figure 7 (left) Representative TEM images of oxide minerals(a) This mineral is intepreted as a detrital Ti oxide owing to itsrounded nature SAED shows that this grain is composed ofmultiple crystals (b) Aggregates of Ti oxides with grains10 ndash20 nm in diameter The aggregates are interpreted asauthigenic (c) Euhedral outline and zoning indicate that thisFe-Ti oxide grain is authigenic

Vol 52 No 3 2004 Microtextures of palygorskite-rich sediments 271

Thus caution should be used in interpreting whole-rock geochemical data from these sediments for prove-nance and other studies because the distribution of Ca SrP and rare earth elements (REE) are strongly controlled bythe distribution of apatite Therefore whole-rock studies

using REE and Sr isotopes may be of limited use tointerpret provenance of these deposits in future studiesbut may possibly be of some use in understandingphosphate mobilization The ``roll-front weatheringrsquorsquo asdescribed by Merkl (1989) is an oxidation process whichimplies a fluctuation in a paleo-water table and may be thecause of higher concentrations of hydroxylapatite in thetan clay Study of REE and Sr isotopes may provideinsight into this mobilization process

Phosphate complexes are well known to interferewith sorption of organic molecules on clay mineralsurfaces (Theng 1974 Solomon et al 1971 Solomon1968 Solomon et al 1968) particularly with respect topalygorskite Aluminum in the octahedral sheet of 21ribbons at the edges of palygorskite fibers normally actas electron acceptors (Theng 1974 Solomon 1968Solomon and Rosser 1965) In the presence ofphosphate however these surface sites become deacti-vated by phosphate and polyphosphate (Theng 1974Solomon 1968) More recent work has shown thatphosphate complexes decrease sorption of herbicides onpillared clay and crystal violet montmorillonite(Polubesova et al 2002) Gimsing and Borggaard(2002a 2002b) have shown that phosphate interfereswith sorption of glyphosate in clay minerals

The identification of hydroxylapatite in these sedi-ments possibly explains the differences in commercialgrade between the blue clay and the tan clay Palygorskitefibers may be naturally coated with phosphate as a resultof diagenesis during the roll-front weathering processAlternatively because of the small size of crystals (30 to100 nm) and moderate solubility of hydroxylapatite (logKSP of 849 Brown 1960) hydroxylapatite may readilydissolve when these clays are used in applicationsinvolving water or mixed organic-water fluids Thereforepalygorskite-rich sediments such as the tan clay with 3 to5 hydroxylapatite are more likely to produce phosphatecomplexes when interacting with aqueous or mixedorganic-aqueous fluids As a result the tan clays have alower sorption capacity than palygorskite-rich sedimentswith less or no hydroxylapatite

Oxides

Although the oxide minerals generally comprise lt5of the bulk mineralogy of the clay units studied theyshow a range of diversity that suggests most are ofauthigenic origin Most of the Ti oxides are interpretedas authigenic phases based on textures which involvesmectite and palygorskite The phyllosilicates are gen-erally intimately associated with the oxide grains andthe oxides often have delicate features that mechanicaltransport would probably disrupt or destroy Detritaloxides are common and all of the Fe-Ti oxides that areinterpreted as detrital grains are probably derived fromthe Appalachian Piedmont although they may be eolian

Oxides occur in the tan and blue clays but they aresomewhat more abundant and diverse in the tan clay

Figure 8 (a) TEM image of aggregate of cassiteri te withindividual crystals ~15 nm in diameter (b) Cassiterite aggre-gate showing multiple single crystals in the aggregate (c) EDSspectra from a cassiterite aggregate showing Sn and O peaks theC and Cu are from C coating and Cu substrate

272 Krekeler Guggenheim and Rakovan Clays and Clay Minerals

The tan clay is believed to involve the ``roll-front typeweatheringrsquorsquo process (Merkl 1989) and this implies thatwater has percolated through the unit to producealteration and oxidation of the tan clay We interpretthe bluetan contact and adjacent tan and blue clay unitsas reflecting a paleo-water table where the tan clayexisted in the vadose zone and was subject to drying andre-wetting as the water table fluctuated This enabled theelemental components for the oxide minerals to bemobilized and oxidized in the tan clay as would beexpected in a vadose zone The higher concentrations ofoxide minerals in the tan clay are consistent with thisinterpretation This may be the cause of the colordifference between the tan and blue clays Alternativelythe oxidation state of cations in the palygorskite orsmectite may be the cause of color variation and may belinked to a fluctuating paleo-base level

Cassiterite which appears restricted to the lowermosttan clay unit is believed to be an authigenic phase owingto the small intergrown crystals and euhedral cross-sections The restricted stratigraphic occurrence at thebase of the paleo-vadose zone in the tan clay alsosuggests that it is authigenic Tin was probablymobilized throughout the section and was re-precipitatedduring fluctuations of the water table

CONCLUSIONS

The textures of the minerals at the Pittman quarrysection indicate that there was an evolving and complexmineralogical and geochemical system during and afterdeposi tion of the palygorski te deposits of theHawthorne Textures indicate that several differentreactions occurred to form these sediments such asmultiple stages of growth of palygorskite overgrowth ofsmectite and I-S on detrital clay particles dissolutionand re-precipitation of hydroxylapatite and precipitationof oxide minerals These reactions occurred in geologi-cal environments on scales ranging from the depth of thewater column to nanometers

This study documents smectite overgrowths ondetrital particles in a palygorskite deposit and thisunusual texture is probably a result of a stratifiedwater column that existed in the Apalachicola embay-ment The findings of our study are consistent withenvironmental interpretations of Patterson (1974)Weaver and Beck (1977) Weaver (1984) and Merkl(1989) Our model may be tested by the analysis ofmicrofossils that may indicate mixed freshwater andmarine assemblages and organic geochemistry whichmay yield molecular evidence of environmentallyspecific bacteria

ACKNOWLEDGMENTS

We thank W F Moll for valuable discussions and JRoth and A Nichols of the Research Resources Center atthe University of Illinois at Chicago for assistance with

electron microscopy We thank the US National ScienceFoundation for support under grants EAR0001122 andEAR0001251

REFERENCES

Arthur MA and Sagemon BB (1994) Marine black shalesA review of depositional mechanisms and significance ofancient deposits Annual Review Earth and PlanetarySciences 22 499 ndash551

Bickmore B Bosbach D Hochella MF Jr Charlet L andRufe E (2001) In situ atomic force microscopy study ofhectorite and nontronite dissolution Implicat ions forphyllosilicate edge surface structures and dissolutionmechanisms American Mineralogist 86 411ndash423

Bremner JM (1975) Faecal pellets glauconite phosphoriteand bedrock from the Kunene-Walvis continental marginTechnical Report ndash South African National Committee forOceanographic Research Marine Geology Programme 7Progress Report for the Year pp 59 ndash67

Brown WE (1960) Behaviour of slightly soluble calciumphosphate as revealed by phase equilibrium calculationsSoil Science 90 51 ndash57

Canfield DE (1994) Factors influencing organic carbonpreservation in marine sediments Chemical Geology 114315ndash329

Chamley H (1989) Clay Sedimentology Springer-VerlagNew York 623 pp

Demaison GJ and Moore GT (1980) Anoxic environmentsand oil source bed genesis American Association ofPetroleum Geologists Bulletin 64 1179 ndash1209

Galan E and Carretero MI (1999) A new approach tocompositional limits for sepiolite and palygorskite Claysand Clay Minerals 47 399 ndash409

Gimsing AL and Borggaard OK (2002a) Competitiveadsorption and desorption of glyphosate and phosphate onclay silicates and oxides Clay Minerals 37 509ndash515

Gimsing AL and Borggaard OK (2002b) Effect ofphosphate on the adsorption of glyphosate on soils claymi ne ra l s and ox id e s In t e rna t iona l J our na l o fEnvironmental Analytical Chemistry 82 545 ndash552

Grim RE and Johns WD (1954) Clay mineral investigationsof sediments in the northern Gulf of Mexico Clays and ClayMinerals 2nd National Conference Pergamon New Yorkpp 81 ndash103

Huggett JM (1996) Aluminosilicate diagenesis in a Tertiarysandstone-mudrock sequence from the central North SeaUK Clay Minerals 31 523 ndash536

Huggett JM and Shaw HF (1997) Field emission electronmicroscopy ndash a high resolution technique for the study ofclay minerals in sediments Clay Minerals 32 197 ndash203

Jones B and Galan E (1988) Sepiolite and palygorskite Pp631ndash674 in Hydrous Phyllosillicates (SW Bailey editor)Reviews in Mineralogy 19 Mineralogical Society ofAmerica Washington DC

Kim J Peacor DR Tessier D and Elass F (1995) Atechnique for maintaining texture and permanent expansionof smectite interlayers for TEM observations Clays andClay Minerals 43 51 ndash57

Krekeler MPS (2004) Improved constraints on sedimentaryenvironments of palygorskite deposits of the HawthorneFormation southern Georgia from a detailed study of acore Clays and Clay Minerals 52 253 ndash262

Lamboy M (1982) Importance des pelotes fecales commeorigine des grains de phosphate lrsquoexemple du gisement deGaf sa (Tunisie) Comptes-R endus des Seances delrsquo Academie des Sciences Serie 2 Mecanique-PhysiqueChimie Sciences de lrsquoUnivers Sciences de la Terre 295595ndash600

Vol 52 No 3 2004 Microtextures of palygorskite-rich sediments 273

Oxides

Oxide minerals are common at the Pittman QuarryOxide minerals in the tan clay comprise an estimated3 ndash5 whereas these minerals comprise ~1 ndash3 in theblue clay The oxides are Ti-rich or Fe-Ti phases withthree textures being recognized (Figure 7) in smectite-dominated regions and in palygorskite-dominatedregions Nearly all of the grains are lt01 mm in diameterThe most common Ti oxide texture is one of smallaggregates usually lt200 nm in average diameter Theseaggregates consist of small crystals typically 5 ndash15 nmin width Where these aggregates occur they arecommonly separated by distances of 200 to 3000 nmAnother Fe-Ti oxide texture involves grains that are

typically 01 to 003 mm in diameter The grains aretypically equant and consist of crystal domains rangingin size from 5 nm to 700 nm or more The least-commonTi oxides observed are crystals or groups of crystals thatare usually square in profile These grains are~100 ndash200 nm on edge The EDS data indicate thatthese phases commonly have a limited (lt15) Fecontent These crystals may be zoned nearly perfector almost free of defects

Aggregates of cassiterite occur also in trace quan-tities in the clays studied (Figure 8) Cassiterite wasobserved primarily in the tan clay near the contact withthe blue clay Grains of ~200 nm in diameter arecommon and they may consist of several small crystalstypically 10 ndash20 nm in diameter Chemical analysisindicates that the cassiterite is pure Sn and O

