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The Colors of Sillimanite

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The Colors of Sillimanite AmericanMineralogist, Volume 67, pages749-761 ' 1982 The colors of sillimanite GeoncB R. RossunN Division of Geological and Planetary Sciences California Institute of Technology Pasadena, C alifurnia 9I 1251 Eownnn S. Gnew nuo W. A. Dot-use Department of Earth and Space Sciences U niv e rsity of C alifurnia Los Angeles, California 90024 Abstract Sillimanite occurs in three colored varieties: yellow, brown, and blue. The yellow color is characteristic of unaltered single crystals of sillimanite from high-grade metamorphic rocks and associatedpegmatites. Such sillimanitescontain up to 1.8 wt.VoFe2O3and up to 0.3/o Cr2O3.The yellow color is due to Fe3+,or in a few cases'to Cr3*. The ion dominant in the optical spectrum of sillimanite containing negligible Cr3* is Fe3* in tetrahedral coordination. However, Mdssbauerdata suggestthat about 80%of the iron is in octahedral coordination and only 2O%in tetrahedral coordination. The salient features in the optical spectrum from Fe3* are absorption bands at 462, 44O,and 412nm in a, 616,474, and 438 Cr3* nm in 7, and 361nm in c and 7. No evidencefor Fe2+was found. Absorptionfrom occurs near 620 and 423 nn. The brown variety is also formed in high-grademetamorphic rocks and associated pegmatites; some brown sillimanite is chatoyant from abundant acicular inclusions oriented parallel to c. Brown sillimanite generally contains I or more wt.Vo Fe2O3.Optical absorption spectra of brown sillimanite include some of the iron -452 features characteristic of yellow sillimanite and prominent bands at nm and 542 nm. These last two bands may be due to incipient exsolution of an iron-rich phase' which appears to constitute the inclusions. Blue sillimanite has been documented from crustal xenoliths in basalt at two localities and from alluvial deposits at two other localities.'Blue sillimanite contains no more than I wt.%oFezOr. The prominent absorption features, -836 restrictedto 7, consistof two bandsat 595and nm. The spectraare similarto thoseof blue kyanite. As with kyanite, it is likely that the coloration is the result of intervalence chargetransfer although the intensity ofthe absorption bands is not correlated with either Fe or Ti content. Introduction ously communicated the results of a study of the optical absorption of a blue and a yellow sillimanite Sillimanite, AlzSiOs, occurs in three principal and observed that the absorption spectra of these colored varieties-yellow, brown, and blue. The varieties are fundamentally different. They suggest- yellow and brown varieties are widespreadwhereas ed that the yellow color is largely due to Fe3* in the blue variety is a mineralogicalcuriosity that has tetrahedralcoordination' On the other hand, Hilen- been well-documented from only four localities ius (1979) reported an absorption spectrum of a worldwide. Iron, chromium and titanium are the yellow sillimanite and concluded that iron in the six- most abundantminor elementsin sillimanite and the coordinate site was responsiblefor the color. most likely to be responsible for the color. Grew In a study of the electron paramagneticresonance and Rossman(1976a, b) and Grew (1976)had previ- of OconeeCounty, South Carolina, sillimanite,Le Marshall et al. (197t) observed signalsfrom two 'ContributionNo. 3655 crystallographically inequivalent Fe3* ions and 0003-004)v82l070E-0749$02.00 749 750 ROSSMAN ET AL.: COLORS OF SILLIMANITE basedthe interpretationof their dataon the assump- Table L Chemical composition and localities of yellow-green tion that Fe3+ occurred in both the six-coordinate sillimanite and four-coordinatesites. The present study, an Sdople 12345 6 extensionof our earlierwork, attemptsto provide a A1203 - 62.21 6L.46 6r.74 60.E4 systematicexamination of a number of sillimanites 62.17 5102 - 36.19 - which span the range of color variation. It is the 36.77 36.52 37.19 purpose I{ax 1.33 0.85 1,87 L,97, 0.59 of this study to examinethe origin of these Fe203 Ave 1.28 0,8s r.78 t,79 0.62 t.4l colors, and to relate the colors to the petrologic Mln 1.22 0.84 1.63 7.70 0.56 environmentin which sillimaniteforms. Tt02 0.01 0.02 0.03 0,02 0.02 0.02 CrZ03 0.003** 0.21* 0.08 0.00 0.1st 0.00 Nunber of Methods potnts (pEobe analysee)104893 Samplesof sillimanitefor this study were select- ed from (1) granulite-faciesrocks collectedin Ant- (2) Retnbolt gil16, Antarctlca (70'2E'S, 72"27,Er. Crew #556 and NltNH arctica, crustal xenoliths collectedat Kilbourne 1,37011. Froo peg@ttte ln granullte-facles rocks. Hole, Molodezhdaya Statlon, Antarctica (67'40rS, 45'50'E), Gtew ll2748, New Mexico, U.S.A. and Bournac,Haute- Fron garnet-blotlte quertzofeldepathlc gnelss. Analysls: crew Loire, France, (3) (1980). and from specimensobtained Kllbourne Hole, New