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THE HYDROUS COMPONENT of SILLIMANITE 8R3

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THE HYDROUS COMPONENT of SILLIMANITE 8R3 American Mineralogist, Volume 74, pages812-817, 1989 The hydrous componentof sillimanite ANroN Brn-LNo* Gnoncn R. RossvreN Division of Geological and Planetary Sciences,California Institute of Technology,Pasadena, California 91125, U.S.A. Eow.q.nnS. Gnnw Department of Geological Sciences,University of Maine, Orono, Maine 04469, U.S.A. Ansrnlcr Polarized infrared spectra of a suite of sillimanite samplesfrom high-grade regionally metamorphosedrocks and associatedquartz veins and pegmatites(upper-amphibolite to pyroxene granulite facies), from xenoliths in basaltic rocks, and from alluvial deposits indicate that hydroxyl is the dominant hydrous speciesbound in sillimanite. Absorption bands at 3556, 3329, 3300, and 3248 cm-L are characteristic of many of the samples. Heating experiments indicate that only above 700'C is weight loss primarily due to loss 'C. of structurally bound OH. Complete dehydration required heating to 1400 The max- imum content of bound OH in the sillimanites was 0.02 wto/oHrO equivalent. The inten- sities of the OH features generally decreasewith increasing temperatures estimated for metamorphism, consistentwith the expectationthat water activities decreasewith increas- ing temperature. However, notable exceptionsto this trend suggestthat secondaryhydra- tion at the unit-cell scaleis also a viable explanation for OH incorporation in sillimanite. INrnooucrrol,l ExpnnrvrnNTAl DETAILS Trace amounts of water have been reported in a num- The sillimanites listed in Table I originate from high- ber of nominally anhydrous minerals, including the grade regionally metamorphosed rocks and associated AlrSiO, polymorphs. Sillimanite analysesindicate up to quartz veins and pegmatites, from xenoliths in basaltic 0.72wto/oHrO (Aramakiand Roy, 1963;Deer etal., 1982', rocks, and from alluvial deposits (Rossmanet al., 1982 Beranet al., I 983). Hydroxyl absorptionbands have been Beran et al.. 1983rGrew and Rossman,1985). The re- observedin the infrared spectraof the three AlrSiO' min- gional metamorphic sillimanites formed under condi- erals(Beran ar'd Tnmann, I 969; Wilkins and Sabine,I 973; tions of the upper-amphibolite facies (sample numbers Beranet a1.,1983; Beran and G6tzinger,1987). However, 6(?), 7, 8, 9, 10, I l, 13, and 27), hornblendegranulite the mechanism by which OH is accommodated in the facies(1, 2,4, and29) and pyroxenegranulite facies (12, AlrSiO5 structure is not well understood, and the extent 25, and 30; Grew, 1980, 1982;Lal et al., 1987).The ofOH incorporationis unknown.Beran et al. (1983)pro- conditions for sample 27 were deducedfrom Thompson posedthat the hydroxide incorporation in sillimanite was (1979)and Crawford and Mark (1982,Fig. 2). Sample6, a charge-compensatingresponse to an Al deficiency on which H[lenius (1979)reported to be from "Brevig, Nor- the A(2) site, whereasHllenius (1979) suggestedthat Fe'z* way" (an old spelling of Brevik, H. Annersten, personal + OH- substitution for A13* * O':- could be a mecha- communication, 1988) is tentatively included with the nism. Incorporation of even small quantities of water by amphibolite-facies group. The sample was most likely either mechanismcould have substantialeffects of AlrSiO' collected in the Kongsberg-Bamble atea, a dominantly relations analogousto the shifts in the andalusite-silli- amphibolite-facies complex where metamorphic grade manite transition that Kerrick and Speer (1988) attrib- locally reachesthe granulite facies(e.g., Barth and Reitan, uted to minor amounts of Fe3*. 1963; Touret, l97 l). Annersten (personalcommunica- The objectives of the present study were to determine tion, 1988) has reported that quartz and cordierite, but whether hydroxyl is commonly incorporated in silliman- not K-feldspar, accompaniesthe sillimanite (cf. H6lenius, ite from a variety of parageneses,to determine whether 1979). Sample 30 is included with the pyroxene granu- the OH content reflects the environment of formation, lite-facies rocks becausethis sillimanite-bearing rock is and to assessthe possible effect on AlrSiOs phase rela- associatedwith sapphirine-quartz rocks. trons. Metamorphic temperaturesfor the Antarctic pyroxene granulite facies(Napier Complex)are estimatedto be 800- 900 "C (sample12) and at least900 "C (sample25) with Pn,o11 P,.o,(e.g., Grew, 1980, 1981a;Sheraton et a1., * Permanent address:Institut fiir Mineralogie und Kristallo- graphie, Universitlit Wien, Dr. Karl Lueger-Ring 1, .4-1010 Vi- 1987) and for the Indian (Paderu) terrane, 800-900 "C enna, Austria. (sample30) (Grew, 1982;Lal et al., 1987).Temperatures 0003-004x/89/0708-o8l 2$02.00 812 BERAN ET AL.: THE HYDROUS COMPONENT OF SILLIMANITE 8r3 TABLE1. Sillimaniteinfrared absorbance in the OH reoton Band position(cm r) No.. Sample Locality Originl I GRR273 ReinboltHills, Antarctica HG 0.127 0.037 0.060 0.184 0.000 2 GRR439 MolodezhnayaStation, Antarctica HG 0 096 0.105 0.105 0.166 0.000 J GRR 385 KilbourneHole, New Mexico X 0.096 0.000 0.000 0.000 0.000 4 GRR 380 BensonMines, New York HG 0.140 0.000 0.089 0.242 0.076 A+ GRR 1585 Brevik, Norway UA 0.056 0.000 0.000 0.000 0 000 7 GRR 384 Willimantic,Connecticut UA 0.000 0.000 0.000 0.000 0.000 8 GRR547 Vernon, Connecticut UA 0 175 0.000 0.197 0.263 0.110 9 GRR383 Guilford(?), Connecticut UA 0.112 0.000 0.117 0.166 0.057 10 GRR 450 OconeeCounty, South Carolina UA 0.172 0.000 0 215 0.215 0.189 1'l GRR 382 Sardinia(?) ? 