American Mineralogist, Volume 73, pages 487499, 1988

Experimental investigation of the effect of oxygen fugacity on ferric-ferrousratios and unit-cell parameters of four natural clinoamphiboles

Cpr,r,q. A. Cr-own,* Ronnnr K. PoppDepartment of Geology, Texas A&M University, College Station, Texas 77843, U.S.A'

SrBvnN J. FnIrzDepartment of Earth and Atmospheric Science, Purdue University, West Lafayette, Indiana 47907, U.S.A.

Ansrucr

To investigate the relation between oxygen fugacity, ferric-ferrous ratio (R), and unit-cell parameters, four natural clinoamphiboles (grunerite, tschermakitic hornblende, mag-nesio-hornblende, and a riebeckite-arfvedsonite solid solution) were reacted at 650'C, Ikbar at oxygen fugacities defined by several solid oxygen-buffer assemblages. In order toproduce more highly oxidized samples, heating in air at 700 'C was also carried out.Variation in R is accomplished mainly by the oxidation-dehydrogenation equilibrium

Fe2* + OH : Fe3* * 02 + t/z}{r,

but the results suggest that other mechanisms may also be involved. All four amphibolesexhibited systematically higher ferric-ferrous ratios with increasing 6, of equilibration.Equilibrium R values were achieved relatively rapidly and could be readily restored toonginal values by treatment at the appropriate buffer. In some cases, a metastable equi-librium of ferric-ferrous ratio was achieved before the amphibole decomposed to otherFe3*-bearing phases.

Of the four amphiboles, grunerite is apparently the least able to accommodate Fe3*within its crystal structure and decomposes at relatively higher oxygen fugacities.

The value of a sin B decreases systematically uniformly as the Fe3* content of tscher-makitic hornblende, magnesio-hornblende, and riebeckite increases, reflecting increasingFe3* in the octahedral cation sites. The variation in a sin B ofgrunerite is significantly lessthan for the other three amphiboles. The variation in b for the two hornblendes suggeststhat Fe3* produced by oxidation is not strongly ordered into the M(2) site.

INrnolucrroN

Amphiboles are the most chemically complex of themajor rock-forming mineral groups. This complexityarises from the wide variation of unique structural sitesavailable for cations. Fe is a major component in manynaturally occurring amphibole solid solutions, but thefactors that control the proportions of Fe3* and Fe2* inany given amphibole are still very poorly understoodquantitatively.

Two types of experiments have been carried out in at-tempts to produce variation in the ferric-ferrous ratios inamphiboles: heating of amphiboles in air, and hydro-thermal synthesis and/or treatment of amphiboles at oxy-gen fugacities defined by the standard solid oxygen buff-ers. The earlier air-heating studies were carried out mainlyon abestiform amphiboles, but more recently, nonasbes-tiform varieties have been so-treated (see Hawthorne,198 I , 1983, for reviews). Hydrothermal studies to delim-it physical properties and the pressure-temperature-oxy-

* Present address: Department of Geological Sciences, TheUniversity of Texas at Austin, Austin, Texas 78712, U.S.A.

0003-004x/88/0506-0487$02.00

gen fugacity stability have been carried out on a widevariety of bulk compositions representing the majorchemical groups of amphiboles (e.g., Gilbert et al., 1982).

The results of the existing experimental studies haveshown that changes in chemical compositions and phys-ical properties occur as a function ofoxygen fugacity andtemperature. Oxidation of Fe in an amphibole results ini-tially from the formation of oxy-amphibole componentby a dehydrogenation reaction ofthe type

Fe2* * OH- : Fe3* + Ot + V2H2, (l)

in which O' replaces OH- in the O(3) anion site of theamphibole. This process, first suggested by the results ofBarnes (1930), has also been proposed to be operative inother hydrous minerals such as the micas (e.9., Wones,1963; Vedder and Wilkins, 1969). Most studies in syn-thetic systems are complicated by the fact that 1000/o yieldsof amphibole are not achieved. Therefore, changes ineither the proportions or compositions of the nonamphi-bole phases may cause variations in amphibole cornpo-sitions. That is, changes observed in physical propertiesmay result from changes in amphibole bulk composition

487

488 CLOWE ET AL.: Fe3+/Fe'z+ AND UNIT-CELL PARAMETERS OF CLINOAMPHIBOLES

TABLE 1. Compositions of starting amphiboles

Tschermakitic Magnesio-Grunerite hornblende hornblende

rolite grade amphibolite (sample 73-318, Spear, 1982); a mag-nesio-hornblende from Strickland quarry, Portland, Vermont,for which petrologic information has not been published (Na-tional Museum of Natural History specimen A6169); and a rie-beckite-arfvedsonite solid solution, hereafter referred to as rie-beckite, from a granitic pegmatite (Scofield and Gilbert, 1982).

Separation techniques

In order to ensure close to 100o/o purity in the starting mate-rials, each amphibole sample was subjected to a variety of sep-aration techniques. All samples were disaggregated, washed inacid, subjected to isodynamic magnetic and heavy-liquid sepa-ration techniques, and hand picked to produce 99-1000/o purity.Additional details ofthe separation techniques are given in Clowe(1987). The final grain sizes ofsamples are reported in Table 1.With the exception of a small proportion of crushed grains, therewas no change in amphibole gmin size during runs.

Phase identification and characterization

Electron-microprobe analysis of the starting materials and se-lected run products was carried out using the nnI- instrument inthe Department of Geological Sciences at Virginia Tech, Blacks-burg, Virginia. The analytical procedures and operating condi-tions were essentially identical to those described by Solberg andSpeer (l 982).

Fe3+/Fe2+ ratios were determined using a single-dissolutiontechnique applicable to small sample sizes (Fritz and Popp, 1985).Sample aliquots ranging from 0.95 to 11.96 mg were digestedwith HF and H'SO. in the presence of o-phenanthroline, afterwhich Na-citrate and boric acid solutions were added. The so-lutions were then analyzed colorimetrically for FeO. Total Fe asFerO, was determined by atornic absorption spectrophotometryafter further dilution of the samples with the quartz blank usedin the FeO determination. Duplicate solutions were made foreach sample in the colorimetric analysis for FeO, and each ofthose solutions was diluted and analyzed twice for FerO,. Ferric-ferrous ratios, reported as molar Fe3*/(Fe3* + Fe'?*) and abbre-viated R, are considered to be precise to +0.01 (Fritz and Popp,1 985).

A complication arises in electron-microprobe analyses ofphasesthat contain oxy-amphibole component because total Fe is gen-erally reported as FeO and the formulas are normalized to 23oxygens. For an end-member grunerite, for example, this pro-cedure is equivalent to expressing the chemical formula in termsof oxides with one molecule of water present in the stucture, i.e.,

Fe'SirO',(OH), : TFeO' 8SiOz'H:O.

The formulas are normalized to 23 oxygens because the electronmicroprobe cannot analyze for H, and therefore, the amount ofoxygen necessary to form "hydrogen oxide" is not included inthe normalized formula. In the case of a totally dehydrogenatedgrunerite, it is necessary to normalize microprobe analyses to 24rather than 23 oxygens, i.e.,

Fel"Fel-SirO.o : 5FeO FerOr' 8SiOr.