DISCUSSION

Phyllosilicates

The proportion of birdrsquos-nest texture and bundles ofpalygorskite filling apparent voids is generally uniformin the tan and blue clay strata These textures coexist andare intimately related However they are interpreted asreflecting two separate phases of growth The birdrsquos-nesttexture is interpreted as an accumulation of palygorskitefibers precipitating from the water column In contrastthe bundles of palygorskite fibers are interpreted as alater-stage pore filling that occurred early in diagenesisperhaps at the sediment-water interface The textures donot suggest extensive compression or compaction ofsediment Thus the birdrsquos-nest texture of palygorskiteindicates that the hydraulic regime near the sediment-water interface was very stable and sedimentation rateswere low This supports the general environmentalinterpretations of Patterson (1974) Weaver and Beck(1977) Weaver (1984) and Merkl (1989)

The fine-grained smectite in these samples isauthigenic Regions rich in fine-grained smectite haveparticle orientations that are semi-random to anastomos-ing and do not have a preferred orientation as expectedfor smectite settling from suspension Fine-grainedsmectite textures are discordant with larger smectiteparticles that are clearly detrital eg the detrital smectitein Figure 3b Furthermore in no instance were texturesobserved indicating the preservation of detrital smectitefloccules For example small smectite particles do notradiate or are parallel to the (001) face of other smectiteparticles

Analysis by EDS of palygorskite and smectite showthat the chemical composition of these phases is similar(Table 1) This similarity suggests that changes in watercomposition are responsible for the conditions ofprecipitation for smectite or palygorskite Water chem-istry variations involving changes in Eh and pH forexample may be readily modified with an influx of freshwater or an inundation of seawater The Apalachicola

Figure 3 (a) TEM image showing fine-grained authigenicsmectite particles (b) TEM image showing large detritalsmectite particle

268 Krekeler Guggenheim and Rakovan Clays and Clay Minerals

embayment which was a narrow body of water wouldbe expected to have undergone extensive and rapidchanges in water chemistry owing to its narrowgeometry

An important textural relationship observed in thePittman Quarry samples is the epitaxial overgrowth ofsmectite and I-S on detrital illite and kaolinite particlesSimilar epitaxial relationships for illite and illite-smectite have been found in mudrocks by Huggett andShaw (1997) and Huggett (1996)

Merkl (1989) interpreted SEM images as evidence ofa transition between smectite palygorskite and kaolinite

Although these transformations are plausible and mayexist (eg palygorskite and smectite) in these sedimentsthe TEM textural data of this study indicate that theassociations of smectite with kaolinite and illite arerelated by epitaxy

The composite grains of epitaxial growth of smectiteon detrital kaolinite and illite may be related to astratified water column within the ApalachicolaEmbayment We define stratified water column as aregion of freshwater derived from continental dischargefrom rivers that overlies a water mass that is marine orhas elevated salinity Epitaxial growth of smectite is

Figure 4 TEM images of composite grains of detrital kaolinite and illite with smecite and I-S epitaxy (a) Kaolinite particlesurrounded by I-S (b) Small illite particle with I-S on one side of particle (c) Illite particle with I-S growth on both sides of grain(d) Kaolinite particle showing contact between smectite rich-illite smectite and kaolinite Kaolinite particles in these images arebeam damaged owing to the sensitivity of kaolinite to the electron beam

Vol 52 No 3 2004 Microtextures of palygorskite-rich sediments 269

most extensively developed on small detrital particlesThese particles were probably transported by freshwaterand deposited as individual crystals and not as flocculesFreshwater transport impedes flocculation whereassaline water enhances flocculation especially of smec-tite (Chamley 1989 Porrenga 1966 Grim and Johns1954 Shepard and Moore 1954) No textures wereobserved to indicate that composite grains are sedimen-tary aggregates

Detrital particles of kaolinite and illite are suffi-ciently small to be suspended in the water column for anextended period During this period smectite begins toprecipitate epitaxially on the detrital particles Assmectite crystallizes and adds mass to the particleeventually the particle becomes sufficiently large tosettle from the suspension producing the compositegrain textures observed

The possible formation of smectite or I-S oncomposite grains only after illite or kaolinite particleshave settled from suspension is not likely because thetextures of surrounding matrix of smectite and palygors-kite are discordant with the composite grain Smectiteparticles and palygorskite fibers surrounding compositegrains commonly have random or nearly random textureComposite grains are also much larger than thesurrounding matrix of smectite and palygorskite

Figure 5 TEM image of a detrital particle of illite in matrix ofsmectite

Figure 6 (right) TEM images of textures of hydroxylapatite(a) This rounded oval aggregate of hydroxylapatite is inter-preted as a fecal pellet (b) Cluster of apatite crystals in matrix ofpalygorskite fibers (c) Near atomic resolution structural imageof apatite along the [001] direction The lsquodiamondrsquo patternspacing is ~95 AEcirc and corresponds to the cell dimensions ofhydroxylapatite

270 Krekeler Guggenheim and Rakovan Clays and Clay Minerals

Smectite overgrowths in composite grains are oftenthicker and more coherent with fewer stacking defectsthan the smaller less ordered matrix of smectite Thistexture suggests that smectite and I-S in composite grainsmust have a different origin than the matrix smectite

This model of the water column is consistent with theresults of Patterson (1974) Weaver (1984) and Merkl(1989) Merkl (1989) suggested a gradient in salinitybased on the prevalence of smectite in the northeasternportion of the Apalachicola embayment and the morepalygorskite-rich sediments in the southwestern regionsof the embayment Such a distribution suggests astratified water column Patterson (1974) observedexamples of very organic-rich strata within the paly-gorskite clays of the Hawthorne Formation and thesestrata also suggest a stratified water column Stratifiedwater columns are known to be an important mechanismto accumulate organic matter in the sedimentary record(eg Murphy et al 2000 Canfield 1994 Arthur andSagemon 1994 Tyson and Pearson 1991 Pendersonand Calvert 1990 Demaison and Moore 1980)

Phosphates

Hydroxylapatite is the only recognized phosphatemineral occurring in the Pittman Quarry samples Thetextures of the hydroxylapatite suggest that much of thephosphate is derived from fecal pellets The largeraggregates of hydroxylapatite have well defined bound-aries are ovoid in shape and are generally consistent insize with that expected of fecal pellets of smallorganisms Fecal pellets from zooplankton and otherorganisms are believed to be an important mechanismfor concentrating and depositing phosphate in othersedimentary environments (Lamboy 1982 Porter andRobbins 1981 Robbins and Porter 1980 Bremmer1975) The sedimentary source of phosphate is inter-preted as being from fecal material from zooplankton orpossibly small arthropods

The three textures for hydroxylapatite suggest adissolution and precipitation process involving phos-phate originally from the fecal pellets The intimateassociations of single crystals and small clusters ofapatite crystals and the relative decrease in theprevalence of these textures away from the largeaggregate phosphate grains is interpreted as reflectinga process involving a concentration mechanism Adissolution and re-precipitation mechanism would beexpected to mobilize the phosphate to produce localapatite concentrations

Figure 7 (left) Representative TEM images of oxide minerals(a) This mineral is intepreted as a detrital Ti oxide owing to itsrounded nature SAED shows that this grain is composed ofmultiple crystals (b) Aggregates of Ti oxides with grains10 ndash20 nm in diameter The aggregates are interpreted asauthigenic (c) Euhedral outline and zoning indicate that thisFe-Ti oxide grain is authigenic

Vol 52 No 3 2004 Microtextures of palygorskite-rich sediments 271

Thus caution should be used in interpreting whole-rock geochemical data from these sediments for prove-nance and other studies because the distribution of Ca SrP and rare earth elements (REE) are strongly controlled bythe distribution of apatite Therefore whole-rock studies

using REE and Sr isotopes may be of limited use tointerpret provenance of these deposits in future studiesbut may possibly be of some use in understandingphosphate mobilization The ``roll-front weatheringrsquorsquo asdescribed by Merkl (1989) is an oxidation process whichimplies a fluctuation in a paleo-water table and may be thecause of higher concentrations of hydroxylapatite in thetan clay Study of REE and Sr isotopes may provideinsight into this mobilization process

Phosphate complexes are well known to interferewith sorption of organic molecules on clay mineralsurfaces (Theng 1974 Solomon et al 1971 Solomon1968 Solomon et al 1968) particularly with respect topalygorskite Aluminum in the octahedral sheet of 21ribbons at the edges of palygorskite fibers normally actas electron acceptors (Theng 1974 Solomon 1968Solomon and Rosser 1965) In the presence ofphosphate however these surface sites become deacti-vated by phosphate and polyphosphate (Theng 1974Solomon 1968) More recent work has shown thatphosphate complexes decrease sorption of herbicides onpillared clay and crystal violet montmorillonite(Polubesova et al 2002) Gimsing and Borggaard(2002a 2002b) have shown that phosphate interfereswith sorption of glyphosate in clay minerals

The identification of hydroxylapatite in these sedi-ments possibly explains the differences in commercialgrade between the blue clay and the tan clay Palygorskitefibers may be naturally coated with phosphate as a resultof diagenesis during the roll-front weathering processAlternatively because of the small size of crystals (30 to100 nm) and moderate solubility of hydroxylapatite (logKSP of 849 Brown 1960) hydroxylapatite may readilydissolve when these clays are used in applicationsinvolving water or mixed organic-water fluids Thereforepalygorskite-rich sediments such as the tan clay with 3 to5 hydroxylapatite are more likely to produce phosphatecomplexes when interacting with aqueous or mixedorganic-aqueous fluids As a result the tan clays have alower sorption capacity than palygorskite-rich sedimentswith less or no hydroxylapatite

Oxides

Although the oxide minerals generally comprise lt5of the bulk mineralogy of the clay units studied theyshow a range of diversity that suggests most are ofauthigenic origin Most of the Ti oxides are interpretedas authigenic phases based on textures which involvesmectite and palygorskite The phyllosilicates are gen-erally intimately associated with the oxide grains andthe oxides often have delicate features that mechanicaltransport would probably disrupt or destroy Detritaloxides are common and all of the Fe-Ti oxides that areinterpreted as detrital grains are probably derived fromthe Appalachian Piedmont although they may be eolian

Oxides occur in the tan and blue clays but they aresomewhat more abundant and diverse in the tan clay

Figure 8 (a) TEM image of aggregate of cassiteri te withindividual crystals ~15 nm in diameter (b) Cassiterite aggre-gate showing multiple single crystals in the aggregate (c) EDSspectra from a cassiterite aggregate showing Sn and O peaks theC and Cu are from C coating and Cu substrate

