the mineralogy of some deposits of kaolinized volcanic ash from the slate belt of north carolina

8/7/2019 the mineralogy of some deposits of kaolinized volcanic ash from the slate belt of north carolina

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THE MINERALOGY OF SOME DEPOSITS OFKAOLINIZED VOLCANIC ASH FROM THESLATE BELT OF NORTH CAROLINAJesrnn L. SrucrBy, North Carolina State College.

INrnonuctroNThe Slate Belt of North Carolina embracesa region which variesin width from 8 to 50 miles! and extends across the center of theState in a northeast-southwest direction. The belt reaches itsmaximum width near the center of the State and narrows to thenortheast and southwest. The rocks of this area, becauseof their

well developed cleavage, have been called slates. The wholeSlate Belt has not been mapped in detail, but field investigation,petrographic study, and chemicalanalysesof the rocks from differ-ent parts of the belt have shown that the slates are of volcanicorigin. In the areasstudied the slatesare composedof the followingmaterials: (1) flows of rhyolite, dacite, andesite and relatedrocks; (2) beds of volcanic ash, tufi and breccia of the samegeneralcomposition as the flow materials; (3) beds of slate composedofthe finer ash and tufi materials and land waste.The metamorphic forceswhich changed hesematerials to slatesdid not act uniformly throughout the whole area. In the northernhalf of the Slate Belt the rocks are highly slaty and schistosewith a well developedcleavage hat dips steeply to the southeast,while in the southern half the cleavage s in may areas less welldeveloped and dips to the northwest. In a number of places,particularly along the SeaboardRailway between Wadesboro andMonroe, original bedding planes are well preserved.Scattered throughout the Slate Belt, but especially common inits southeastern portion in Moore, Montgomery and Richmondcounties, are beds of a residual gray to white clay commonly re-ferred to as kaolin. This material while quite similar throughoutthe area varies from place to place. Some outcrops grade down-ward into a massiveand jointed material while others are distinctlyschistosebeneath the surface. This study dealswith the difierencebetween the massiveand schistoseohases. In all the deposits theclay hasa solt, smooth eel,but gri i ty material s much ,nor. .o--

mon in someoccurrences han in others. The bedsare all distinctlylenticular in shape and irregular in size, and vary in areal extentfrom a few square yards to several acres and in depth up to 25 or30 eet.


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TH E AM EMCAN M I NERALOGISTAs early as 1897Riesr studied and tested samplesof this material

from Richmond and Montgomery counties which yielded uponwashing from 15 to 40 per cent of clay substance. Tests showedthat this clay substancecompared favorably with standard kaolin.

Bayley2 while studying the kaolin of North Carolina in 1918visited Montgomery and Richmond counties and describedsomeof the deposits. Describing the Overton deposit near the IolaGold Mine at Candor he says: "The sample furnished is a loose,very light pinkish gray, gritty, flour-like mixture of very fine quartzand kaolin.xxxxllnder the microscope he principal constituentsarequartz grains of all sizes from the most minute to those 0.2millimeter in length. Perhaps the greatest number have diametersbetween 0.05 and 0.06 millimeter. Beiides these are a few whiteopaque grains with straight edges that may be altered Ieldspargrains and a fair quantity of small kaolinite particles of about thesize of the smallest quartz particles. Occasionally there is ashred of kaolinite 0.06 millimeter long but most particles are lessthan 0.004 millimeter across."

Describing the Eames Prospect near Mount Gilead he says.:"The sample looks very much like that from the Iola Mine atCandor. It is a very fine, gritty, flour-like material of a very palegrayish white color. The crude material is made up mainly ofsmall quartz grains with diameters between 0.02 and 0.M milli-meter. In addition there are a few particles of rutile, hydromicaand strained feldspar and a very few tiny plates of kaolinite. Theqtartz is in little sharp'edged splinters, in subangular grains, invery irregular shapedparticles and in a very few cases n roundedgrains. Evidently the material has not been carried far from itssource. It may be a residual mass, like that at the Iola Minefrom which most of the kaolinite has beenremoved." He concludesthat the material probably has been formed from a fine-grained,volcanic rock.