Thus, normalization of the formula of an amphibole containinga significant oxy-component to 23 oxygens results in a total ox-ide weight percent that is too low by a factor that represents theadditional oxygen present as FerO. rather than as FeO. To cor-rect an analysis based on 23 oxygens to the proper number ofoxygens, (1) the total elemental weight percent of Fe is multipliedby the ferric-ferrous ratio (R) to determine the absolute elemen-tal weight percent ofFe in each valence state, (2) the elemental

Riebeckite

sio, 49.19Tio, 0.03Al,o3 0.51FeO 44.30Fe,O" N.D.MnO 0.66MgO 3.21CaO O.32Na,O 0 04K,O N.D.F 0 .17c l o . 12

Sum 98.55

43.440.60

13.8212.533.700.259.80

11.571 .450.510 .13N .D .

97.80

6.411 .590.830.071 .540.420.042 . 1 61 8 30.410.09

47 050.31

10.2412.541.050.74

12.241 1 .931 . 1 30.590.72N.D.

98.54

6.871 . 1 30.640.041 .54o .120 0 92.671 .870.320 . 1 1

0.3323.00

51 .390.780.80

21.671 1 . 8 50.710.250 .158.871 .541 .97

N .D .99.98

8 0 0

0 .150.092.831 .400.090.060.032.680.30

0.9823 00

0.0623.00

si'vAlvrAl

TiFe,*Fe3*MnMgCaNaKclFo

MaxMin

7.940.060.040.015.99

0 0 90.780.060 . 1 0

0.010.09

23.00

Grain size (mm)1.0 x 0.025 0.19 x 0.025 0.5 x 0.1 0.15 x 0.050.1 x 0.025 0.075 x 0.075 0.1 x 0.2 0.1 x 0.02

Nofe; N.D. : not detected. Chemical compositions (in wt% oxides andas atoms per 23 oxygens) and grain sizes of amphiboles used in experi-ments. Chemical compositions were determined by electron microprobe.Ferric-ferrous ratio determined by wet-chemical analysis. Amphibole com-positions were normalized to 23 oxygens because of unknown oxy-am-ohibole content.

as oxygen fugacity varies, rather than solely from changesin the ferric-ferrous ratio. For most synthetic hydrother-mal studies, precise chemical compositions and, moreimportantly, ferric-ferrous ratios of the amphiboles havenot been determined routinely.

This study reports the results of experiments designedto elucidate further ferric-ferrous equilibrium in amphi-boles. Four natural amphiboles were annealed over a rangeofoxygen fugacities at constant temperature and pressureto determine the variation in Fe3* content and in unit-cell parameters. Natural amphiboles were chosen in orderto avoid the complication of less than 1000/o yields insynthetic systems and also to provide crystals large enoughfor single-crystal structure refinements. Results that de-scribe the variation ofoptical properties as a function offerric-ferrous ratio and the results of crystal-structure re-finements have been presented elsewhere (Phillips et al.,1986;Clowe and Popp, 1987;Phil l ips et al., 1988).

ExpnnrvrnurAl METHoDS

Four natural clinoamphiboles representing the three majoramphibole chemical groups (iron-magnesium, calcic, and sodicamphiboles) were selected for the study. These amphiboles in-clude a grunerite from metamorphosed iron formation (sample1, Klein, 1964); a tschermakitic hornblende from a kyanite-stau-

CLOWE ET AL.: Fe3+/Fe2+ AND UNIT-CELL PARAMETERS OF CLINOAMPHIBOLES

TneLe 2. Compositions of amphibole run products

489

AOE 3AG AEO 3AG AOE24 AOE 11A AEO . I1A AOE 14 AOE 19 AOE 32 AOE 32 AOE 31

sio,Tio,Al,o3FeOFerO"MnOMgoCaONaroKrOFcl

Sum

49.260.o20.46

36.997.820.643.210.330.07N.D.0 .170 . 1 1

99 08

7 8 20 . 1 0

0.014.910.940 0 90.760.06o.02

0.030.09

23 00

49.910.05o.14

44.06N.D.0.673.220.320.060.010.080 . 1 1

98.63

8.04

0.030.015.94

0.090.780.060.02

0.030.04

23.00

43.700.59

13.6411 .035.500.28

10.001 1 . 6 11 .400.510 .18N .D .98.44

6.401 .600.760.071.360.610.042 .181 .820.410.09

0.0923.00

44.120.66

1 3 . 1 812.333.86o.32

10.3311.731.240.47o.47N.D.

98.31

6.461 .540 7 50.081 .510.430.042251 8 40.350.09

0.0323.00

47.590.298.23

12.82N.A.0.79

12.811 1.860.880.470.620.01

96.37

7.090.910.540.031.59

0.102.851.890.250.090.010.62

23.00

51.010.941.05

14.3720.30o.70o.220.158.871.462.00N.D.

101 .08

7.780.19

0 . 1 11.832.340.090.050.032.620.29

1.0023.00

50.050.870.71

23.929.330.700.210 .188.641.351 .98

N.D.97.94

8.O2

0 . 1 40 . 1 13.211 . 1 30 .100.050.032.68o.27

0.9723.00

S iIVAIurAl

TiFe2*Fe3*MnMgCaNaKclFo

7.980.020.070.015.010.960.090.780.060.o2

0.030.09

23.48

6.431.570.800.071.360.610.042.191.830.410.09

0.0923.10

7.940.060.140 . 1 1't.872.390.090.050.032.680.30

1.0023.49

Note: N.D. : not detected; N.A : not available. Chemical compositions (in wt% oxides and in atoms per formula unit normalized to given numberof oxygens) of selected run products (see Table 3 for details). Samples with > 1 0% oxy-amphibole component have been normalized to the appropriatenumber of oyxgens per formula unit (see text for discussion).

weight percents are conyerted to oxide weight percents by mul-tiplication by the proper conversion factor, (3) the oxide weightpercents are converted to moles, and (4) the formula is normal-ized to the required number of oxygens (23 plus one-half thenumber of Fe3* ions present as oxy-amphibole component performula unit). The effect of the correction for grunerite is shownin the two columns labeled AOE 34, G in Table 2. The grunerite,which originally contained no Fe3*, was oxidized in air, but therewas no evidence for decomposition to secondary phases. Thus,its chemical composition should remain constant except for lossof H and oxidation of a portion of the Fe2*. The corrected for-mula of the oxidized grunerite based on 23.48 oxygens is vir-tually identical to that ofthe unoxidized starting material (Table1), whereas the formula normalized to 23.00 oxygens containsoverall lower atomic amounts. It is important to note that thismethod of correcting chemical formulas presumes knowledge ofthe amount of Fe3+ present as oxy-amphibole component. Be-cause Fe3+ can enter the amphibole structure by processes otherthan the formation of oxy-amphibole component (see Discus-sion), the amount of oxy-amphibole component is not necessar-ily the same as the number of ferric irons per formula unit. Forexample, in the case of the tschermakitic hornblende startingmaterial, which originally contained Fe3* (Table 1), the chemicalformula was based on 23 oxygens because the amount of oxy-amphibole was unknown. However, when the tschermakitichornblende was oxidized without decomposing, the chemicalformula could be corrected to the increased number ofoxygensthat represents the increase in Fe3* as oxy-amphibole component(columns labeled AOE l1A, Table 3).