272 Krekeler Guggenheim and Rakovan Clays and Clay Minerals

The tan clay is believed to involve the ``roll-front typeweatheringrsquorsquo process (Merkl 1989) and this implies thatwater has percolated through the unit to producealteration and oxidation of the tan clay We interpretthe bluetan contact and adjacent tan and blue clay unitsas reflecting a paleo-water table where the tan clayexisted in the vadose zone and was subject to drying andre-wetting as the water table fluctuated This enabled theelemental components for the oxide minerals to bemobilized and oxidized in the tan clay as would beexpected in a vadose zone The higher concentrations ofoxide minerals in the tan clay are consistent with thisinterpretation This may be the cause of the colordifference between the tan and blue clays Alternativelythe oxidation state of cations in the palygorskite orsmectite may be the cause of color variation and may belinked to a fluctuating paleo-base level

Cassiterite which appears restricted to the lowermosttan clay unit is believed to be an authigenic phase owingto the small intergrown crystals and euhedral cross-sections The restricted stratigraphic occurrence at thebase of the paleo-vadose zone in the tan clay alsosuggests that it is authigenic Tin was probablymobilized throughout the section and was re-precipitatedduring fluctuations of the water table

CONCLUSIONS

The textures of the minerals at the Pittman quarrysection indicate that there was an evolving and complexmineralogical and geochemical system during and afterdeposi tion of the palygorski te deposits of theHawthorne Textures indicate that several differentreactions occurred to form these sediments such asmultiple stages of growth of palygorskite overgrowth ofsmectite and I-S on detrital clay particles dissolutionand re-precipitation of hydroxylapatite and precipitationof oxide minerals These reactions occurred in geologi-cal environments on scales ranging from the depth of thewater column to nanometers

This study documents smectite overgrowths ondetrital particles in a palygorskite deposit and thisunusual texture is probably a result of a stratifiedwater column that existed in the Apalachicola embay-ment The findings of our study are consistent withenvironmental interpretations of Patterson (1974)Weaver and Beck (1977) Weaver (1984) and Merkl(1989) Our model may be tested by the analysis ofmicrofossils that may indicate mixed freshwater andmarine assemblages and organic geochemistry whichmay yield molecular evidence of environmentallyspecific bacteria

ACKNOWLEDGMENTS

We thank W F Moll for valuable discussions and JRoth and A Nichols of the Research Resources Center atthe University of Illinois at Chicago for assistance with

electron microscopy We thank the US National ScienceFoundation for support under grants EAR0001122 andEAR0001251

REFERENCES

Arthur MA and Sagemon BB (1994) Marine black shalesA review of depositional mechanisms and significance ofancient deposits Annual Review Earth and PlanetarySciences 22 499 ndash551

Bickmore B Bosbach D Hochella MF Jr Charlet L andRufe E (2001) In situ atomic force microscopy study ofhectorite and nontronite dissolution Implicat ions forphyllosilicate edge surface structures and dissolutionmechanisms American Mineralogist 86 411ndash423

Bremner JM (1975) Faecal pellets glauconite phosphoriteand bedrock from the Kunene-Walvis continental marginTechnical Report ndash South African National Committee forOceanographic Research Marine Geology Programme 7Progress Report for the Year pp 59 ndash67

Brown WE (1960) Behaviour of slightly soluble calciumphosphate as revealed by phase equilibrium calculationsSoil Science 90 51 ndash57

Canfield DE (1994) Factors influencing organic carbonpreservation in marine sediments Chemical Geology 114315ndash329

Chamley H (1989) Clay Sedimentology Springer-VerlagNew York 623 pp

Demaison GJ and Moore GT (1980) Anoxic environmentsand oil source bed genesis American Association ofPetroleum Geologists Bulletin 64 1179 ndash1209

Galan E and Carretero MI (1999) A new approach tocompositional limits for sepiolite and palygorskite Claysand Clay Minerals 47 399 ndash409

Gimsing AL and Borggaard OK (2002a) Competitiveadsorption and desorption of glyphosate and phosphate onclay silicates and oxides Clay Minerals 37 509ndash515

Gimsing AL and Borggaard OK (2002b) Effect ofphosphate on the adsorption of glyphosate on soils claymi ne ra l s and ox id e s In t e rna t iona l J our na l o fEnvironmental Analytical Chemistry 82 545 ndash552

Grim RE and Johns WD (1954) Clay mineral investigationsof sediments in the northern Gulf of Mexico Clays and ClayMinerals 2nd National Conference Pergamon New Yorkpp 81 ndash103

Huggett JM (1996) Aluminosilicate diagenesis in a Tertiarysandstone-mudrock sequence from the central North SeaUK Clay Minerals 31 523 ndash536

Huggett JM and Shaw HF (1997) Field emission electronmicroscopy ndash a high resolution technique for the study ofclay minerals in sediments Clay Minerals 32 197 ndash203

Jones B and Galan E (1988) Sepiolite and palygorskite Pp631ndash674 in Hydrous Phyllosillicates (SW Bailey editor)Reviews in Mineralogy 19 Mineralogical Society ofAmerica Washington DC

Kim J Peacor DR Tessier D and Elass F (1995) Atechnique for maintaining texture and permanent expansionof smectite interlayers for TEM observations Clays andClay Minerals 43 51 ndash57

Krekeler MPS (2004) Improved constraints on sedimentaryenvironments of palygorskite deposits of the HawthorneFormation southern Georgia from a detailed study of acore Clays and Clay Minerals 52 253 ndash262

Lamboy M (1982) Importance des pelotes fecales commeorigine des grains de phosphate lrsquoexemple du gisement deGaf sa (Tunisie) Comptes-R endus des Seances delrsquo Academie des Sciences Serie 2 Mecanique-PhysiqueChimie Sciences de lrsquoUnivers Sciences de la Terre 295595ndash600

Vol 52 No 3 2004 Microtextures of palygorskite-rich sediments 273

embayment which was a narrow body of water wouldbe expected to have undergone extensive and rapidchanges in water chemistry owing to its narrowgeometry

An important textural relationship observed in thePittman Quarry samples is the epitaxial overgrowth ofsmectite and I-S on detrital illite and kaolinite particlesSimilar epitaxial relationships for illite and illite-smectite have been found in mudrocks by Huggett andShaw (1997) and Huggett (1996)

Merkl (1989) interpreted SEM images as evidence ofa transition between smectite palygorskite and kaolinite

Although these transformations are plausible and mayexist (eg palygorskite and smectite) in these sedimentsthe TEM textural data of this study indicate that theassociations of smectite with kaolinite and illite arerelated by epitaxy

The composite grains of epitaxial growth of smectiteon detrital kaolinite and illite may be related to astratified water column within the ApalachicolaEmbayment We define stratified water column as aregion of freshwater derived from continental dischargefrom rivers that overlies a water mass that is marine orhas elevated salinity Epitaxial growth of smectite is

Figure 4 TEM images of composite grains of detrital kaolinite and illite with smecite and I-S epitaxy (a) Kaolinite particlesurrounded by I-S (b) Small illite particle with I-S on one side of particle (c) Illite particle with I-S growth on both sides of grain(d) Kaolinite particle showing contact between smectite rich-illite smectite and kaolinite Kaolinite particles in these images arebeam damaged owing to the sensitivity of kaolinite to the electron beam

Vol 52 No 3 2004 Microtextures of palygorskite-rich sediments 269

most extensively developed on small detrital particlesThese particles were probably transported by freshwaterand deposited as individual crystals and not as flocculesFreshwater transport impedes flocculation whereassaline water enhances flocculation especially of smec-tite (Chamley 1989 Porrenga 1966 Grim and Johns1954 Shepard and Moore 1954) No textures wereobserved to indicate that composite grains are sedimen-tary aggregates

Detrital particles of kaolinite and illite are suffi-ciently small to be suspended in the water column for anextended period During this period smectite begins toprecipitate epitaxially on the detrital particles Assmectite crystallizes and adds mass to the particleeventually the particle becomes sufficiently large tosettle from the suspension producing the compositegrain textures observed

The possible formation of smectite or I-S oncomposite grains only after illite or kaolinite particleshave settled from suspension is not likely because thetextures of surrounding matrix of smectite and palygors-kite are discordant with the composite grain Smectiteparticles and palygorskite fibers surrounding compositegrains commonly have random or nearly random textureComposite grains are also much larger than thesurrounding matrix of smectite and palygorskite

Figure 5 TEM image of a detrital particle of illite in matrix ofsmectite

Figure 6 (right) TEM images of textures of hydroxylapatite(a) This rounded oval aggregate of hydroxylapatite is inter-preted as a fecal pellet (b) Cluster of apatite crystals in matrix ofpalygorskite fibers (c) Near atomic resolution structural imageof apatite along the [001] direction The lsquodiamondrsquo patternspacing is ~95 AEcirc and corresponds to the cell dimensions ofhydroxylapatite

270 Krekeler Guggenheim and Rakovan Clays and Clay Minerals

Smectite overgrowths in composite grains are oftenthicker and more coherent with fewer stacking defectsthan the smaller less ordered matrix of smectite Thistexture suggests that smectite and I-S in composite grainsmust have a different origin than the matrix smectite

This model of the water column is consistent with theresults of Patterson (1974) Weaver (1984) and Merkl(1989) Merkl (1989) suggested a gradient in salinitybased on the prevalence of smectite in the northeasternportion of the Apalachicola embayment and the morepalygorskite-rich sediments in the southwestern regionsof the embayment Such a distribution suggests astratified water column Patterson (1974) observedexamples of very organic-rich strata within the paly-gorskite clays of the Hawthorne Formation and thesestrata also suggest a stratified water column Stratifiedwater columns are known to be an important mechanismto accumulate organic matter in the sedimentary record(eg Murphy et al 2000 Canfield 1994 Arthur andSagemon 1994 Tyson and Pearson 1991 Pendersonand Calvert 1990 Demaison and Moore 1980)

Phosphates

Hydroxylapatite is the only recognized phosphatemineral occurring in the Pittman Quarry samples Thetextures of the hydroxylapatite suggest that much of thephosphate is derived from fecal pellets The largeraggregates of hydroxylapatite have well defined bound-aries are ovoid in shape and are generally consistent insize with that expected of fecal pellets of smallorganisms Fecal pellets from zooplankton and otherorganisms are believed to be an important mechanismfor concentrating and depositing phosphate in othersedimentary environments (Lamboy 1982 Porter andRobbins 1981 Robbins and Porter 1980 Bremmer1975) The sedimentary source of phosphate is inter-preted as being from fecal material from zooplankton orpossibly small arthropods

