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PARAGONITE PSEUDOMORPHS AFTER KYANITE from TURKEY HEAVEN MOUNTAIN, CLEBURNE COUNTY, ALABAMA1 Tnonnron L. Noarnnnu, Geological Su

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PARAGONITE PSEUDOMORPHS AFTER KYANITE from TURKEY HEAVEN MOUNTAIN, CLEBURNE COUNTY, ALABAMA1 Tnonnron L. Noarnnnu, Geological Su THE AMERICAN MINERALOGIST, VOL, 50, MAY_JIJ'}IE, 1965 PARAGONITE PSEUDOMORPHS AFTER KYANITE FROM TURKEY HEAVEN MOUNTAIN, CLEBURNE COUNTY, ALABAMA1 TnonNroN L. Noarnnnu, GeologicalSuraey of Alabama, Uniaersity, Alabama. ABSTRACT Paragonite pseudomorphs after kyanite have been found in the Turkey Heaven Moun- tain kyanite prospects. The pseudomorphs range in size from microscopic grains to mega- scopic prisms, which resemble andalusite crystals Optical, o-ray and petrographic studies indicate direct hysterogenic paragonite alteration of the kyanite. INrnolucrroN An investigationof kyanite prospectsin Sections22 and29,T.17 5., R. 11 E., at Turkey Heaven Mountain, Cleburne County, Alabama, discloseda unique mineral occurrence.The mineral assemblageincludes hysterogenicparagonite pseudomorphsafter kyanite, which resemble andalusite.The geneticphysicochemical parameters are consideredas a function of retrogrademetamorphism within a restrictive water environ- ment. GBor-ocrc SB:rrrNc The Turkey Heaven Mountain areais underlain by two metamorphic rock units: 1) the WedoweeFormation, a highly resistantgraphite-mica schist,and 2) a unit of the Ashland Mica Schist,an extensivelyweathered garnet-muscovite schist. Fresh rock exposuresindicate a retrograde metamorphic cycle from an almandine-amphibolite facies to a green- schist facies. At the crest of Turkey Heaven Mountain, the Wedowee Formation over-liesthe Ashland Mica Schist.The resistantgraphite-mica schist of the WedoweeFormation and associatedquartz veins contribute to the physiographic configuration of the area. The generalregional strike in the Alabama crystalline belt is about N. 45o E., and the dip is generally southeast.The crystalline area has undergoneprimary deformationfrom the stressesapplied generallyfrom the southeast,which producedisoclinal folding and thrust faulting to the northwest. Geologic studies at Turkey Heaven Mountain indicate a secondperiod of deformation in which the stressdirection can be ex- pressedaS a northeast shear couple that formed nonplanar, non-cylin- drical left lateral drag folds. Accompanying shear planes parallel the schistosity of the first deformation. 1 Publication authorized by the State Geologist, Geological Survey of Alabama' 718 P A RAGON I T]':. A FT T:,RKY A N I T'E Frc. 1. Photomicrograph showing paragonization of kyanite crystals along (010) cleavageplanes (X56). The kyanite zone follows the sigmoidaldrag fold configurationof the secondarydeformation, and is restrictedto the fracture and shearzones that subparallel the schistosity. The quartz-kyanite veins dip steeply to the southeastand transectthe schistosity.The veinsare prominent along the central and southernpart of the Mountain where they are localized in closeproximity to the Wedowee-Ashlandcontact. Crystalloblasts and thin blades of kyanite are oriented parallel to the schistosity.They are found on the west and southwestside of the Moun- tain in the Ashland Nlica Schist immediately under the Wedowee- Ashland contact. Away from the Wedowee-Ashlandcontact the sizeand regularity of the kyanite veins and abundance of crystalloblasts rapidly diminish. Retrogressive alteration of the kyanite to paragonite is most pro- nouncedwhere the greatestsecondary deformation is observed.Near the hinge of the horizontal drag fold, paragonization of the kyanite is nearly complete.At the crestof the fold, kyanite, paragonite,and quartz veins are not present.Toward the flanks of the fold paragonizationdecreases and the ratio of unaltered kyanite to paragonite increases. 72O THORNTONL. NEATIIDRY The paragonite pseudomorphsrange in size from microscopicscales suggestiveof sericite, to elongate,oblate prisms 8 to 10 inches long, chiefly elliptical in crosssection. The pseudomorphsresemble andalusite instead of the parent polymorph kyanite. The individual paragonite prisms are composedof massive,randomll.