Chemical analyses of the amphiboles used in the experimentsare given in Table 1. Chemical zoning and significant impuritiesboth within and between grains were not observed. Agreementbetween these analyses and those reported previously in the lit-

erature is excellent. Previous analysis for the magnesio-horn-blende has not been reported. The analysis ofriebeckite-arfved-sonite reported in Scofield and Gilbert (1982) contains 0.3 Liper formula unit, which is not analyzed by the electron mrcro-probe and, therefore, is not included in the table. The formulasin Table t have been corrected to 23 oxygens, because the per-cent oxy-component is unknown. As discussed below, the pres-ence of Fe3* in an amphibole does not necessarily imply thepresence of oxy-component.

Phase identification and unit-cell parameters were determinedusing a Philips X-ray diffractorneter with Ni-filtered CuK. ra-diation. Peak locations used to calculate unit-cell parameterswere collected from four scans (two oscillations) collected at 0.5"/min using synthetic calcium fluoride as an internal standard,except in the case of riebeckite, for which synthetic MgAlrO4spinel was used in order to avoid significant overlap of peaks.Centers ofpeaks were located at % peak height. Reflections wereindexed in comparison to the calculated powder patterns in Borgand Smith (1969). Unit-cell parameters, reported in Table 3,were obtained by least-squares refinement (Evans et al.,1963),weighting all peaks equally.

Apparatus

Hydrothermal nrns were carried out in horizontally mountedRene-4l pressure vessels (e.g., Ernst, 1968, Fig. 6) using eitherAr or methane as the pressure medium. Pressures were generallymaintained within + 10 bars, with a maximum variation of 30bars recorded. Temperature gradients within the vessels werecalibrated previously at elevated temperature and pressure, andare 3'or less over a 2.54-cm capsule. Temperature variations inmost runs were + 1 'C, with a maximum variation of +4 oC.

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CLOWE ET AL.: Fe3+/Fe2+ AND UNIT-CELL PARAMETERS OF CLINOAMPHIBOLES 491

was calibrated with the same temperature-measuring system usedin the hydrothermal runs.

Oxygen fugacity in the hydrothermal runs was controlled usingthe standard solid buffer techniques (e.g., Huebner, 1971). Themagnetite-hematite (MH), nickel-nickel oxide (NNO), fayalite-magnetite-quartz (FMQ), and graphite-methane (CCH.) buferswere used. In addition, some samples were heated in air to achievea much more highly oxidizing atmosphere than MH. A few runswere made at the more reducing iron-wiistite buffer, but thatapproach was abandoned because of the rapid loss of H, gasfrom the capsules. In runs as short as one day at 650 "C and Ikbar, H, diffusion resulted in the complete loss of water fromthe buffer assemblage, even when the graphic-methane systemwas used as the pressure medium.

In hydrothermal runs, the amphibole charges (approximately0.10 g) plus a measured amount of distilled-deionized water (ap-proximately 20 pL) were sealed into an inner AgroPdro capsule(2.54 cm x 3.0 mm outside diameter x 2.4 mm inside diameter)or into a Pt capsule (2.54 cm x 3.0 mm outside diameter x 2.6mm inside diameter) in the case of runs made at the NNO buffer.The inner capsule was sealed into an outer Au capsule (3.81cm x 4.75 mm outside diameter x 4.0 mm inside diameter)containing approximately 0.25 g of the buffer assemblage, and50- to 75-pL distilled-deionized water. For runs made at thegraphite-methane buffer, the sealed Ag-Pd capsule was loadeddirectly into the bomb with a small graphite filler rod, and thesystem was pressurized with methane. For samples heated in air,approximately 0. I 5 g of charge was loaded into a small Au boat,which was placed in the preheated muffie furnace.

Upon completion of hydrothermal runs, the vessels were re-moved from the fumaces and initially quenched in a stream ofcompressed air and then immersed in water. Using this proce-dure, the vessels reached a temperature ofless than 50 "C in lessthan 5 min. After opening the capsules, the presence of water inboth the buffer and charge was verified by heating each of thecapsules in a small bottle on a hot plate and noting the conden-sation on the upper walls of the bottle. The l-atm nrns weresimply removed from the furnace and allowed to cool to roomtemperature.

In attempts to demonstrate steady-state conditions and theapproach to equilibrium, run times were varied from I to 12 dfor the hydrothermal runs, and from 0.5 to 4.75 h for the runsin air. In addition, products of some experiments were re-run atdifferent conditions in order to reverse the efects of oxidationor reduction.

Rnsur,rsResults of experiments are presented in Table 3, which

gives experimental run conditions, a description of runproducts, R : molar Fe3*/(Fe3* + Fe2*) values, and unit-cell parameters of selected amphiboles. Chemical com-positions of amphiboles from selected mns are given inTable 2.

Except for the air-annealing experiments, which wererun at 700 oC, all experiments were carried out at 1 kbarand 650 oC. Temperature was not varied in this series ofexperiments in order to avoid the complication producedby variation in oxygen fugacity ofthe buffers with chang-ing temperature. As a result, because of the different ther-mal stabilities of the four amphiboles, decomposition ofamphibole was observed in a number of cases. Treatmentof two amphiboles at the same buffer but at two different

temperatures, so that each mineral lies within its ownstability field, would result in a different oxygen fugacityin each run. Thus, an additional variable would be addedto the interpretation of results, as compared to isothermalexperiments.

Ferric-ferrous ratios and phase relations

Grunerite. As expected, air treatment produced the mosthighly oxidized grunerite, with R increasing from zero inthe starting material to 0.16, 0.27, and 0.31 after treat-ment for 0.5,2.0, and 4.75 h, respectively. These valuesrepresent 02- occupancy of 48,81, and 93 mo10/o of theO(3) site, respectively, assuming that only the dehydro-genation mechanism (Eq. l) is operative. X-ray and op-tical analysis of run products indicate no decompositionto secondary phases, even though the grunerite, as a hy-drous phase, is clearly unstable in air at elevated temper-ature. Peak broadening was observed in the X-ray patternof the 4.75-h run, which may represent the initial stagesof decomposition. The chemical composition of the am-phibote in 0.5-h run (AOE3AG, Table 2) is virtuallyidentical with that of the starting material, except for R,after the analysis is corrected for the presence of oxy-component.

In runs made at the MH buffer, the sample decom-posed to a presumably Mg-enriched amphibole * ironoxides * silica, behavior that is consistent with previousexperimental studies in Fe-Mg amphibole systems(Forbes, 1977; Popp et aI., 1977). Ferric-ferrous ratioswere not determined for these runs because of the con-taminating effect of additional Fe-bearing phases on theanalytical technique.