The three textures for hydroxylapatite suggest adissolution and precipitation process involving phos-phate originally from the fecal pellets The intimateassociations of single crystals and small clusters ofapatite crystals and the relative decrease in theprevalence of these textures away from the largeaggregate phosphate grains is interpreted as reflectinga process involving a concentration mechanism Adissolution and re-precipitation mechanism would beexpected to mobilize the phosphate to produce localapatite concentrations

Figure 7 (left) Representative TEM images of oxide minerals(a) This mineral is intepreted as a detrital Ti oxide owing to itsrounded nature SAED shows that this grain is composed ofmultiple crystals (b) Aggregates of Ti oxides with grains10 ndash20 nm in diameter The aggregates are interpreted asauthigenic (c) Euhedral outline and zoning indicate that thisFe-Ti oxide grain is authigenic

Vol 52 No 3 2004 Microtextures of palygorskite-rich sediments 271

Thus caution should be used in interpreting whole-rock geochemical data from these sediments for prove-nance and other studies because the distribution of Ca SrP and rare earth elements (REE) are strongly controlled bythe distribution of apatite Therefore whole-rock studies

using REE and Sr isotopes may be of limited use tointerpret provenance of these deposits in future studiesbut may possibly be of some use in understandingphosphate mobilization The ``roll-front weatheringrsquorsquo asdescribed by Merkl (1989) is an oxidation process whichimplies a fluctuation in a paleo-water table and may be thecause of higher concentrations of hydroxylapatite in thetan clay Study of REE and Sr isotopes may provideinsight into this mobilization process

Phosphate complexes are well known to interferewith sorption of organic molecules on clay mineralsurfaces (Theng 1974 Solomon et al 1971 Solomon1968 Solomon et al 1968) particularly with respect topalygorskite Aluminum in the octahedral sheet of 21ribbons at the edges of palygorskite fibers normally actas electron acceptors (Theng 1974 Solomon 1968Solomon and Rosser 1965) In the presence ofphosphate however these surface sites become deacti-vated by phosphate and polyphosphate (Theng 1974Solomon 1968) More recent work has shown thatphosphate complexes decrease sorption of herbicides onpillared clay and crystal violet montmorillonite(Polubesova et al 2002) Gimsing and Borggaard(2002a 2002b) have shown that phosphate interfereswith sorption of glyphosate in clay minerals

The identification of hydroxylapatite in these sedi-ments possibly explains the differences in commercialgrade between the blue clay and the tan clay Palygorskitefibers may be naturally coated with phosphate as a resultof diagenesis during the roll-front weathering processAlternatively because of the small size of crystals (30 to100 nm) and moderate solubility of hydroxylapatite (logKSP of 849 Brown 1960) hydroxylapatite may readilydissolve when these clays are used in applicationsinvolving water or mixed organic-water fluids Thereforepalygorskite-rich sediments such as the tan clay with 3 to5 hydroxylapatite are more likely to produce phosphatecomplexes when interacting with aqueous or mixedorganic-aqueous fluids As a result the tan clays have alower sorption capacity than palygorskite-rich sedimentswith less or no hydroxylapatite

Oxides

Although the oxide minerals generally comprise lt5of the bulk mineralogy of the clay units studied theyshow a range of diversity that suggests most are ofauthigenic origin Most of the Ti oxides are interpretedas authigenic phases based on textures which involvesmectite and palygorskite The phyllosilicates are gen-erally intimately associated with the oxide grains andthe oxides often have delicate features that mechanicaltransport would probably disrupt or destroy Detritaloxides are common and all of the Fe-Ti oxides that areinterpreted as detrital grains are probably derived fromthe Appalachian Piedmont although they may be eolian

Oxides occur in the tan and blue clays but they aresomewhat more abundant and diverse in the tan clay

Figure 8 (a) TEM image of aggregate of cassiteri te withindividual crystals ~15 nm in diameter (b) Cassiterite aggre-gate showing multiple single crystals in the aggregate (c) EDSspectra from a cassiterite aggregate showing Sn and O peaks theC and Cu are from C coating and Cu substrate

272 Krekeler Guggenheim and Rakovan Clays and Clay Minerals

The tan clay is believed to involve the ``roll-front typeweatheringrsquorsquo process (Merkl 1989) and this implies thatwater has percolated through the unit to producealteration and oxidation of the tan clay We interpretthe bluetan contact and adjacent tan and blue clay unitsas reflecting a paleo-water table where the tan clayexisted in the vadose zone and was subject to drying andre-wetting as the water table fluctuated This enabled theelemental components for the oxide minerals to bemobilized and oxidized in the tan clay as would beexpected in a vadose zone The higher concentrations ofoxide minerals in the tan clay are consistent with thisinterpretation This may be the cause of the colordifference between the tan and blue clays Alternativelythe oxidation state of cations in the palygorskite orsmectite may be the cause of color variation and may belinked to a fluctuating paleo-base level

Cassiterite which appears restricted to the lowermosttan clay unit is believed to be an authigenic phase owingto the small intergrown crystals and euhedral cross-sections The restricted stratigraphic occurrence at thebase of the paleo-vadose zone in the tan clay alsosuggests that it is authigenic Tin was probablymobilized throughout the section and was re-precipitatedduring fluctuations of the water table

CONCLUSIONS

The textures of the minerals at the Pittman quarrysection indicate that there was an evolving and complexmineralogical and geochemical system during and afterdeposi tion of the palygorski te deposits of theHawthorne Textures indicate that several differentreactions occurred to form these sediments such asmultiple stages of growth of palygorskite overgrowth ofsmectite and I-S on detrital clay particles dissolutionand re-precipitation of hydroxylapatite and precipitationof oxide minerals These reactions occurred in geologi-cal environments on scales ranging from the depth of thewater column to nanometers

This study documents smectite overgrowths ondetrital particles in a palygorskite deposit and thisunusual texture is probably a result of a stratifiedwater column that existed in the Apalachicola embay-ment The findings of our study are consistent withenvironmental interpretations of Patterson (1974)Weaver and Beck (1977) Weaver (1984) and Merkl(1989) Our model may be tested by the analysis ofmicrofossils that may indicate mixed freshwater andmarine assemblages and organic geochemistry whichmay yield molecular evidence of environmentallyspecific bacteria

ACKNOWLEDGMENTS

We thank W F Moll for valuable discussions and JRoth and A Nichols of the Research Resources Center atthe University of Illinois at Chicago for assistance with

electron microscopy We thank the US National ScienceFoundation for support under grants EAR0001122 andEAR0001251

REFERENCES

Arthur MA and Sagemon BB (1994) Marine black shalesA review of depositional mechanisms and significance ofancient deposits Annual Review Earth and PlanetarySciences 22 499 ndash551

Bickmore B Bosbach D Hochella MF Jr Charlet L andRufe E (2001) In situ atomic force microscopy study ofhectorite and nontronite dissolution Implicat ions forphyllosilicate edge surface structures and dissolutionmechanisms American Mineralogist 86 411ndash423

Bremner JM (1975) Faecal pellets glauconite phosphoriteand bedrock from the Kunene-Walvis continental marginTechnical Report ndash South African National Committee forOceanographic Research Marine Geology Programme 7Progress Report for the Year pp 59 ndash67

Brown WE (1960) Behaviour of slightly soluble calciumphosphate as revealed by phase equilibrium calculationsSoil Science 90 51 ndash57

Canfield DE (1994) Factors influencing organic carbonpreservation in marine sediments Chemical Geology 114315ndash329

Chamley H (1989) Clay Sedimentology Springer-VerlagNew York 623 pp

Demaison GJ and Moore GT (1980) Anoxic environmentsand oil source bed genesis American Association ofPetroleum Geologists Bulletin 64 1179 ndash1209

Galan E and Carretero MI (1999) A new approach tocompositional limits for sepiolite and palygorskite Claysand Clay Minerals 47 399 ndash409

Gimsing AL and Borggaard OK (2002a) Competitiveadsorption and desorption of glyphosate and phosphate onclay silicates and oxides Clay Minerals 37 509ndash515

Gimsing AL and Borggaard OK (2002b) Effect ofphosphate on the adsorption of glyphosate on soils claymi ne ra l s and ox id e s In t e rna t iona l J our na l o fEnvironmental Analytical Chemistry 82 545 ndash552

Grim RE and Johns WD (1954) Clay mineral investigationsof sediments in the northern Gulf of Mexico Clays and ClayMinerals 2nd National Conference Pergamon New Yorkpp 81 ndash103

Huggett JM (1996) Aluminosilicate diagenesis in a Tertiarysandstone-mudrock sequence from the central North SeaUK Clay Minerals 31 523 ndash536

Huggett JM and Shaw HF (1997) Field emission electronmicroscopy ndash a high resolution technique for the study ofclay minerals in sediments Clay Minerals 32 197 ndash203

Jones B and Galan E (1988) Sepiolite and palygorskite Pp631ndash674 in Hydrous Phyllosillicates (SW Bailey editor)Reviews in Mineralogy 19 Mineralogical Society ofAmerica Washington DC

Kim J Peacor DR Tessier D and Elass F (1995) Atechnique for maintaining texture and permanent expansionof smectite interlayers for TEM observations Clays andClay Minerals 43 51 ndash57

Krekeler MPS (2004) Improved constraints on sedimentaryenvironments of palygorskite deposits of the HawthorneFormation southern Georgia from a detailed study of acore Clays and Clay Minerals 52 253 ndash262

Lamboy M (1982) Importance des pelotes fecales commeorigine des grains de phosphate lrsquoexemple du gisement deGaf sa (Tunisie) Comptes-R endus des Seances delrsquo Academie des Sciences Serie 2 Mecanique-PhysiqueChimie Sciences de lrsquoUnivers Sciences de la Terre 295595ndash600

Vol 52 No 3 2004 Microtextures of palygorskite-rich sediments 273

most extensively developed on small detrital particlesThese particles were probably transported by freshwaterand deposited as individual crystals and not as flocculesFreshwater transport impedes flocculation whereassaline water enhances flocculation especially of smec-tite (Chamley 1989 Porrenga 1966 Grim and Johns1954 Shepard and Moore 1954) No textures wereobserved to indicate that composite grains are sedimen-tary aggregates

Detrital particles of kaolinite and illite are suffi-ciently small to be suspended in the water column for anextended period During this period smectite begins toprecipitate epitaxially on the detrital particles Assmectite crystallizes and adds mass to the particleeventually the particle becomes sufficiently large tosettle from the suspension producing the compositegrain textures observed