Nerunn or DEposrrs aNp MBrnoDS oE SrUDYDuring the past two or three years interest has been revived in

theseclays and attempts have beenmade to find uses or them' Inaddition to the occurrencesstudied by Ries and Bayley other de-

r Ries, H., Clay Deposits and CIay Industry in North Carolina:SuroeyBull.13, pp. 64-70, 1897.2Bayley, W. S., Kaolins of North Carolina: N. C. GeoI.Suney126-127. 1925.

N. C. Geol..80i l1..29,p.


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JOURNAL MINERALOGICAL SOCIETY OF AMEMCA 255posits have been prospected, three of which are describedbelow.One of these Iies along Drowning Creek in the western edge ofMoore and the eas+ernedge of Montgomery counties, some 5miles west of Eagle Springs. Another lies in Randolph countv4 miles west of Staley and about 25 miles south of Greensboro.The third deposit is located in Granville county about 2 milesnorth of Stem and some20 miles south of the Virginia line. Carefulfield investigation of the three areas ndicates,that each deposit isassociated with and occurs in acid volcanic rocks that consistchiefly of ash and fine tuff.Sevensampleswere collected rom these hree deposits or study,5 of which came rom the depositon Drowning Creek n Moore andMontgomery counties, and one each from the deposits nearStaley, Randolph county and Stem, Granville county. Wetscreen tests, partial chemical analyses and microscopic exami-nations weremade.For screentests samplesof each clay were crushed to disinte-grate any lumps present, care being taken to avoid grinding orreducing the natural grain size. Porlions of exactly 300grams eachwere weighedout dnd put into 500cc. of water to which a few dropsof NHrOH were added. The samples were left over night andscreened through a nest of hand screensconsisting of the 150mesh screen above the 200 mesh. The results of the screen testsare given in Table 1, and partial chemical analyses are recordedin Table 2.

Microscopic study was'carried out by mounting the clays inindex liquids which recently had been standardized. A small par-ticle of the material was placed on a glassslide and crushedwith thethumb nail, a few dropsof the index liquid were added and a coverglassplacedover the mount. The minerals presentwere determinedby indices of refraction and general optical properties, includinginterference igureswhere possible.DBscnrp:rrou or Saupr-Bs

Drowning Creek has cut through the deposit in Moore andMontgomery counties to the parent material below, and by de-flection against its southern bank hasproduced a blufi some 15 feethigh. Al l the evidenceobtained indicates that the clay is residual.The outcrop is jointed but much freer from schistose structurethan deposits farther north in the Slate Belt. The cfay is gray to


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256 TEE AM ERICAN MI NERA.LOGISTwhite in color while the bed rock just above the water level in thecreek s gray to yellow. About 1000 eet to the south of the streambed rock is exposedat the surface as a crudely stratified, gray topinkish colored material.

Five sampleswere taken from this deposit for study as follows:one sample from the parent rock just above the water level of thestream, one sample of the bed rock about 1000 feet south of thestream, and three samples at different levels from the streambluff.

in a fine grained matrix. Quartz is a common mineral occurring assplinters and sharp edged grains. Feldspar is present in partlyaltered grains. Small amounts of epidote, kaolinite, sericite andiron oxidesare present n a fine-grainedground masssomeof whichis non-polarizing.

Sampl,e2. The bed rock from the north end of the deposit isacrudely stratified material which contains gray and chocolatecolored layers. It is extremely fine-grained and sufficientlyweathered to break readily in one'shands and crumbles nto a pink

oxides, kaolinite, and sericite in a mass either non-polarizing ortoo fine to identify.

SompleS. A sampleof the clay taken from the bluff two or threefeet above the parent rock is a soft, gritty, rather white, flour-likematerial free from lumps and fragments. The microscopeshowsqtJartzcommon in angular, splinter-edgedand curved faced grains,the greater part of which vary between 0.02 and 0.04 millimeter indiameter. Kaolinite is abundant in shreds and plates about thesizeof the smaller quartzgrains, while fan and worm shapedmassestwo or three times the sizeof the largest qtartz grains are common'


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JOURNAL MINERALOGICAL SOCIETY OF AMERICA 257A few grains of halloysite, sericite, hydromica, rutile and titaniteare alsopresent.