A small amount (<2o/o) of opaque phases was observedoptically in the 4-d run at the NNO buffer. Previous stud-ies of phase relations (e.g., Gilbert et al., 1982) indicatethat Fe-rich amphiboles should decompose to Mg-en-riched amphibole containing approximately 600/o gmner-ite component by the liberation of magnetite and quartzat the conditions employed here. It is concluded that theamphibole was in the incipient stages of decomposition,but that the 4-d run time was not sufficient for a signifi-cant extent ofreaction (Popp et al.,1977). The relativelysmall extent of decomposition results in both a ferric-ferrous ratio (Table 3) and a chemical composition(AOE24, Table 2) that are not significantly different fromthose of the starting material.

Runs carried out at the CCH. buffer resulted in nochange in R as compared to the unoxidized starting ma-terial. Within the precision of measurement, no Fe3* wasdetected. A weak olivine reflection was observed in prod-ucts of one of the runs, but decomposition of the amphi-bole to small amounts of olivine is not unexpected (Gil-bert et al., 1982). Starting material for one of the CCH.runs was the 0.5-h air-oxidized material (R : 0. l6). Res-toration of the ferric-ferrous ratio to essentially zero,within error of measurement, in 4 d demonstrates thatthe formation of oxy-amphibole component is easily re-versible.

492 CLOWE ET AL.: Fe3+/Fe2+ AND UNIT-CELL PARAMETERS OF CLINOAMPHIBOLES

Tschermakitic-Hornblende

Ferric-Ferrous Ratio (R)

0.20 0.30. r l l

Enor = 10.01

777)0.40 0.50

, t l0.10

l l

o s.m.

G M 4 d . €

GM 8d

AtoGM 4d . r l

FMQ 12d .-{

NNO 4d '{

NNO 8d O*

MH 4d O-

MH 8d O-

G M t o M H s d f

A700"C 30m

Fig. 1. Results ofexperiments on tschermakitic hornblende. Circles represent ferric-ferrous ratio ofunaltered starting material.Tips of arrows indicate ferric-ferrous ratio in run products; precision of measurement is +0.01. Boxes represent runs in whichstarting materials were products of previous experiments. Abbreviations: s.m. : unaltered starting material; GM : graphite-

methane; FMQ:fayal i te-magneti te-quartz; NNO:nickel-nickeloxide;MH:magneti te-hematite; A:air;d:days; m:mtnutes.

In the products of the air and NNO runs, some gru-nerite grains were observed to exhibit small, widelyspaced, dark striations perpendicular to the long axis ofthe crystal when viewed optically before crushing. Opticalexamination was unable to reveal whether the featureswere fractures or inclusions, but electron-microprobeanalysis, including dot maps, indicated no compositionalvariations. This phenomenon was also observed in rie-beckite treated at the graphite-methane buffer.

Tschermakitic hornblende. The tschermakitic horn-blende exhibited continuous variation in ferric-ferrousratio as a function of increasing oxygen fugacity. Runresults are shown graphically in Figure 1. Air oxidationfor 0.5 h produced an increase in R from 0.21 in theuntreated sample to 0.47, which represents oxidation of0.52 Fe per formula unit. The 4- and 8-d runs at the fo,of the MH buffer resulted in statistically identical R val-ues of 0.30 and 0.31, respectively, with no evidence fordecomposition. The composition of the amphibole in the8-d run did not change, except for R, after correction forthe increased oxy-amphibole component (AOE I lA, Ta-ble 2).

Samples treated at the NNO buffer for 4 d (R : 0.19)

and 8 d (R:0.22), and at the FMQ buffer for 12 d (R :

0.18) resulted in little change in ferric-ferrous ratio rela-tive to the starting material. A small amount of quartz(<30/0) was detected optically in the 8-d NNO run, butthe chemical composition of the amphibole (AOE l4,Ta-ble 2) did not change significantly. Whether the presenceof quartz indicates an original quartz impurity or actualdecomposition is unclear. In an experimental investiga-tion at pressures above 7 kbar, Oba (1978) observedqvartz, clinopyroxene, and garnet in the dehydration as-semblage of tschermakite. However, data on the stabilityrelations in the temperature-pressure range of this studyare not available.

Treatment of tschermakitic hornblende at the CCH.bufer produced the somewhat reduced ferric-ferrous ra-tios of 0. 1 6 after 4 d, and 0. 1 5 after 8 d.

Treatment of the air-oxidized sample at the CCH4 buff-er for 4 d reduced R from 0.47 to 0. 14, which agrees wellwith the values of 0. 15 and 0. 16 obtained using unoxi-dized starting material. Re-annealing the amphibole froma CCH. run at the MH buffer for 4 d resulted in a valueof R : 0.30, which is identical with that obtained bystarting with the original amphibole. In contrast, treat-

CLOWE ET AL.: Fe3+/Fe2+ AND UNIT-CELL PARAMETERS OF CLINOAMPHIBOLES 493

ment of the air-oxidized sample at MH for 4 d resultedin partial decomposition of the amphibole to liberate ap-proximately l0o/o hematite and quartz.

Magnesio-hornblende. The magnesio-hornblende issomewhat similar in composition to the tschermakitichornblende, containing 1.66 vs. 1.96 total Fe per formulaunit, respectively. However, the magrresio-hornblendecontains 0.65 fewer Al per formula unit, and has a lowerinitial ferric-ferrous ratio of 0.07 vs. 0.21. As with thetschermakitic hornblende, a systematic variation in R wasobserved over the range of oxygen fugacities employed(Table 3).

The most highly oxidized amphibole was obtained byai r -heat ing: R:0.31 af ter 0.5 h, R:0.50 af ter 1.5 h.Very small amounts of opaques (< lolo), presumably ironoxide, were observed optically in the air-heated runs.Treatment at the MH bufer for 8 d resulted in the for-mation ofpyroxene and a spinel phase in the charge, asidentified by X-ray difraction. Electron-microprobeanalysis (AOE 19, Table 2) reveals that the oxidized am-phibole is somewhat depleted in Al and Fe, and enrichedin Si and Mg relative to the starting material. Whetherthis amphibole composition represents an equilibriumvalue is unknown; treatment for longer times could resultin further decomposition and changes in composition.

As with the tschermakitic hornblende, runs of 8 d atthe NNO and FMQ buffers produced little change in R.The resulting amphiboles are slightly more oxidized ascompared to the ferric-ferrous ratio of 0.07 in the startingmaterial, i.e., R : 0. I I for NNO, and R : 0. l2 for FMQ.

Treatment of the original sample at the CCH. bufferfor 8 d resulted in total reduction (R : 0.00) of the Fe inthe amphibole. Treatment of the air-heated sample at thesame conditions resulted in run products with R : 0.09,but the small amount of opaque phases present in the air-heated sample was not resorbed during the run at loweroxygen fugacity. The presence of l0lo magnetite in thesample would increase R of the bulk sample by 0.025.Thus, subtraction of the effect of the magnetite results inan amphibole with R in the range 0.06 to 0.07, as com-pared to the value of 0.00 obtained from treatment of theoriginal starting material at the CCH. buffer.