The possible formation of smectite or I-S oncomposite grains only after illite or kaolinite particleshave settled from suspension is not likely because thetextures of surrounding matrix of smectite and palygors-kite are discordant with the composite grain Smectiteparticles and palygorskite fibers surrounding compositegrains commonly have random or nearly random textureComposite grains are also much larger than thesurrounding matrix of smectite and palygorskite

Figure 5 TEM image of a detrital particle of illite in matrix ofsmectite

Figure 6 (right) TEM images of textures of hydroxylapatite(a) This rounded oval aggregate of hydroxylapatite is inter-preted as a fecal pellet (b) Cluster of apatite crystals in matrix ofpalygorskite fibers (c) Near atomic resolution structural imageof apatite along the [001] direction The lsquodiamondrsquo patternspacing is ~95 AEcirc and corresponds to the cell dimensions ofhydroxylapatite

270 Krekeler Guggenheim and Rakovan Clays and Clay Minerals

Smectite overgrowths in composite grains are oftenthicker and more coherent with fewer stacking defectsthan the smaller less ordered matrix of smectite Thistexture suggests that smectite and I-S in composite grainsmust have a different origin than the matrix smectite

This model of the water column is consistent with theresults of Patterson (1974) Weaver (1984) and Merkl(1989) Merkl (1989) suggested a gradient in salinitybased on the prevalence of smectite in the northeasternportion of the Apalachicola embayment and the morepalygorskite-rich sediments in the southwestern regionsof the embayment Such a distribution suggests astratified water column Patterson (1974) observedexamples of very organic-rich strata within the paly-gorskite clays of the Hawthorne Formation and thesestrata also suggest a stratified water column Stratifiedwater columns are known to be an important mechanismto accumulate organic matter in the sedimentary record(eg Murphy et al 2000 Canfield 1994 Arthur andSagemon 1994 Tyson and Pearson 1991 Pendersonand Calvert 1990 Demaison and Moore 1980)

Phosphates

Hydroxylapatite is the only recognized phosphatemineral occurring in the Pittman Quarry samples Thetextures of the hydroxylapatite suggest that much of thephosphate is derived from fecal pellets The largeraggregates of hydroxylapatite have well defined bound-aries are ovoid in shape and are generally consistent insize with that expected of fecal pellets of smallorganisms Fecal pellets from zooplankton and otherorganisms are believed to be an important mechanismfor concentrating and depositing phosphate in othersedimentary environments (Lamboy 1982 Porter andRobbins 1981 Robbins and Porter 1980 Bremmer1975) The sedimentary source of phosphate is inter-preted as being from fecal material from zooplankton orpossibly small arthropods

The three textures for hydroxylapatite suggest adissolution and precipitation process involving phos-phate originally from the fecal pellets The intimateassociations of single crystals and small clusters ofapatite crystals and the relative decrease in theprevalence of these textures away from the largeaggregate phosphate grains is interpreted as reflectinga process involving a concentration mechanism Adissolution and re-precipitation mechanism would beexpected to mobilize the phosphate to produce localapatite concentrations

Figure 7 (left) Representative TEM images of oxide minerals(a) This mineral is intepreted as a detrital Ti oxide owing to itsrounded nature SAED shows that this grain is composed ofmultiple crystals (b) Aggregates of Ti oxides with grains10 ndash20 nm in diameter The aggregates are interpreted asauthigenic (c) Euhedral outline and zoning indicate that thisFe-Ti oxide grain is authigenic

Vol 52 No 3 2004 Microtextures of palygorskite-rich sediments 271

Thus caution should be used in interpreting whole-rock geochemical data from these sediments for prove-nance and other studies because the distribution of Ca SrP and rare earth elements (REE) are strongly controlled bythe distribution of apatite Therefore whole-rock studies

using REE and Sr isotopes may be of limited use tointerpret provenance of these deposits in future studiesbut may possibly be of some use in understandingphosphate mobilization The ``roll-front weatheringrsquorsquo asdescribed by Merkl (1989) is an oxidation process whichimplies a fluctuation in a paleo-water table and may be thecause of higher concentrations of hydroxylapatite in thetan clay Study of REE and Sr isotopes may provideinsight into this mobilization process

Phosphate complexes are well known to interferewith sorption of organic molecules on clay mineralsurfaces (Theng 1974 Solomon et al 1971 Solomon1968 Solomon et al 1968) particularly with respect topalygorskite Aluminum in the octahedral sheet of 21ribbons at the edges of palygorskite fibers normally actas electron acceptors (Theng 1974 Solomon 1968Solomon and Rosser 1965) In the presence ofphosphate however these surface sites become deacti-vated by phosphate and polyphosphate (Theng 1974Solomon 1968) More recent work has shown thatphosphate complexes decrease sorption of herbicides onpillared clay and crystal violet montmorillonite(Polubesova et al 2002) Gimsing and Borggaard(2002a 2002b) have shown that phosphate interfereswith sorption of glyphosate in clay minerals

The identification of hydroxylapatite in these sedi-ments possibly explains the differences in commercialgrade between the blue clay and the tan clay Palygorskitefibers may be naturally coated with phosphate as a resultof diagenesis during the roll-front weathering processAlternatively because of the small size of crystals (30 to100 nm) and moderate solubility of hydroxylapatite (logKSP of 849 Brown 1960) hydroxylapatite may readilydissolve when these clays are used in applicationsinvolving water or mixed organic-water fluids Thereforepalygorskite-rich sediments such as the tan clay with 3 to5 hydroxylapatite are more likely to produce phosphatecomplexes when interacting with aqueous or mixedorganic-aqueous fluids As a result the tan clays have alower sorption capacity than palygorskite-rich sedimentswith less or no hydroxylapatite

Oxides

Although the oxide minerals generally comprise lt5of the bulk mineralogy of the clay units studied theyshow a range of diversity that suggests most are ofauthigenic origin Most of the Ti oxides are interpretedas authigenic phases based on textures which involvesmectite and palygorskite The phyllosilicates are gen-erally intimately associated with the oxide grains andthe oxides often have delicate features that mechanicaltransport would probably disrupt or destroy Detritaloxides are common and all of the Fe-Ti oxides that areinterpreted as detrital grains are probably derived fromthe Appalachian Piedmont although they may be eolian

Oxides occur in the tan and blue clays but they aresomewhat more abundant and diverse in the tan clay

Figure 8 (a) TEM image of aggregate of cassiteri te withindividual crystals ~15 nm in diameter (b) Cassiterite aggre-gate showing multiple single crystals in the aggregate (c) EDSspectra from a cassiterite aggregate showing Sn and O peaks theC and Cu are from C coating and Cu substrate

272 Krekeler Guggenheim and Rakovan Clays and Clay Minerals

The tan clay is believed to involve the ``roll-front typeweatheringrsquorsquo process (Merkl 1989) and this implies thatwater has percolated through the unit to producealteration and oxidation of the tan clay We interpretthe bluetan contact and adjacent tan and blue clay unitsas reflecting a paleo-water table where the tan clayexisted in the vadose zone and was subject to drying andre-wetting as the water table fluctuated This enabled theelemental components for the oxide minerals to bemobilized and oxidized in the tan clay as would beexpected in a vadose zone The higher concentrations ofoxide minerals in the tan clay are consistent with thisinterpretation This may be the cause of the colordifference between the tan and blue clays Alternativelythe oxidation state of cations in the palygorskite orsmectite may be the cause of color variation and may belinked to a fluctuating paleo-base level

Cassiterite which appears restricted to the lowermosttan clay unit is believed to be an authigenic phase owingto the small intergrown crystals and euhedral cross-sections The restricted stratigraphic occurrence at thebase of the paleo-vadose zone in the tan clay alsosuggests that it is authigenic Tin was probablymobilized throughout the section and was re-precipitatedduring fluctuations of the water table

CONCLUSIONS

The textures of the minerals at the Pittman quarrysection indicate that there was an evolving and complexmineralogical and geochemical system during and afterdeposi tion of the palygorski te deposits of theHawthorne Textures indicate that several differentreactions occurred to form these sediments such asmultiple stages of growth of palygorskite overgrowth ofsmectite and I-S on detrital clay particles dissolutionand re-precipitation of hydroxylapatite and precipitationof oxide minerals These reactions occurred in geologi-cal environments on scales ranging from the depth of thewater column to nanometers

This study documents smectite overgrowths ondetrital particles in a palygorskite deposit and thisunusual texture is probably a result of a stratifiedwater column that existed in the Apalachicola embay-ment The findings of our study are consistent withenvironmental interpretations of Patterson (1974)Weaver and Beck (1977) Weaver (1984) and Merkl(1989) Our model may be tested by the analysis ofmicrofossils that may indicate mixed freshwater andmarine assemblages and organic geochemistry whichmay yield molecular evidence of environmentallyspecific bacteria

ACKNOWLEDGMENTS

We thank W F Moll for valuable discussions and JRoth and A Nichols of the Research Resources Center atthe University of Illinois at Chicago for assistance with

electron microscopy We thank the US National ScienceFoundation for support under grants EAR0001122 andEAR0001251

REFERENCES

Arthur MA and Sagemon BB (1994) Marine black shalesA review of depositional mechanisms and significance ofancient deposits Annual Review Earth and PlanetarySciences 22 499 ndash551

Bickmore B Bosbach D Hochella MF Jr Charlet L andRufe E (2001) In situ atomic force microscopy study ofhectorite and nontronite dissolution Implicat ions forphyllosilicate edge surface structures and dissolutionmechanisms American Mineralogist 86 411ndash423

Bremner JM (1975) Faecal pellets glauconite phosphoriteand bedrock from the Kunene-Walvis continental marginTechnical Report ndash South African National Committee forOceanographic Research Marine Geology Programme 7Progress Report for the Year pp 59 ndash67

Brown WE (1960) Behaviour of slightly soluble calciumphosphate as revealed by phase equilibrium calculationsSoil Science 90 51 ndash57

Canfield DE (1994) Factors influencing organic carbonpreservation in marine sediments Chemical Geology 114315ndash329

Chamley H (1989) Clay Sedimentology Springer-VerlagNew York 623 pp

Demaison GJ and Moore GT (1980) Anoxic environmentsand oil source bed genesis American Association ofPetroleum Geologists Bulletin 64 1179 ndash1209

Galan E and Carretero MI (1999) A new approach tocompositional limits for sepiolite and palygorskite Claysand Clay Minerals 47 399 ndash409

Gimsing AL and Borggaard OK (2002a) Competitiveadsorption and desorption of glyphosate and phosphate onclay silicates and oxides Clay Minerals 37 509ndash515

Gimsing AL and Borggaard OK (2002b) Effect ofphosphate on the adsorption of glyphosate on soils claymi ne ra l s and ox id e s In t e rna t iona l J our na l o fEnvironmental Analytical Chemistry 82 545 ndash552