t4extco, U.S.A. Gtew #76-5-I. FroE crustel from museumand private (Tables xenollth ln core of beselElc boob. AneLysls: Grew (19E0). collections l-3). Benson Mines, New York, U.S.A. NMN}{115586, Froil nagnetite- Compositionsof sillimanite were determined quattz-sLIIlMnlte lron-ore. Anely616: Grew (1980). on Foreflnger Polnt, Antarctice (67'37'S,48'04'E). crew #2113, Fron carbon coated thin sections coerae - gtained s1lllEnl.te-orthopyroxene-cordlertte-blotite 1 and isolated grains at sapphirlne rock. the electron microprobe facilities at UCLA and Brevlg, Nomay. Analyalst Halentus (1979). Caltech. The average, maximum, and minimum Table entriea ln welght percenL. valuesfor Fe2O3for (Tables Analyaeg ete electron microprobe, excepE: each sample I and 3) * Energy dlsperslve analysls on scannlng electron dcroscope (ED{) ** are basedon analysesat four to six points on three Edlsslon specrrotraphtc adalysls (ESA). Also by ESA on nuuber l: 0.0062 Be and 0.0052 V. to six grainsin each thin sectionof the sillimanite- T X-ray fluorescence anelysls: 0.012 V; ltr absent, Abbrevietlons: NMNH: Natlonal Museun of Netural llistory, bearingrock (sample 15, Table 3, consistsof two Soith6onlan Ine!itution. grainsbroken from one piece).The TiO2contents of samples14 and 19 are basedon an analysisat one point per sample.The tabulatedanalyses are aver- Sample1 is part of a cm-sizedsingle crystal from a agesofboth referencedliterature analyses and new subconcordantgarnetiferous pegmatite lens in gar- data. Optical spectrawere obtainedusing methods net biotite quartzofeldspathicgneiss. Fe2O3con- describedby Rossman(1975). e-Values are in units tents of the pegmatiticsillimanite from this locality of liters per mole per cm. The intensitiespresented vary from crystal to crystal (0.76 to 1.30 wt.% in Figure 2 were obtained by constructing a line Fe2O3,Grew, 1980)and the Fe2O3contents in Table tangentto local minima on the sidesof the absorp- I were obtained on the piece used in the optical tion band. There are consequently uncertainties absorptionwoqk. associatedwith the valuesobtained for bandswhich Sample 5 is from granulite-faciesrocks of the were shoulderson steeplyrising baselines,such as Archean Napier complex at Forefinger Point in the the bandsat 390nm and 361nm in a and at 361nm westernpart of CaseyBay, Enderby Land, Antarc- in 7. Mossbauerspectra were obtainedon about210 tica (Grew and Manton, 1979; c/. sample 12 of mg of acid-washedhand-picked grains. The sample brown sillimanite,below). It was collectedfrom a 57Fe densitieswere about0.012 mg per cm2.Isomer layer 1.5 m thick of aluminousgranulite containing shifts are reported relative to iron in pd. orthopyroxene,sillimanite, cordierite, biotite, and sapphirine;other rock types present at Forefinger Yellow sillimanite Point are pyroxene-hornblende granulite, garnet- quartz-feldspargneiss, and charnockiticgneiss. Sil- Mode of occurrence and composition limanitein sample6 from Brevig, Norway, is asso- We have analyzed samples of yellow sillimanite ciatedwith quartz, K-feldspar,biotite, hematiteand from six localities(Table l). Samplesl, 2, and,4 are magnetite(Hilenius, 1979).Samples 1,2, 4,5,and6 from granulite-facies rocks in Antarctica and the arerepresentative of prismatic,Fe2O3-bearing, silli- Adirondack Mountains, New York (Grew, 1980, manite that is found in many high-grade metamor- 1981a),The sillimanitein samplesI and 2 is assoc! phic terrains. ated with ilmenite, and that in 4, with magnetite. Sample 3 is from a sillimanite-quartz-K-feld- ROSSMAN ET AL.: COLORS OF SILLIMANITE spar-ilmenite-magnetite gneiss xenolith containing depth of color and the intensitiesof all other absorp- abundantglass, which was found amongthe ejecta tion bands,except the 616nm 7 band (the dominant at Kilbourne Hole maar, New Mexico (Grew, 1979, causeofabsorption in the 620nm region),generally 1980).Xenoliths of quartzofeldspathicgneiss such increasewith iron content (Fig. 2). Consequently, as sample3 and the samplescontaining blue silli- we attributeall the absorptionbands except 616 nm manite(see below) may be derivedfrom a granulite- to iron. The absorptionof sample3 is anomalously faciescomplex in the lower third of the earth'scrust low at most wavelengths.We doubt if this is due to under Kilbourne Hole (Padovani and Carter, compositionalinhomogeneity because it was ob- 1977a).During their incorporation in the basalt host servedin multiple optical and chemicalanalyses of and transport to the earth's surface, the xenoliths fragmentsof sample3. -616 have been partially fused either by heating or de- The intensity of the nm 7 band varies compression,and subsequentlyquenched (Pado- among the different samples. The band is less vani and Carter, 1977b;Grew, 1979). intenserelative to the other features in the spectrum of samplea Gig. 3). Although its
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