Riebeckite. The riebeckite-arfvedsonite solid solutionhad an initial R value of 0.33 as compared to 0.20 forideal arfvedsonite and 0.40 for ideal riebeckite. In addi-tion, it contained 0.98 F per formula unit, so that themaximum possible amount of dehydrogenation is limitedrelative to the other three amphiboles studied here.

Treatment of the original sample in air for 0.5 h re-sulted in the liberation of a small amount of free quartz(<2o/o). The resulting amphibole may be slightly silicadeficient (AOE 32, Table 2), but the difference is at thelevel of detection. Because the anall'tical technique forferric-ferrous ratio is not affected by the presence of ad-ditional non-Fe-bearing phases (Fritz and Popp, 1985), avalue of R can be determined for the oxidized riebeckite.The resulting value of R (0.56) requires that 0.97 atomsof Fe2t were oxidized. That is, of the 1.02 atoms of OH

available per formula unit, nearly all were converted too2-.

In an 8-d run at the MH buffer, riebeckite decomposednearly completely to hematite, magnetite, quattz, and ac-mite. These results are in agteement with those observedin the synthetic system (Ernst, 1962),inwhich riebeckite-arfvedsonite solid solutions are unstable down to tem-peratures as low as 500'C at the oxygen fugacity definedby the MH buffer.

Runs carried out at the FMQ and CCH. buffers for 8d resulted in slightly reduced ferric-ferrous ratios of0.28and0.26, respectively, with no optical or X-ray evidencefor decomposition. Based on the results of Ernst (1962),the amphibole should lie within its stability field at theseconditions. In contrast to the results ofErnst, Owen (1985)reported arfvedsonitic amphibole to be unstable at 7 50oC, I kbar, andfo, defined by the FMQ buffer, but theresults cannot be directly compared to those obtainedhere at 650 "C. The most reduced of the riebeckites re-quires reduction of 0.27 Fe3* per formula unit relative tothe starting material.

Unit-cell parameters

Unit-cell parameters of selected amphibole run prod-ucts determined from powder X-ray diffraction are givenin Table 3. The numbers in parentheses in the table rep-resent one-standard-error values calculated by the least-squares computer program. The relatively larger stan-dard-error values for the air-treated samples probably aredue to the factthal the crystallinity of those samples hasbeen affected by incipient stages ofdecomposition.

As a simple test for precision, unit-cell parameters forthe grunerite, as determined by several different tech-niques and by several different individuals, are comparedin Table 4. The grunerite used for this study was obtainedas a hand sample of schist from C. Klein, who has madethe amphibole available to workers for a variety of dif-ferent kinds of analyses (e.g., Finger, 1969;Dyar, 1984).Shown in Table 4 are five different determinations, twoobtained by different individuals using a powder diffrac-tometer as described above, two obtained from powderphotographs utilizing an internal standard, and one ob-tained from an automated single-crystal difractometer.In general, the agreement among the values in Table 4 isgood, but the data suggest that doubling ofthe standard-error values, a practice advocated by some researchers,gives better agreement for comparing the results of thedifferent analysts.

DrscussroN

In the discussion that follows, it is assumed that thereader is familiar with the basic features of the amphibolecrystal structure, the nomenclature for cation and anionsites, and the general scheme of partitioning of cationsamong the sites according to size and valence. Excellentreviews ofthe subject are given in Ernst (1968), Carneronand Papike (1979), and Hawthorne (1981, 1983).

CLOWE ET AL.: Fe3+/Fe2+ AND UNIT-CELL PARAMETERS OF CLINOAMPHIBOLES

Treue 4. Comparison of unit-cell parameters

Method ofAnalyst X-ray diffraction (A")

R

f)(A)b

(A)(A)

C. A. Clowe"M W. Phillips (pers. comm.)Klein (1964)Finger (1 969)M A. Cusimano (pers. comm.)

1 8.397(4)1 8.397(3)1 8.380(7)18.393(2)18.374(17)

Nofe.' Comparison of unit-cell parameters of grunerite (Klein, sample 1 , 1964) determined by several different methods and experimenters.- Values as given in Table 3.

diffractometersingle crystalpowder photopowder photodiffractometer

9.556(2)9.570(2)9.s62(2)9.5642(7)9.568(5)

5.347(7)5.342(2)5.338(4)s.3388(3)5.33s(1 6)

101.904(35) 919.855(961)101 .94(3) 920.2(8)101.86(3) 918.2(7',)101.892(3) 919.0(2)101.828(48) 918.067(2.058)

Ferric-ferrous ratios and phase relations

Because no single value ofR has been approached fromopposite directions, the strict reversal ofequilibrium hasnot been documented in any of the runs in this study.However, the relatively rapid changes in R, the time in-dependence offinal R values in the hydrothermal runs,and the fact that R values can be restored after havingbeen altered (Fig. l) lend support to the assumption thatan approach to equilibrium has been obtained.

Clearly, there are differences in the behavior ofthe fourdifferent amphiboles. Air oxidation produces the mosthighly oxidized samples in all four minerals. As hydrousphases, amphiboles are unstable in air at 700 "C. Theslight, but still significant broadening of peaks in the X-raypatterns, particularly in runs longer than 0.5 h, may rep-resent incipient stages of decomposition. Longer-term air-heating runs described elsewhere (Hodgson et al., 1965;Patterson and O'Connor, 1966) have resulted in decom-position. It is, thus, concluded that equilibrium is notapproached in any ofthe air-heating runs. The Fe3* con-tent of the amphiboles should increase with longer runtimes until complete dehydrogenation occurs, completeoxidation of Fe is achieved, or the amphibole decom-poses. Therefore, differences in R values of the four am-phiboles heated in air are most likely related to kineticfactors and to their different initial Fe2* contents.

At the oxygen fugacity of the MH buffer, all the am-phiboles except tschermakitic hornblende decomposed toassemblages containing Fe3*-bearing oxides, or in the caseof riebeckite, Fe3*-bearing pyroxene. The original tscher-makitic hornblende (R : 0.21) did not decompose in runsof up to 8 d, but treatment of the more highly oxidizedair-heated sample (R : 0.47) at MH did result in decom-position to an assemblage containing iron oxides. Theseresults suggest that the amount of Fe3* required in am-phibole by this relatively high oxygen fugacity cannot beaccommodated within the crystal structure, and decomposi-tion to other Fe3*-bearing phases results. The attainmentof an apparent metastable equilibrium in tschermakitichornblende indicates that the equilibrium ferric-ferrousratio is achieved more rapidly than the rate at which de-composition reactions occur. The fact that decomposi-tion is not observed in the more highly oxidized air-heat-ed samples is considered to be a result of the significantly

shorter run times and the fluxing effect of water in thehydrothermal runs.

The NNO and FMQ buffers produce little, or only slight,variation in ferric-ferrous ratio relative to all four startingmaterials. Most significant is the fact that grunerite, un-like the other three varieties, contains no Fe3* at the fo,defined by NNO.