Grim RE and Johns WD (1954) Clay mineral investigationsof sediments in the northern Gulf of Mexico Clays and ClayMinerals 2nd National Conference Pergamon New Yorkpp 81 ndash103

Huggett JM (1996) Aluminosilicate diagenesis in a Tertiarysandstone-mudrock sequence from the central North SeaUK Clay Minerals 31 523 ndash536

Huggett JM and Shaw HF (1997) Field emission electronmicroscopy ndash a high resolution technique for the study ofclay minerals in sediments Clay Minerals 32 197 ndash203

Jones B and Galan E (1988) Sepiolite and palygorskite Pp631ndash674 in Hydrous Phyllosillicates (SW Bailey editor)Reviews in Mineralogy 19 Mineralogical Society ofAmerica Washington DC

Kim J Peacor DR Tessier D and Elass F (1995) Atechnique for maintaining texture and permanent expansionof smectite interlayers for TEM observations Clays andClay Minerals 43 51 ndash57

Krekeler MPS (2004) Improved constraints on sedimentaryenvironments of palygorskite deposits of the HawthorneFormation southern Georgia from a detailed study of acore Clays and Clay Minerals 52 253 ndash262

Lamboy M (1982) Importance des pelotes fecales commeorigine des grains de phosphate lrsquoexemple du gisement deGaf sa (Tunisie) Comptes-R endus des Seances delrsquo Academie des Sciences Serie 2 Mecanique-PhysiqueChimie Sciences de lrsquoUnivers Sciences de la Terre 295595ndash600

Vol 52 No 3 2004 Microtextures of palygorskite-rich sediments 273

Smectite overgrowths in composite grains are oftenthicker and more coherent with fewer stacking defectsthan the smaller less ordered matrix of smectite Thistexture suggests that smectite and I-S in composite grainsmust have a different origin than the matrix smectite

This model of the water column is consistent with theresults of Patterson (1974) Weaver (1984) and Merkl(1989) Merkl (1989) suggested a gradient in salinitybased on the prevalence of smectite in the northeasternportion of the Apalachicola embayment and the morepalygorskite-rich sediments in the southwestern regionsof the embayment Such a distribution suggests astratified water column Patterson (1974) observedexamples of very organic-rich strata within the paly-gorskite clays of the Hawthorne Formation and thesestrata also suggest a stratified water column Stratifiedwater columns are known to be an important mechanismto accumulate organic matter in the sedimentary record(eg Murphy et al 2000 Canfield 1994 Arthur andSagemon 1994 Tyson and Pearson 1991 Pendersonand Calvert 1990 Demaison and Moore 1980)

Phosphates

Hydroxylapatite is the only recognized phosphatemineral occurring in the Pittman Quarry samples Thetextures of the hydroxylapatite suggest that much of thephosphate is derived from fecal pellets The largeraggregates of hydroxylapatite have well defined bound-aries are ovoid in shape and are generally consistent insize with that expected of fecal pellets of smallorganisms Fecal pellets from zooplankton and otherorganisms are believed to be an important mechanismfor concentrating and depositing phosphate in othersedimentary environments (Lamboy 1982 Porter andRobbins 1981 Robbins and Porter 1980 Bremmer1975) The sedimentary source of phosphate is inter-preted as being from fecal material from zooplankton orpossibly small arthropods

The three textures for hydroxylapatite suggest adissolution and precipitation process involving phos-phate originally from the fecal pellets The intimateassociations of single crystals and small clusters ofapatite crystals and the relative decrease in theprevalence of these textures away from the largeaggregate phosphate grains is interpreted as reflectinga process involving a concentration mechanism Adissolution and re-precipitation mechanism would beexpected to mobilize the phosphate to produce localapatite concentrations

Figure 7 (left) Representative TEM images of oxide minerals(a) This mineral is intepreted as a detrital Ti oxide owing to itsrounded nature SAED shows that this grain is composed ofmultiple crystals (b) Aggregates of Ti oxides with grains10 ndash20 nm in diameter The aggregates are interpreted asauthigenic (c) Euhedral outline and zoning indicate that thisFe-Ti oxide grain is authigenic

Vol 52 No 3 2004 Microtextures of palygorskite-rich sediments 271

Thus caution should be used in interpreting whole-rock geochemical data from these sediments for prove-nance and other studies because the distribution of Ca SrP and rare earth elements (REE) are strongly controlled bythe distribution of apatite Therefore whole-rock studies

using REE and Sr isotopes may be of limited use tointerpret provenance of these deposits in future studiesbut may possibly be of some use in understandingphosphate mobilization The ``roll-front weatheringrsquorsquo asdescribed by Merkl (1989) is an oxidation process whichimplies a fluctuation in a paleo-water table and may be thecause of higher concentrations of hydroxylapatite in thetan clay Study of REE and Sr isotopes may provideinsight into this mobilization process

Phosphate complexes are well known to interferewith sorption of organic molecules on clay mineralsurfaces (Theng 1974 Solomon et al 1971 Solomon1968 Solomon et al 1968) particularly with respect topalygorskite Aluminum in the octahedral sheet of 21ribbons at the edges of palygorskite fibers normally actas electron acceptors (Theng 1974 Solomon 1968Solomon and Rosser 1965) In the presence ofphosphate however these surface sites become deacti-vated by phosphate and polyphosphate (Theng 1974Solomon 1968) More recent work has shown thatphosphate complexes decrease sorption of herbicides onpillared clay and crystal violet montmorillonite(Polubesova et al 2002) Gimsing and Borggaard(2002a 2002b) have shown that phosphate interfereswith sorption of glyphosate in clay minerals

The identification of hydroxylapatite in these sedi-ments possibly explains the differences in commercialgrade between the blue clay and the tan clay Palygorskitefibers may be naturally coated with phosphate as a resultof diagenesis during the roll-front weathering processAlternatively because of the small size of crystals (30 to100 nm) and moderate solubility of hydroxylapatite (logKSP of 849 Brown 1960) hydroxylapatite may readilydissolve when these clays are used in applicationsinvolving water or mixed organic-water fluids Thereforepalygorskite-rich sediments such as the tan clay with 3 to5 hydroxylapatite are more likely to produce phosphatecomplexes when interacting with aqueous or mixedorganic-aqueous fluids As a result the tan clays have alower sorption capacity than palygorskite-rich sedimentswith less or no hydroxylapatite

Oxides

Although the oxide minerals generally comprise lt5of the bulk mineralogy of the clay units studied theyshow a range of diversity that suggests most are ofauthigenic origin Most of the Ti oxides are interpretedas authigenic phases based on textures which involvesmectite and palygorskite The phyllosilicates are gen-erally intimately associated with the oxide grains andthe oxides often have delicate features that mechanicaltransport would probably disrupt or destroy Detritaloxides are common and all of the Fe-Ti oxides that areinterpreted as detrital grains are probably derived fromthe Appalachian Piedmont although they may be eolian

Oxides occur in the tan and blue clays but they aresomewhat more abundant and diverse in the tan clay

Figure 8 (a) TEM image of aggregate of cassiteri te withindividual crystals ~15 nm in diameter (b) Cassiterite aggre-gate showing multiple single crystals in the aggregate (c) EDSspectra from a cassiterite aggregate showing Sn and O peaks theC and Cu are from C coating and Cu substrate

272 Krekeler Guggenheim and Rakovan Clays and Clay Minerals

The tan clay is believed to involve the ``roll-front typeweatheringrsquorsquo process (Merkl 1989) and this implies thatwater has percolated through the unit to producealteration and oxidation of the tan clay We interpretthe bluetan contact and adjacent tan and blue clay unitsas reflecting a paleo-water table where the tan clayexisted in the vadose zone and was subject to drying andre-wetting as the water table fluctuated This enabled theelemental components for the oxide minerals to bemobilized and oxidized in the tan clay as would beexpected in a vadose zone The higher concentrations ofoxide minerals in the tan clay are consistent with thisinterpretation This may be the cause of the colordifference between the tan and blue clays Alternativelythe oxidation state of cations in the palygorskite orsmectite may be the cause of color variation and may belinked to a fluctuating paleo-base level

Cassiterite which appears restricted to the lowermosttan clay unit is believed to be an authigenic phase owingto the small intergrown crystals and euhedral cross-sections The restricted stratigraphic occurrence at thebase of the paleo-vadose zone in the tan clay alsosuggests that it is authigenic Tin was probablymobilized throughout the section and was re-precipitatedduring fluctuations of the water table

CONCLUSIONS

The textures of the minerals at the Pittman quarrysection indicate that there was an evolving and complexmineralogical and geochemical system during and afterdeposi tion of the palygorski te deposits of theHawthorne Textures indicate that several differentreactions occurred to form these sediments such asmultiple stages of growth of palygorskite overgrowth ofsmectite and I-S on detrital clay particles dissolutionand re-precipitation of hydroxylapatite and precipitationof oxide minerals These reactions occurred in geologi-cal environments on scales ranging from the depth of thewater column to nanometers

This study documents smectite overgrowths ondetrital particles in a palygorskite deposit and thisunusual texture is probably a result of a stratifiedwater column that existed in the Apalachicola embay-ment The findings of our study are consistent withenvironmental interpretations of Patterson (1974)Weaver and Beck (1977) Weaver (1984) and Merkl(1989) Our model may be tested by the analysis ofmicrofossils that may indicate mixed freshwater andmarine assemblages and organic geochemistry whichmay yield molecular evidence of environmentallyspecific bacteria

ACKNOWLEDGMENTS

We thank W F Moll for valuable discussions and JRoth and A Nichols of the Research Resources Center atthe University of Illinois at Chicago for assistance with

electron microscopy We thank the US National ScienceFoundation for support under grants EAR0001122 andEAR0001251

REFERENCES

Arthur MA and Sagemon BB (1994) Marine black shalesA review of depositional mechanisms and significance ofancient deposits Annual Review Earth and PlanetarySciences 22 499 ndash551

Bickmore B Bosbach D Hochella MF Jr Charlet L andRufe E (2001) In situ atomic force microscopy study ofhectorite and nontronite dissolution Implicat ions forphyllosilicate edge surface structures and dissolutionmechanisms American Mineralogist 86 411ndash423

Bremner JM (1975) Faecal pellets glauconite phosphoriteand bedrock from the Kunene-Walvis continental marginTechnical Report ndash South African National Committee forOceanographic Research Marine Geology Programme 7Progress Report for the Year pp 59 ndash67

Brown WE (1960) Behaviour of slightly soluble calciumphosphate as revealed by phase equilibrium calculationsSoil Science 90 51 ndash57