Sample 4. A sample of the clay taken about midway betweenthe top and bottom of the blufi is a grayish white, rather gritty,flour-like material containing a few yellow streaks probably dueto iron stain. Quartz is abundant in irregular and angular grains0.01 to 0.03 millimeter in diameter. Kaolinite is common in tinyshredsand flakes and sparingly in excellentplates. Small amountsof sericite,hydromica and rutile are alsopresent.Sample 5. A sample taken from just beneath the soil is dis-tinctly gray in color, rather gritty and f ree from lumps. The mostcommon mineral identified is quartz. It occurs n irregular grainsof dust-like size hat vary from 0.003 to 0.02 millimeter in diameter.Much fine-grainedmaterial like kaolin is present though no definiteflakes or fan structures were seen. Small amounts of sericite, somehydromica and titanite are present in a fine-grained dirty groundmass much of which is non-polarizing.The deposit near Staley, Randolph County, occursas the hang-ing wall of a pyrophyllite body, but some 50 feet away from themineralized zone in which the pyrophylliLe is found. It has beenprospectedby a trench 40 feet long and a pi t 20 feet deep. In thepit the material has a well developed schistosestructure whichdips steeply to the northwest.

Sample d. A representative specimen from this pi t crumblesreadily in one's hands to a very fine, flour-like material, free fronlany gritty feel and is light gray to white in color. Microscopic ex-amination shows quartz abundant as dust-like fragments with amaximum diameter of 0.02 millimeter. Kaolin is sparingly presentin tiny particles a very few of which are doubtless distinct flakesof kaolinite. Sericite is abundant in characteristic scales andfibers. Rutile is common in needles and elbow twins, whiletitanite is occasionally ound.In the deposit near Stem, Granville County, the material asexposed n a small pit on the side of a hill has an extreme schistosestructure that dips steeply to the southeast.Sample 7. A hand specimenfrom this pi t crushes readily to afine-grained, lour-like material that is free from any gritty feel andis light gray to white in color. Under the microscopequartz andsericite are the only minerals readily determined. The shapeandsizeof the grains are much like thosealready described,particularlyin sample 6, and need not be repeated.


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THE AM ERICAN M I NERALOGISTTasr,B 1. WBr ScneBNTnsrs ol Smlpr-us

1PrnCrnrJ ?

Pr'n PrnCnNr CnNr

4 5 6PBn PBn PrnCnxr Cnwr Cnwr1

PBnCrNrOn 150MeshOn 200MeshTotalthrough200Mesh

29.93 2.86 29 .201 . 4 6 0 2 0 2 . 7 6

68.61 96 .94 68.04

30 .23 6 . t6 0 .40 2 .161 56 2 93 0 .53 0 .50

68.21 90.91 99.07 97 34Tanlr 2. Cnrurcal Awer-yses r SAupr,rs*

Sios 59.90 64.70Alzo:*FezOa 29 20 26.00Ign.Loss 7 90 5 50Na:oKzOTotal 97.1O 96.20

70.40 74.40 75.7022.00 19.20 19.104.60 4.40 4.60

97.00 98.00 99.40

73 .20 67 1018 .70 22 .904 .72 3 302 . 0 15.6496.12 100.95

* Analysesy G.R. Shelton, eramic hemist, ureau f Standards, olumbusBranch,Columbus, hio.Suuuanv ANDCoNCLUSToNS

Field evidence shows that the three deposits described lie inareasof acid volcanic rocks. Microscopic examination of samplesI and 2 reveals a material which, though much weathered, hasmany characteristics of a fine tuff or volcanic ash. The other 5samplesdue to weathering show little resemblancemicroscopicallyto true fragmental materials. Ilowever, the general similarity ofal l the materials and their associationwith acid volcanic rocks in-dicate that they were originally fine-grained tuff or volcanic ash.The present mineral composition of these deposits seems o berelated to the amount of metamorphism they have undergone.In the northern part of the Slate Belt where metamorphism hasbeen severe he fine-grainedmaterials doubtlesshave been changeddirectly to sericite schist, while farther south where dynamicforceswere ess ntense they have weathered to impure kaolin.

the mineralogy of some deposits of kaolinized volcanic ash from the slate belt of north carolina

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