The different behavior of the four amphiboles may berelated partially to the two mechanisms by which Fe3* isaccommodated within the amphibole crystal structure. Inthe first mechanism, formation of oxy-amphibole com-ponent by oxidation-dehydrogenation (Eq. 1), the in-creased charge that results from oxidation ofFe2* to Fe3*is compensated by replacement of the same number ofunivalent hydroxyls by divalent oxygens. As discussed indetail elsewhere (Phillips et a1., 1988), dehydrogenationproduces significant underbonding at the O(3) site, whichin turn, induces compensational mechanisms to offset theunderbonding. For example, changes in octahedral oc-cupancies, shortening of M-O bond lengths, displace-ment of univalent cations from M(4) to the A site, andincreased interaction of A-site cations with O(3) accom-pany oxidati on-dehydrogenation.

In the second mechanism, herein termed Al substitu-tion, Fe3* behaves analogously to Al3* in octahedral cat-ion sites. Octahedral Al3* is accommodated in amphi-boles either (l) bv a coupled tschermakitic-typesubstitution in which replacement of divalent cations byAl3* in octahedral sites is coupled with substitution ofAl3+ for Si4+ in the tetrahedral sites or (2) by a substitutionin which replacement of Ca2* by a univalent cation in theM(4) sites is coupled with substitution of Al for divalentcations in the octahedral sites, as in the case of glauco-phane (NarMg.AlrSirOrr(OH)r) or alumino-winchite(NaCaMg"AlSi8Orr(OH)r). The extent to which each ofthe two mechanisms determines the Fe3* content of anygiven natural amphibole is unknown, but publishedchemical analyses of amphiboles suggest that both pro-cesses are involved in many natural samples (Popp andPhillips, 1987).

Reduction of Fe3* present as oxy-component in an am-phibole can be accomplished by a hydrogenation reac-tion; that is, the reverse of Equation l. The extent towhich Fe3* that is present as a result of Al substitutioncan be reduced is less well known. Semet (1973) docu-

mented the complete reduction of Fe in magnesio-has-tingsite (NaCarMgFe3*Al,Si6Orr(OH)r) to l00o/o Fe2* andconcluded that one excess H was present in the structurein order to produce the required charge balance. Thepresence ofexcess OH has also been reported in syntheticamphiboles (Witte et al., 1969; Maresch and Langer,1976). as well as in natural calcic and subcalcic varieties(Leake, 1968).

Some conclusions can be made based on the two dif-ferent mechanisms for incorporation of Fe3* into amphi-boles. The experimental results indicate that grunerite cancontain a considerable amount of Fe3* under short-term,nonequilibrium conditions (i.e., the air-heated samples),but can accommodate only a very limited amount of Fe3"under equilibrium conditions. No Fe3* was observed tobe present in any of the hydrothermal runs made in thisstudy, although analysis of amphibole Fe3* content of runsmade at the MH buffer was not possible. Because gru-nerites, in general, contain only small amounts of Na andAl, the Al substitution mechanism cannot produce a sig-nificant Fe3* content. The extent to which oxy-amphibolecomponent can form in grunerite may also be limited. Ifthe presence ofA-site cations is required to relieve a por-tion of the imbalance of bond strengths caused by O'z- inthe O(3) site, the relative absence of Na in this and mostnaturally occurring grunerites may severely limit theamount of oxy-component that can form. The result ofthe local charge imbalance may be that grunerites eitherresist oxidation of Fe2*, as observed in the NNO run, orat higher oxygen fugacities, decompose to Fe3*-bearingoxides so that the Fe3t in the system is accommodated inoxides rather than in the amphibole. Previous experi-mental studies (e.g., Popp et al., 1977) have documentedthat Fe-Mg amphiboles are restricted to very Mg-richcompositions at the oxygen fugacity defined by the MHbuffer, which is likely to be related to the inability of thisgroup of amphiboles to accommodate Fe3*. Low Fe3tcontents are also observed in naturally occurring Fe-Mgamphiboles (Fig. 2 in Robinson et al., 1982; Deer et al.,l 963) .

The magnesio-hornblende is more reduced than thetschermakitic hornblende for all the buffer conditions andis totally reduced (R : 0.00) at the CCH. buffer as com-pared to the value ofR:0.15 obtained for the tscher-makitic hornblende. These results suggest that the origi-nal Fe3* content of the magnesio-hornblende may havebeen virtually all oxy-component, whereas that of thetschermakitic hornblende may have been due to both oxy-component and Al substitution.

A simple comparative thermodynamic analysis canprovide some information regarding the comparative be-havior of the two hornblendes. The log form of the equi-librium constant for Equation 1 is

log K: log 4.":* * log c.*, * r/zlogfr, ",gapg2+- log anru.""r, (2)

where/r, is the fugacity of H, in the system, and the terms

495

4, represent the activities ofthe appropriate componentsin the amphibole solid solution (a."r*, the activity of Fe3*component; 4oxy, oxy-amphibole component; 4r"z*, Fe2tcomponent; and 4hyd,o*y, hydroxy-amphibole component).Because the values of/r, for the buffer assemblages canbe calculated, Equation 2 canbe used to compare calcu-lated and observed compositions for a single amphiboletreated over a range of buffers, or two different amphi-boles treated at the same buffer, provided activity-com-position relations for the amphiboles are known. The ma-jor compositional diferences between the two hornblendesare that the magnesio-hornblende (1) contains a largeramount of F in O(3) (0.33 vs. 0.06 per formula unit), and(2) contains a smaller amount of octahedral Al (0.64 vs.0.83 pfu). Given these compositional differences, somesemiquantitative conclusions can be made, provided thatthe activity coefficients for the two minerals are unity orare assumed to be similar. For unit activity coefficients,the activity of hydroxy-amphibole component can be ex-pressed as

an"o*" : X 'ot ' (3)

where X is the mole fraction of OH in the O(3) site.Therefore, from Equations 2 and 3, for two amphibolesequilibrated at the same buffer, the lower activity of hy-droxy-component in the magnesio-hornblende requires alower ferric-ferrous ratio at constant activity of oxy-com-ponent. That is, if the activity of oxy-component is thesame in the two minerals, the lower activity of hydroxy-component requires a lower ferric-ferrous ratio. The ac-tivities ofFe3* and Fe2* can be expressed as

Qp4* : X2prt*1yrrll4":*tr..rt:>Xo",*G.,rrrll (4)

Qps2+ : X2."r*1*1r;X2u.r*1u12;XE z*641t;, (5)

where the X terms refer to the mole fraction of the ionsin the individual cation sites. Fe3* and Al are stronglypartitioned into the M(2) site in amphiboles (e.9., Haw-thorne, 1983), and thus effectively reduce the number ofsites on which Fe2* can mix. For example, if M(2) weretotally occupied by Al and Fe3*, Fe2* could mix on onlythree rather than five sites, whereas Fe3* could still mixon all five. A consequence of mixing on fewer sites is anincrease in activity, i.e., the mole fraction is raised to alower power. Therefore, if two amphiboles containedidentical amounts of Fe2*, that with the higher Al contentwould have a higher activity of Fe2*. From Equation 2,a higher activity ofFe2* requires a higher activity ofFe3*at constant ahyd,o,y, a"*y, and fH2. Thus, increased Al con-tent favors a higher ferric-ferrous ratio, given that all oth-er variables are constant.