Canfield DE (1994) Factors influencing organic carbonpreservation in marine sediments Chemical Geology 114315ndash329

Chamley H (1989) Clay Sedimentology Springer-VerlagNew York 623 pp

Demaison GJ and Moore GT (1980) Anoxic environmentsand oil source bed genesis American Association ofPetroleum Geologists Bulletin 64 1179 ndash1209

Galan E and Carretero MI (1999) A new approach tocompositional limits for sepiolite and palygorskite Claysand Clay Minerals 47 399 ndash409

Gimsing AL and Borggaard OK (2002a) Competitiveadsorption and desorption of glyphosate and phosphate onclay silicates and oxides Clay Minerals 37 509ndash515

Gimsing AL and Borggaard OK (2002b) Effect ofphosphate on the adsorption of glyphosate on soils claymi ne ra l s and ox id e s In t e rna t iona l J our na l o fEnvironmental Analytical Chemistry 82 545 ndash552

Grim RE and Johns WD (1954) Clay mineral investigationsof sediments in the northern Gulf of Mexico Clays and ClayMinerals 2nd National Conference Pergamon New Yorkpp 81 ndash103

Huggett JM (1996) Aluminosilicate diagenesis in a Tertiarysandstone-mudrock sequence from the central North SeaUK Clay Minerals 31 523 ndash536

Huggett JM and Shaw HF (1997) Field emission electronmicroscopy ndash a high resolution technique for the study ofclay minerals in sediments Clay Minerals 32 197 ndash203

Jones B and Galan E (1988) Sepiolite and palygorskite Pp631ndash674 in Hydrous Phyllosillicates (SW Bailey editor)Reviews in Mineralogy 19 Mineralogical Society ofAmerica Washington DC

Kim J Peacor DR Tessier D and Elass F (1995) Atechnique for maintaining texture and permanent expansionof smectite interlayers for TEM observations Clays andClay Minerals 43 51 ndash57

Krekeler MPS (2004) Improved constraints on sedimentaryenvironments of palygorskite deposits of the HawthorneFormation southern Georgia from a detailed study of acore Clays and Clay Minerals 52 253 ndash262

Lamboy M (1982) Importance des pelotes fecales commeorigine des grains de phosphate lrsquoexemple du gisement deGaf sa (Tunisie) Comptes-R endus des Seances delrsquo Academie des Sciences Serie 2 Mecanique-PhysiqueChimie Sciences de lrsquoUnivers Sciences de la Terre 295595ndash600

Vol 52 No 3 2004 Microtextures of palygorskite-rich sediments 273

Thus caution should be used in interpreting whole-rock geochemical data from these sediments for prove-nance and other studies because the distribution of Ca SrP and rare earth elements (REE) are strongly controlled bythe distribution of apatite Therefore whole-rock studies

using REE and Sr isotopes may be of limited use tointerpret provenance of these deposits in future studiesbut may possibly be of some use in understandingphosphate mobilization The ``roll-front weatheringrsquorsquo asdescribed by Merkl (1989) is an oxidation process whichimplies a fluctuation in a paleo-water table and may be thecause of higher concentrations of hydroxylapatite in thetan clay Study of REE and Sr isotopes may provideinsight into this mobilization process

Phosphate complexes are well known to interferewith sorption of organic molecules on clay mineralsurfaces (Theng 1974 Solomon et al 1971 Solomon1968 Solomon et al 1968) particularly with respect topalygorskite Aluminum in the octahedral sheet of 21ribbons at the edges of palygorskite fibers normally actas electron acceptors (Theng 1974 Solomon 1968Solomon and Rosser 1965) In the presence ofphosphate however these surface sites become deacti-vated by phosphate and polyphosphate (Theng 1974Solomon 1968) More recent work has shown thatphosphate complexes decrease sorption of herbicides onpillared clay and crystal violet montmorillonite(Polubesova et al 2002) Gimsing and Borggaard(2002a 2002b) have shown that phosphate interfereswith sorption of glyphosate in clay minerals

The identification of hydroxylapatite in these sedi-ments possibly explains the differences in commercialgrade between the blue clay and the tan clay Palygorskitefibers may be naturally coated with phosphate as a resultof diagenesis during the roll-front weathering processAlternatively because of the small size of crystals (30 to100 nm) and moderate solubility of hydroxylapatite (logKSP of 849 Brown 1960) hydroxylapatite may readilydissolve when these clays are used in applicationsinvolving water or mixed organic-water fluids Thereforepalygorskite-rich sediments such as the tan clay with 3 to5 hydroxylapatite are more likely to produce phosphatecomplexes when interacting with aqueous or mixedorganic-aqueous fluids As a result the tan clays have alower sorption capacity than palygorskite-rich sedimentswith less or no hydroxylapatite

Oxides

Although the oxide minerals generally comprise lt5of the bulk mineralogy of the clay units studied theyshow a range of diversity that suggests most are ofauthigenic origin Most of the Ti oxides are interpretedas authigenic phases based on textures which involvesmectite and palygorskite The phyllosilicates are gen-erally intimately associated with the oxide grains andthe oxides often have delicate features that mechanicaltransport would probably disrupt or destroy Detritaloxides are common and all of the Fe-Ti oxides that areinterpreted as detrital grains are probably derived fromthe Appalachian Piedmont although they may be eolian

Oxides occur in the tan and blue clays but they aresomewhat more abundant and diverse in the tan clay

Figure 8 (a) TEM image of aggregate of cassiteri te withindividual crystals ~15 nm in diameter (b) Cassiterite aggre-gate showing multiple single crystals in the aggregate (c) EDSspectra from a cassiterite aggregate showing Sn and O peaks theC and Cu are from C coating and Cu substrate

272 Krekeler Guggenheim and Rakovan Clays and Clay Minerals

The tan clay is believed to involve the ``roll-front typeweatheringrsquorsquo process (Merkl 1989) and this implies thatwater has percolated through the unit to producealteration and oxidation of the tan clay We interpretthe bluetan contact and adjacent tan and blue clay unitsas reflecting a paleo-water table where the tan clayexisted in the vadose zone and was subject to drying andre-wetting as the water table fluctuated This enabled theelemental components for the oxide minerals to bemobilized and oxidized in the tan clay as would beexpected in a vadose zone The higher concentrations ofoxide minerals in the tan clay are consistent with thisinterpretation This may be the cause of the colordifference between the tan and blue clays Alternativelythe oxidation state of cations in the palygorskite orsmectite may be the cause of color variation and may belinked to a fluctuating paleo-base level

Cassiterite which appears restricted to the lowermosttan clay unit is believed to be an authigenic phase owingto the small intergrown crystals and euhedral cross-sections The restricted stratigraphic occurrence at thebase of the paleo-vadose zone in the tan clay alsosuggests that it is authigenic Tin was probablymobilized throughout the section and was re-precipitatedduring fluctuations of the water table

CONCLUSIONS

The textures of the minerals at the Pittman quarrysection indicate that there was an evolving and complexmineralogical and geochemical system during and afterdeposi tion of the palygorski te deposits of theHawthorne Textures indicate that several differentreactions occurred to form these sediments such asmultiple stages of growth of palygorskite overgrowth ofsmectite and I-S on detrital clay particles dissolutionand re-precipitation of hydroxylapatite and precipitationof oxide minerals These reactions occurred in geologi-cal environments on scales ranging from the depth of thewater column to nanometers

This study documents smectite overgrowths ondetrital particles in a palygorskite deposit and thisunusual texture is probably a result of a stratifiedwater column that existed in the Apalachicola embay-ment The findings of our study are consistent withenvironmental interpretations of Patterson (1974)Weaver and Beck (1977) Weaver (1984) and Merkl(1989) Our model may be tested by the analysis ofmicrofossils that may indicate mixed freshwater andmarine assemblages and organic geochemistry whichmay yield molecular evidence of environmentallyspecific bacteria

ACKNOWLEDGMENTS

We thank W F Moll for valuable discussions and JRoth and A Nichols of the Research Resources Center atthe University of Illinois at Chicago for assistance with

electron microscopy We thank the US National ScienceFoundation for support under grants EAR0001122 andEAR0001251

REFERENCES

Arthur MA and Sagemon BB (1994) Marine black shalesA review of depositional mechanisms and significance ofancient deposits Annual Review Earth and PlanetarySciences 22 499 ndash551

Bickmore B Bosbach D Hochella MF Jr Charlet L andRufe E (2001) In situ atomic force microscopy study ofhectorite and nontronite dissolution Implicat ions forphyllosilicate edge surface structures and dissolutionmechanisms American Mineralogist 86 411ndash423

Bremner JM (1975) Faecal pellets glauconite phosphoriteand bedrock from the Kunene-Walvis continental marginTechnical Report ndash South African National Committee forOceanographic Research Marine Geology Programme 7Progress Report for the Year pp 59 ndash67

Brown WE (1960) Behaviour of slightly soluble calciumphosphate as revealed by phase equilibrium calculationsSoil Science 90 51 ndash57

Canfield DE (1994) Factors influencing organic carbonpreservation in marine sediments Chemical Geology 114315ndash329

Chamley H (1989) Clay Sedimentology Springer-VerlagNew York 623 pp

Demaison GJ and Moore GT (1980) Anoxic environmentsand oil source bed genesis American Association ofPetroleum Geologists Bulletin 64 1179 ndash1209

Galan E and Carretero MI (1999) A new approach tocompositional limits for sepiolite and palygorskite Claysand Clay Minerals 47 399 ndash409

Gimsing AL and Borggaard OK (2002a) Competitiveadsorption and desorption of glyphosate and phosphate onclay silicates and oxides Clay Minerals 37 509ndash515

Gimsing AL and Borggaard OK (2002b) Effect ofphosphate on the adsorption of glyphosate on soils claymi ne ra l s and ox id e s In t e rna t iona l J our na l o fEnvironmental Analytical Chemistry 82 545 ndash552

Grim RE and Johns WD (1954) Clay mineral investigationsof sediments in the northern Gulf of Mexico Clays and ClayMinerals 2nd National Conference Pergamon New Yorkpp 81 ndash103

Huggett JM (1996) Aluminosilicate diagenesis in a Tertiarysandstone-mudrock sequence from the central North SeaUK Clay Minerals 31 523 ndash536

Huggett JM and Shaw HF (1997) Field emission electronmicroscopy ndash a high resolution technique for the study ofclay minerals in sediments Clay Minerals 32 197 ndash203

Jones B and Galan E (1988) Sepiolite and palygorskite Pp631ndash674 in Hydrous Phyllosillicates (SW Bailey editor)Reviews in Mineralogy 19 Mineralogical Society ofAmerica Washington DC