-oriented iaminar piates of scales. The pseudomorphs occur individuaily in a ground mass of granular to coarsely crystalline qvartz and as contiguous growths on kyanite aggregate.Other minerals identified include rutile, muscovite, and amorphous massesof pyrolusite. Graphite inclusionsand coatings wereobserved on pseuomorphsadjacent to the WedoweeFormation. ExpnnrrrBNrAI, PRocEDUREAND RBsur,rs The physical properties of the paragonite pseudomorphs,in hand specimens,are: blue-gray to greenish-grayinterior colorl vitreous to pearly Iuster with an exterior goldenor blue sheen;fine granular texture and sometimeslaminar plates or microscopicmica books; hardness2'5: and specificgravity 2.85+.05. Optical propertieswere determined on cry-stalgrains microscopically sorted and selectedfrom separatedmaterials. Sink-float was used to separatethe unaltered kyanite and occludedrutle grains from the mica and the mica from the quartz. The mica mineralswere lvashedand dried and carefullyhand picked under the microscopefor oil-immersionstudy. Optical data (sodiumlight) are asfollows: (-) 2V ca. 40o d 1.586 p 1.598 a 1.606 1-a .020 These data are in closeagreement with McCormick (1934)in his study of paragonite from the type locality, Pizzo Forno, Ticino, Switzerland. X-ray powderdiffraction determinations, by the U. S. Bureau of Mines, Tuscaloosa,Alabama, confirmed the paragonite. Diffraction patterns (2' /min) weremade on both orientedand randomly preparedslides using a GeneralElectric XRD-3 diffraction unit with CuKa radiation' Double diffraction peaks were resolved by slow scanning (.2"/min) into parago- nite and muscovite peaks. The diffraction values for the basal spacing of the two micas are indexedon the (002)rMlines. Values for the d(002)n{ Iines,based on the (004), (006),and (008)lines were calculatedand com- pared. X-ray diffraction values for material taken from different pseudo- morphsare shownin Table 1. Petrographic examination of a sequenceof thin sectionscut normal to the c-axis of the relict kyanite blades, indicates that the kyanite alters to paragonitewithout intermediatecompounds. The paragoniteforms with PARAGONITE AF'TER KVANITE 721 I'arlr 1. d(002)rM Dere oN CorxrsrnNr pen,q.coNrrn eun Muscovrrr OccunnrNc rN PsruoononpHs AT.TERKvaNrrr rnotr TunrBt: Hoevnn MouNrarN, Cr-rnunNB Couxry, Ar,,q.n.lu,q,1 SampleNo. Muscovite Paragonite A Difierence Remarks2 10 03 9 710 .JZ Oriented by sedimen- tation slow scan3 9.96 9 680 .28 Random packed slorv scan3 3 10.012 9 662 380 Random packed 4 10 014 9.688 .326 Random packerl 5 9.986 9.662 .324 Random packed 6 10.014 9.688 .326 Oriented by sedimen- tation 7 10 014 9.688 .326 Random packed 8 9.986 9.688 298 Oriented by sedimen- tation I Data concur favorably with Zen and Albee (1964), in their study of paragonite as a thermal indicator of greenschistfacies in pelitic schist. 'zAll dillraction patterns, except as noted, run at 2 degreesper minute. 3 Slow scan at .2of min. a spherulitic texture having a sericitic appearanceconfined to the relict kyanite form and designatedparagonizatioz by Killig (1913).Alteration of kyanite to paragonitestarts at one or both endsof the kyanite crystal and progressestoward the center of the crystal along cleavageplanes. The kyanite is microscopicallypoikiloblastic with anhedralrutile crys- tals.Little qtrartzoccurs in the kyanite or paragonitepseudomorphs. GBNnttc INronpnnrerror.r General physicochemicalconditions leading to the formation and decompositionof kyanite have beendiscussed by Roy and Osborn(1954), Kennedv (1955),Clarke et al. (1957),and Osborn (1957).The kyanite at Turkey Heaven Mountain is believedto have formed under two sets of physicochemicalconditions. The formation of kyanite under stressconcli- tions is indicated by the presenceof strained kyanite crystals oriented parallel to the folded schistosity surfacesof the Ashland Mica Schist. Deposition of kyanite from hydrothermal solutions is indicated by (1) largeporphyroblasts and irregular massesof kyanite occurringrandomly in the host rock, and (2) the occurrenceof kyanite in a prominent quartz- kyanite vein. The major sourceof the kyanite, therefore,is believed,to have beenfrom hydrothermal solutionsderived by regionalmetamorphic differentiation contemporaneouswith the waning phasesof retrograde metamorphism (Neathery, t964) . Of particular interest to the genesisof the Turkey Heaven Mountain 722 THORNTONL NI.:,ATI]ERY paragonite is the stabilit)' field of the k1'anite-paragoniteassociation' Equiiiblium relationshipsin the low soda part of the svstem Na2Q+ AIzOa.SiOz.HzOrvere investigated by Sand et al. (1957),over the tem- peraturerange 290o C. to 700'C. under pressuresup to 30'000psi with restricted water. The mineral assemblageat Turkel'Heaven Mountain closelv conforms to their three-cr1'stalline-phaseassemblage 580o C. to 625" C. at 15,000psi in which qvdrtz and paragoniteexist with k,r'anite available as a transitional phase. Subsequentto the major kyanite emplacement,structural deformation altered the parametersof the existingphysicochemical s1'stem in which kyanite and qtartz were original stable phases.Shearing stresses result- ing from local rotational deformation produced the necessarygrinding and temperature rise to convert the k.v-anite(Kenned,v, 1955)' in the presenceof minor anounts of sodic plagioclase,to the sodium mica, paragonite.Since the stabiiity field of the paragoniteis adjacent to the kyanite polymorph andalusite, hysterogenicalteration of the kyanite produceda paragonitepseudomorph resembling the lower pressure,lower temperature alumino-silicatepoil'me1ph-andalusite' Zen (1960), and Francis (1956)also suggestthat paragoniteinstead
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