It can be concluded that the higher OH and Al contentsof the tschermakitic hornblende are consistent, in a semi-quantitative sense, with its higher ferric-ferrous ratios rel-ative to magnesio-hornblende. Without knowledge of theactual site populations of the cations, more rigorous anal-ysis is not warranted.

The fact that the air-oxidized magnesio-hornblende

CLOWE ET AL.: Fe3+/Fe2+ AND UNIT-CELL PARAMETERS OF CLINOAMPHIBOLES

496 CLOWE ET AL.: Fe3+/Fe2+ AND UNIT-CELL PARAMETERS OF CLINOAMPHIBOLES

cannot be totally reduced, whereas the unoxidized mag-nesio-hornblende can, is considered to be related to thepresence of the iron oxides in the air-heated sample. Thepresence of oxides suggests that mechanisms other thansolely oxidation-dehydrogenation may have been opera-tive (e.g., Addison eI al., 1962a, b; Hodgson et al., 1965).The possibility and identity of such mechanisms are thesubject of continued investigation.

The results of the riebeckite experiments are more dif-ficult to explain in some respects. The ferric-ferrous ratioof the air-oxidized sample (R : 0.56) requires loss of 0.97OH from the starting amphibole composition. Given theoriginal F content of 0.98 F pfu (Table l), the maximumoriginal oxy-component can be no greater than 0.05 oxy-gen pfu. The problem arises in that the riebeckite treatedat the FMQ and CCH. buffers is significantly reducedrelative to the starting material. For example, the mostreduced riebeckite (R : 0.26) requires 0.30 Or- to beconverted to OH-. However, this number of oxygens issignificantly larger than the number present in the chem-ical formula.

Two possible mechanisms could be responsible for theapparent "over-reduction" of Fe3* in riebeckite. Thepresence of excess H in the form of OH within the am-phibole as proposed elsewhere (Leake, 1968; Witte et al.,1969; Semet, 1973; Maresch and Langer, 1976) wouldpermit the existence of excess Fe2t. The likelihood of thisbeing the actual mechanism cannot be evaluated withoutquantitative analyses of water contents or without theapplication of techniques such as infrared spectroscopyor single-crystal structure analyses. Alternatively, Reb-bert and Hewitt (1986) reported results of a similar ex-perimental study of the variation of ferric-ferrous ratiosin synthetic biotites in response to changes in controlled,f"r. Based on the direct measurement of H content of therun products, they concluded that anion vacancies werepresent in the more reduced samples. The presence ofsuch vacancies in amphiboles would allow for excess re-duction ofFe3* over that present as oxy-component, butin the absence ofH analyses, such an explanation is alsototally speculative.

Unit-cell parameters

Changes in unit-cell parameters with changing ferric-ferrous ratio should be related to the size difference be-tween Fe2* and Fe3*, as well as to changes in cation siteoccupancies. The observed variations in unit-cell param-eters can be interpreted to some extent based on the re-sults of Colville et al. (1966), who investigated the rela-tion between unit-cell parameters, bulk chemicalcomposition, and site occupancies in synthetic and nat-ural clinoamphiboles. They concluded that the size of theD unit-cell edge is related to the lateral linking of thetetrahedral chains by the cations occupying the M(4) andM(2) sites. In the case of the calcic and sodic amphiboles,in which M(a) is occupied mainly by Ca and Na, theoccupancy of M(2) exerts the major control on b. In theFe-Mg amphiboles, however, the proportions of Fe and

Mg in M(4) also exert control on b. The size of a sin B isrelated to the unit repeat across facing double chains.Given constant thickness of the tetrahedral chains (i.e.,constant tetrahedral occupancy), a sin B is related to thethickness of the octahedral strip. For constant M(4) con-tent, the occupancy of the M(l), M(2), and M(3) sitesdetermines the size of a sin B. The c unit-cell edge is ameasure of the unit repeat parallel to the tetrahedralchains. Its size is apparently controlled by complex in-teractions of the cations occupying all of the M sites, aswell as the Si:Al ratio. Finally, B is determined mainly bythe occupancy of cations in M(a). These controls on unit-cell parameters describe the general trends observed, butin some cases, the variations in unit-cell parameters cal-culated strictly on the size differences between cations inspecific sites are significantly smaller than, or even op-posite to, those observed in the actual amphiboles (e.g.,Table 4 in Colville et aI., 1966). Such observations sug-gest that other crystal-chemical factors contribute to vari-ations in amphibole unit-cell parameters.

Given the variations in unit-cell parameters (Table 3),some interpretations can be made, but the interpretationis complicated somewhat by the relatively large standarderrors in the unit-cell refinement for the air-heated sam-ples when the standard statistical test for significance atthe two-standard-error level is used.

a sin d. Figure 2 shows the variation in a sin B of theamphiboles normalized to the change in number of ferricirons per formula unit. Positive values represent in-creased amounts ofFe3*, whereas negative values repre-sent reduction of Fe3* relative to the unaltered startingmaterial. The similar slopes for tschermakitic horn-blende, magnesio-hornblende, and riebeckite (0.084,0.062, and 0.060 A per atom ofFes*, respectively) suggestthat the expansion or contraction in the direction per-pendicular to the chains is approximately the same in allthree crystal structures as the amount of Fe3t changes.Assuming that the Fe content of the M(4) site is negligiblefor these three amphiboles, the changes in Fe3* contentmust be accommodated in the M(l), M(2), and M(3) sites,which is then reflected in the size of c sin B.

The response of a sin B to changing Fe3* content isdistinctly different for grunerite. The least-squares curvethrough the data points shows essentially no change overthe range of compositions obtained in the experiments.It is difrcult to envision a mechanism by which replace-ment of a larger cation by a smaller one would result inno change or an increase, but given the very large relativeerror for the most oxidized air-treated run, a decrease ina sin S with increasing Fe3* content is certainly possible.Part of the difference between grunerite and the otherthree minerals may be due to the fact that in grunerite,there is occupancy of Fe in the M(4) site, and thus theoccupancy of M(4) is involved in controlling a sin B. Kleinand Waldbaum (1967) calibrated the variation in unit-cell parameters as a function of Fe-Mg ratio for the cum-mingtonite-grunerite series. The dashed line in Figure 2shows the variation in a sin B calculated from the equa-

CLOWE ET AL.: Fe3+/Fe2+ AND UNIT-CELL PARAMETERS OF CLINOAMPHIBOLES 497

tion of Klein and Waldbaum for replacement of Fe'z* byMg. The variation observed for grunerite is in betteragreement with the calculated curve than with the curvesfor the other three amphiboles, but the analogous behav-ior of Fe3* and Mg2+ is questionable. Colville et al. (1966)were able to explain observed variations in unit-cell pa-rameters with the assumption that the two cations aresimilar in radius. However, in more recent compilationsofionic radii (Shannon and Prewitt,1969), the radius ofFe3t is approximately 100/o smaller than that of Mg. Ideal(M-O) bond lengths in amphiboles (Table 15, Haw-thorne, l98l) also reflect the smaller radius of Fe3* rela-tive to Mg. Increasing Mg content in the natural cum-mingtonites-grunerites is strongly partitioned into theM(l), M(2), and M(3) sites, whereas changes in site oc-cupancies in the oxidized grunerites are unknown. Thesignificantly smaller response of a sin B relative to Fe3*as compared to Mg may indicate that oxidation occurswith little repartitioning of Fe within the cation sites. Insummary, the observed behavior of a sin B in gruneriteis considerably different from that of the three amphi-boles with negligible Fe content in M(4), but an expla-nation for the difference cannot be verified without dataon site populations.