Kim J Peacor DR Tessier D and Elass F (1995) Atechnique for maintaining texture and permanent expansionof smectite interlayers for TEM observations Clays andClay Minerals 43 51 ndash57

Krekeler MPS (2004) Improved constraints on sedimentaryenvironments of palygorskite deposits of the HawthorneFormation southern Georgia from a detailed study of acore Clays and Clay Minerals 52 253 ndash262

Lamboy M (1982) Importance des pelotes fecales commeorigine des grains de phosphate lrsquoexemple du gisement deGaf sa (Tunisie) Comptes-R endus des Seances delrsquo Academie des Sciences Serie 2 Mecanique-PhysiqueChimie Sciences de lrsquoUnivers Sciences de la Terre 295595ndash600

Vol 52 No 3 2004 Microtextures of palygorskite-rich sediments 273

The tan clay is believed to involve the ``roll-front typeweatheringrsquorsquo process (Merkl 1989) and this implies thatwater has percolated through the unit to producealteration and oxidation of the tan clay We interpretthe bluetan contact and adjacent tan and blue clay unitsas reflecting a paleo-water table where the tan clayexisted in the vadose zone and was subject to drying andre-wetting as the water table fluctuated This enabled theelemental components for the oxide minerals to bemobilized and oxidized in the tan clay as would beexpected in a vadose zone The higher concentrations ofoxide minerals in the tan clay are consistent with thisinterpretation This may be the cause of the colordifference between the tan and blue clays Alternativelythe oxidation state of cations in the palygorskite orsmectite may be the cause of color variation and may belinked to a fluctuating paleo-base level

Cassiterite which appears restricted to the lowermosttan clay unit is believed to be an authigenic phase owingto the small intergrown crystals and euhedral cross-sections The restricted stratigraphic occurrence at thebase of the paleo-vadose zone in the tan clay alsosuggests that it is authigenic Tin was probablymobilized throughout the section and was re-precipitatedduring fluctuations of the water table

CONCLUSIONS

The textures of the minerals at the Pittman quarrysection indicate that there was an evolving and complexmineralogical and geochemical system during and afterdeposi tion of the palygorski te deposits of theHawthorne Textures indicate that several differentreactions occurred to form these sediments such asmultiple stages of growth of palygorskite overgrowth ofsmectite and I-S on detrital clay particles dissolutionand re-precipitation of hydroxylapatite and precipitationof oxide minerals These reactions occurred in geologi-cal environments on scales ranging from the depth of thewater column to nanometers

This study documents smectite overgrowths ondetrital particles in a palygorskite deposit and thisunusual texture is probably a result of a stratifiedwater column that existed in the Apalachicola embay-ment The findings of our study are consistent withenvironmental interpretations of Patterson (1974)Weaver and Beck (1977) Weaver (1984) and Merkl(1989) Our model may be tested by the analysis ofmicrofossils that may indicate mixed freshwater andmarine assemblages and organic geochemistry whichmay yield molecular evidence of environmentallyspecific bacteria

ACKNOWLEDGMENTS

We thank W F Moll for valuable discussions and JRoth and A Nichols of the Research Resources Center atthe University of Illinois at Chicago for assistance with

electron microscopy We thank the US National ScienceFoundation for support under grants EAR0001122 andEAR0001251

REFERENCES

Arthur MA and Sagemon BB (1994) Marine black shalesA review of depositional mechanisms and significance ofancient deposits Annual Review Earth and PlanetarySciences 22 499 ndash551

Bickmore B Bosbach D Hochella MF Jr Charlet L andRufe E (2001) In situ atomic force microscopy study ofhectorite and nontronite dissolution Implicat ions forphyllosilicate edge surface structures and dissolutionmechanisms American Mineralogist 86 411ndash423

Bremner JM (1975) Faecal pellets glauconite phosphoriteand bedrock from the Kunene-Walvis continental marginTechnical Report ndash South African National Committee forOceanographic Research Marine Geology Programme 7Progress Report for the Year pp 59 ndash67

Brown WE (1960) Behaviour of slightly soluble calciumphosphate as revealed by phase equilibrium calculationsSoil Science 90 51 ndash57

Canfield DE (1994) Factors influencing organic carbonpreservation in marine sediments Chemical Geology 114315ndash329

Chamley H (1989) Clay Sedimentology Springer-VerlagNew York 623 pp

Demaison GJ and Moore GT (1980) Anoxic environmentsand oil source bed genesis American Association ofPetroleum Geologists Bulletin 64 1179 ndash1209

Galan E and Carretero MI (1999) A new approach tocompositional limits for sepiolite and palygorskite Claysand Clay Minerals 47 399 ndash409

Gimsing AL and Borggaard OK (2002a) Competitiveadsorption and desorption of glyphosate and phosphate onclay silicates and oxides Clay Minerals 37 509ndash515

Gimsing AL and Borggaard OK (2002b) Effect ofphosphate on the adsorption of glyphosate on soils claymi ne ra l s and ox id e s In t e rna t iona l J our na l o fEnvironmental Analytical Chemistry 82 545 ndash552

Grim RE and Johns WD (1954) Clay mineral investigationsof sediments in the northern Gulf of Mexico Clays and ClayMinerals 2nd National Conference Pergamon New Yorkpp 81 ndash103

Huggett JM (1996) Aluminosilicate diagenesis in a Tertiarysandstone-mudrock sequence from the central North SeaUK Clay Minerals 31 523 ndash536

Huggett JM and Shaw HF (1997) Field emission electronmicroscopy ndash a high resolution technique for the study ofclay minerals in sediments Clay Minerals 32 197 ndash203

Jones B and Galan E (1988) Sepiolite and palygorskite Pp631ndash674 in Hydrous Phyllosillicates (SW Bailey editor)Reviews in Mineralogy 19 Mineralogical Society ofAmerica Washington DC

Kim J Peacor DR Tessier D and Elass F (1995) Atechnique for maintaining texture and permanent expansionof smectite interlayers for TEM observations Clays andClay Minerals 43 51 ndash57

Krekeler MPS (2004) Improved constraints on sedimentaryenvironments of palygorskite deposits of the HawthorneFormation southern Georgia from a detailed study of acore Clays and Clay Minerals 52 253 ndash262

Lamboy M (1982) Importance des pelotes fecales commeorigine des grains de phosphate lrsquoexemple du gisement deGaf sa (Tunisie) Comptes-R endus des Seances delrsquo Academie des Sciences Serie 2 Mecanique-PhysiqueChimie Sciences de lrsquoUnivers Sciences de la Terre 295595ndash600

Vol 52 No 3 2004 Microtextures of palygorskite-rich sediments 273

Merkl RS (1989) A sedimentological mineralogical andgeochemical study of the fullerrsquos earth deposits of theMiocene Hawthorne group of south Georgia-north FloridaPhD dissertation Indiana University Bloomington Indiana182 pp

Murphy AE Sagemon BB Holander DJ Lyons TT andBrett CE (2000) Black shale deposition and faunaloverturn in the Devonian Appalachian basin Clast icstarvation seasonal water-column mixing and efficientbiolimiting nutrient cycl ing Paleoceanography 15 280 ndash291

Nagy KL (1994) Application of morphological data obtainedusing scanning force microscopy to quantification of fibrousillite growth rates Pp 204 ndash239 in Scanning ProbeMicroscopy of Clay Minerals (KL Nagy and A Blumeditors) CMS Workshop Lectures 7 The Clay MineralsSociety Bloomington Indiana

Nagy KL and Blum AE editors (1994) Scanning ProbeMicroscopy of Clay Minerals Clay Minerals SocietyWorkshop Lectures 7 239 pp

Patterson SH (1974) Fullerrsquos earth and industrial mineralresources of the Meigs-Attapulgus-Quincy district Georgiaand Florida US Geological Survey Professional PaperP0828 45 pp

Penderson TF and Calvert SE (1990) Anoxia vsProductivity What controls the formation of organic-carbon-rich sediments and sedimentary rocks AmericanAssociation of Petroleum Geologists Bulletin 74 454 ndash466

Polubesova T Nir S Gerstl Z Borisover M and Rubin B(2002) Imazaquin adsorbed on pillared clay and crystalviolet-montmorillonite complexes for reduced leaching insoil Journal of Environmental Quality 331 1657ndash1664

Porrenga DH (1966) Clay minerals in recent sediments of theNiger Delta Clays and Clay Minerals 14th NationalConference Pergamon New York pp 221 ndash233

Porter KG and Robbins EI (1981) Zooplankton fecalpellets link fossil fuel and phosphate deposits Science212 931ndash933

Robbins EI and Porter KG (1980) Geologic importance ofzooplankton fecal pellets in black shale associated with

phosphate deposits Proceedings of the Eleventh AnnualMeeting of the American Association of StratigraphicPalynolgists Phoenix Arizona United States October1978 Palynology 4 249ndash250

Shepard FP and Moore DG (1954) Central Texas coastsedimentation Characteristics of sedimentary environmentrecent history and diagenesis American Association ofPetroleum Geologists Bulletin 39 1463 ndash1593

Solomon DH (1968) Clay minerals as electron acceptors andor electron donors in organic reactions Clays and ClayMinerals 16 31 ndash39

Solomon DH and Rosser MJ (1965) Reactions catalyzed byminerals Part I Journal of Applied Polymer Science 91261

Solomon DH Loft BC and Swift JD (1968) Reactionscatalysed by minerals IV The Mechanism of the benzideneblue reaction on silicate minerals Clay Minerals 7389 ndash397

Solomon DH Swift JD and Murphy AJ (1971) Theacidity of clay minerals in polymerizations and relatedreaction Journal of Macromolecular Science ndash Pure andApplied Chemistry A5 587 ndash601

Theng BKG (1974) The Chemistry of Clay-OrganicReactions John Wiley amp Sons New York 343 pp

Tyson RV and Pearson TH (1991) Modern and ancientcontinental shelf anoxia An overview Pp 1 ndash24 in Modernand Ancient Continental Shelf Anoxia (RV Tyson and THPearson editors) Special Publication 58 GeologicalSociety London

Weaver CE (1984) Origin and geologic implications of thepalygorskite deposits of the SE United States Pp 39 ndash58in Palygorskite-Sepiolite Occurrences Genesis and Uses(A Singer and E Galan editors) Developments inSedimentology 37 Elsevier New York

Weaver CE and Beck KC (1977) Miocene of the SEUnited States a model for chemical sedimentation in aperimarine environment Developments in Sedimentology22 Elsevier New York 234 pp

(Received 22 May 2003 revised 5 January 2004 Ms 794)

274 Krekeler Guggenheim and Rakovan Clays and Clay Minerals