0.292 0.097 0.248 0.529 0.097 12 GRR608 "ChristmasPoint, " Antarctica DA 0.138 0.000 0.000 0.055 0.000 la GRR386 Norwich, Connecticut UA 0.216 0.000 0.216 0 281 0 195 14 GRR31 1 KilbourneHole, New Mexico X 0.000 0.000 0.000 0 000 0.000 17 GRR 509 Mogok, Burma 0 275 0.245 0.200 0.300 0.120 19 GRR 306 Bournac,France X 0 161 0.000 o.117 0.073 0.000 25 GRR 560 Beaverlsland, Antarctica PG 0.000 0.000 0.000 0.000 0.000 zo GRR 1485 Sri Lanka AG 0.053 0.000 0.000 0.032 0.000 27 AB-Gho BrandywineSprings, Delaware UA 0.249 0 111 0.276 0.406 0.087 28 AB-Sml Sri Lanka 0.248 0.552 0.505 0.543 0.000 4J GRR 799 Ellammankovilpatti,India nu 0.283 0.000 o.264 0377 0.000 30$ GRR 802 Paderu,India PG 0.0 0.0 0.0 0.0 0.0 Note.'Allabsorbance values (in absorbanceper millimeter) for * are the Ella orientation. Numbers1-19 are the samplenumbers of Rossmanet al. (1982),which presentsa morecomplete description of thesesamples. Sample 25 is a fragment of a pale-browncrystal with many inclusions.Sample 26 is a cleavageplate from pale-btue,2-cm gem-qualitycrystal. Sample27 is a fragment of a brown crystalwith few fibrousinclusions. lt is part of the sample(Univeisity of Chicagono.2261) used by Winierind Ghose(1979). Samale 28 is a pale-browninclusion-free cleavage plate of a gem-qualitycrystal. Sample 29 is no. 3083D of Grew and Rossman(1985). Sample 30 is no. E2724 of Grew and Rossman(1985). t Origins:AG : alluvialdeposit presumably from granulitefacies; HG : hornblendegranulite facies; PG : pyroxenegranulite facies; UA : upper- amphibolitegrade: X - xenolithic. - + The spectrum of sample 6 in the OH region is so dominatedby includedphases that the tabulatedabsorbance values are rough estimates. Sample 6 is tentatively includedin the upper-amphibolitefacies (see text). $ The detectionlimit for sample30, 0.08 per millimetei,is higherthan for the othersbecause of the thinnessof the sample. of formation of the hornblende granulite-faciessilliman- nace and initially dried for 15 h at ll0 "C. It was then .C ite range from 700 to 800 (p-T data for samples2, 4, removed from the hot zone and immediately allowed to and 29 are given in Grew, 1981b;Bohlen et al., 1985; cool in air to room temperature.After it was weighedand and Grew et al., 1987,respectively) typical of most gran- its spectrum taken, the processwas repeatedat a series ulite-faciesterranes (Bohlen, 1987). Temperatures for the of specifiedtemperatures (250, 500, 750, and 900 'C). amphibolite-faciessillimanite probably did not exceed700 The sample was held at temperature for 1.5 h in each "C, an upper limit of metamorphic temperaturesfor the step. An additional heating step at 900 'C for 15 h did amphibolite facies (e.g.,Bohlen, 1987). The xenolithic not changeeither the weight or the spectrum. A second sillimanites(3, 14, and 19) are interpretedto have orig- fragmentheated at I l0'C (15 h), 700'C (1.5h), 750 "C inated in granulite-faciesrocks at depth and to have been (3 hr), and 900 'C (l 5 h) produced nearly identical results. subsequentlyincorporated in basalt and transported to the Earth's surface by explosive volcanic activity. During Rnsur,rs this transport the xenoliths were subjected to tempera- turesof near 1000'C (e.g.,Kilbourne Hole, Grew, 1979), Spectroscopicfeatures partial melting, and quenching. Provenanceof the allu- A variety of OH-related featuresappear in the region vial sillimanites (samples17,26, and 28) is unknown; from 4000 cm-r to 3000 cm ' in all three polarizations most probably the sillimanite is derived from the gran- of the spectraof sillimanite (e.g.,Fig. l). Although there ulite-facies rocks exposed in the vicinity of the alluvial is much variation among samples involving the minor deposits. features,there are four bands prominent in the Ella spec- Infrared spectrawere obtained on a Nicolet 60SX Fou- trum of many of the samples.They are at 3556, 3329, rier-transform infrared spectrophotometeroperating at 2 3300, and 3248 cm-t (Fig. 2). Additionally, there are weak cm-' resolution. Samples were prepared as doubly pol- bandsat 3205 and 3163 cm 'in the Ella spectraof many ished(010) cleavage plates, 0.5 to 6 mm thick; they were samples.There is alsoa band at 3546cm-' polarizedE llb.
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