D unit-cell edge. The data for grunerite (Table 3) suggesta decrease in b as R increases. Such a decrease is to beexpected because, in grunerite, there is significant Fe2*content in both M(2) and M(4), which, presumably, isoxidized to the smaller Fe3*. As was the case for a sin 0,the observed change in b is considerably less than thatobserved for an analogous amount of replacement of Fe2*by magnesium. The observed decrease of 0.020 A forreplacement of the 1.62 atoms per formula unit comparesto the value of 0. I 20 A for replacement of I .62 atoms inthe Fe-Mg system (Klein and Waldbaum, 1967). In thenatural cummingtonites-grunerites, the increased Mgpartitions very strongly into M(2); thus, the smaller changein D observed here could be explained if Fe3* is not stronglypartitioned into the M(2) site.

In the two hornblendes, there is no evidence for a changein D. For both amphiboles, all values of D except the twoextremes are equal within one standard error, and thoseof the extremes are the same well-within two standarderrors. In general, it has been observed that Fe3* parti-tions strongly into the M(2) site in hornblendes (Cameronand Papike, 1979; Hawthorne, 1981, 1983). The con-stancy of b observed here is interpreted to indicate thatFe3* is not strongly partitioned into that site in the oxi-dized hornblendes. This conclusion is confirmed by Phil-lips et al. (1988), who have shown that both total Fecontent and Fe3* content of the M(2) site decrease in theoxidized tschermakitic hornblendes of this study.

Given the large magnitude of the errors, there is nosignificant change in b for the riebeckite, but Ernst andWai (1970) have documented a general decrease in b ofsodic amphiboles with an increase in oxidation-dehydro-genation.

c unit-cell edge. The factors that control the variation

Riebeckitem = - 0 0 6 0

Mg-hblndem = -0.062

Tsch-hblnde \

m = -0.084

9.300-0.5 0.0 0.5 1.0 1.5 2.0

Change in Atoms of Ferric lron Pfu

Fig.2. Variation in a sin B of the amphiboles as a functionof the change in the number of ferric irons per formula unit. Thevalue 0.0 on the abscissa represents the original starting material.Positive values indicate increase in Fe3* content, whereas nega-tive values represent reduction of Fe3*, relative to the startingmaterial. Slopes (z) are in A/atom of Fe3*. The dashed line forgrunerite shows the change in a sin A that would result fromsubstitution of an equivalent amount of Mg for Fe'?*, rather thanFe3* for Fe2'. See text for details.

in c are understood more poorly on a crystal-chemicalbasis than for the other two unit-cell edges (Colville etal.,1966), so that interpretation ofthe results may be lessmeaningful than for the other parameters. In addition,the data for the c unit-cell edge are considered to be oflesser quality because the least-squares refinement ofc isbased on a relatively small number of peaks as comparedto a and b. A decrease in c with increasing Fe3* contentshould be expected in grunerite, based on the behaviorof Fe-Mg substitutions (Klein and Waldbaum, 1967), butthe observed difference is several times larger than thatin the Fe-Mg system. The relatively imprecise value forthe most oxidized sample may contribute to the differ-ence.

There is no evidence for variation ofthe c unit-cell edgeof the tschermakitic hornblende, but that of the magne-sio-hornblende apparently decreases with increasing Fe3*content. Given the similarities in compositions ofthe twoamphiboles, significant differences would not be expect-ed. Explanation ofthe behavior, ifnot related to the pre-cision of measurement, awaits further crystallographic in-vestigation.

Within the standard errors of the unit-cell refinement,c of riebeckite does not vary with R.

p angle. With the exception of grunerite, the value ofthe interaxial angle B does not vary significantly for theamphiboles (Table 3). This observation is consistent withthe results of Colville et al. (1966), who concluded that B

9.600

9 550

9.500

"? e 4s0

f nooo

498 CLOWE ET AL.: Fe3+/Fe2+ AND UNIT-CELL PARAMETERS OF CLINOAMPHIBOLES

is determined mainly by the identity of the cation in theM(4) site, Ca vs. Na vs. Fe-Mg.

Unit-cell volume. The unit-cell volume is the productofthe other parameters, and thus reflects the variationsof the individual unit-cell parameters discussed above.Except for riebeckite, for which the standard errors arevery large, unit-cell volumes of all the amphiboles de-crease with increasing content of the smaller Fe3*.

CoNcr,usrous

Based on the results obtained in this study, the follow-ing conclusions can be made.

l. Significant variation in Fe3* content can be achievedin amphiboles by variation in oxygen fugacity. Oxida-tion-dehydrogenation equilibrium as described in Equa-tion I can account for most, but not all of the observedbehavior.

2. Steady-state values offerric-ferrous ratio, which arepresumed to approach equilibrium, are attained relative-ly rapidly as compared to rates of decomposition reac-tions. In some cases, metastable Fe3r-Fe2+ equilibrium isachieved prior to decomposition.

3. The Fe3+ content of the magnesio-hornblende ispresent mainly as oxy-amphibole component, whereasthat of the tschermakitic hornblende is present as bothoxy-component and as the result of Al substitution. Thepossible reduction ofFe3* either by the presence ofexcessH over two atoms per formula unit or by the formationofanionic vacancies is possible, but cannot be evaluatedfrom the results obtained here.

4. Under equilibrium conditions, grunerite is inherent-ly less able to accommodate Fe3* than are the other threeamphiboles. Grunerite responds to increased oxygen fu-gacity by either resisting oxidation, or by expelling theoxidized Fe from its crystal structure.

5. Unit-cell parameters respond to changes in ferric-ferrous ratio, but the large errors of refinement of somesamples limit crystal-chemical interpretation. The mostsignificant change observed is a decrease in a sin B ofthetwo hornblendes and riebeckite with increasing R, whichrepresents increasing overall content of Fe3* in M(l),M(2),and M(3). The variation of a sin B with R in grunerite issignificantly different from that of the other three am-phiboles. There is no strong evidence for partitioning ofFe3* into the M(2) site of the oxidized amphiboles, basedon the variation in b.

AcxNowr,nocMENTS

We are indebted to P. Dunn, M. C. Gilbert, C. Klein, and F S. Spearfor providing the amphibole samples, and in some cases analytical data,used in this study. The manuscript was improved considerably by reviewsof M. D. Dyar, M. C. Gilb€rt, M. W. Phillips, M R. Scott, F. S. Spear,and panicularly M. Cho. This research was supported by National ScienceFoundation grant EAR-83 12878.

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MeNuscnrgr REcETvED Mev 28, 1987

M.r.r-uscnrpr AccEprED Jer.rulrv 14, 1988