American Mineralogist, Volume 74, pages812-817, 1989

The hydrous componentof

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 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 , 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 -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-, 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 tabulated absorbancevalues 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. examined through clear areas,as free from inclusions as The absoluteintensities of thesefeatures vary from sam- possible,which varied from 15 x 200 pm to 600 pm in ple to sample.The featureat about 2663 cm-' is most diameter. Heating experiments were conducted in air. A intensein the Ellc spectrum,although this direction gen- fragment of sample 28 was put in a preheatedtube fur- erally could not be measuredwith the (010) cleavageslabs 8r4 BERANET AL.: THE HYDROUSCOMPONENT OF SILLIMANITE

Sillimanite Sillimanite Reinbolt Hills Boumac qFr r U Ll U [J z (E nr tD': (n ''. (E {f D o a a ID (D ti GN

cl o q edoo sgoo 3ooo Zsoo { |,.IAVENUMBER hCVENUMBER Fig. 3. Infrared spectrum of sillimanite (sample 19) with the Fig. I . Infraredspectrum of sillimaniteI plottedfor I .O-mm secondclass (type II) of spectrum,Plotted for 1.O-mmthickness; thicknessin threepolarizations. Ella. that were used for most of the spectroscopicmeasure- ments. For purposesof classification,many of the spectracan Heating experiments be divided into two groups on the basis of the relative Two fragments (16.4 and 5.2 mg) of sample 28 were intensity of the bands in the E lla polarization. In the first subjectedto a stepwiseheating experiment in which the group (type I), there are three prominent bands at 3329, infrared spectrumwas obtained after eachstep. This sam- 3300 and 3248 cm-'(Fig.2, top). Although the relative ple was chosen becauseit is free of inclusions, homoge- intensities of the three component bandsare variable, one neous in OH content, and large enough for multiple ex- of the bands of this triplet is always more intense than periments. The fragments were heated at temperatures the band at 3556 cm-r. In the secondgroup (type II), the from ll0 to 900'C. The total weight loss in each case band at 3556 cm-' is the most intense, and the triplet was about 0.070/o(Table 2). Although there was weight bands are weak or absent (Fig. 3). A few spectra do not loss at each step, the OH triplet bands decreaseonly fall into either group; the spectraof samples7 , 14 and 25 slightly below 500 'C, indicating that the initial weight include none ofthese bands,although OH featuresappear loss below 500'C was not due to the structurally bound at other wavelengths. OH units. Above 700 "C, the decreasein the infrared intensity aI 3329 cm-' and 3248 cm-'correlates with the weight loss. By 900 "C, the OH triplet disappears(Fig. 4). 3244 However, between ll0 and 500 "C, there is a greater Sillimanite weight loss with a much smaller loss of infrared intensity (Fig. on both fragments of \ Ella 5). The heating experiments sample 28 produced nearly quantitatively identical re- U U sults. z oC, (tm Even after heating to I l0 there is a broad infrared m-: tf absorption centerednear 3400 cm-L that is lost at higher o a temperatures. Liquid water absorbs in this spectral re- ID G. gion. After the 900'C treatment, the single OH absorp- 5 tion at 3556 cm-'retains about half of its original inten- sity, and the spectrum resembles that of the type II samples(e.g., samples 3, 12, 19 and 26). Between700 q and 750 oC,a weak band at 3163 cm-' growsnoticeably band f Gig. a). This behavior is reminiscent of the OH HAVENUMBEB that grows at high temperature in the spectra of topaz owing to site re-equilibration (Aines and Rossman, 1985). Fig. 2. The infrared spectra of three sillimanites that show after heating at 1037 the characteristicgroup of OH bands in the Ella polarization. Sample I is not fully dehydrated Top: sample 28. Center: sample 17. Bottom: sample ll. A1l "C for 45 h,judging from the presenceofa weak, residual spectraobtained from (010) platesand plotted for 1.0-mm thick- absorption at 3556 cm-'. All OH features were absent ness. from its spectrum after it was heated to 1400 "C. BERANET AL.: THE HYDROUSCOMPONENT OF SILLIMANITE 815

100 Sillimanite

o80 a o J I -60 o U weight U o z F Gnr *40 0H band tD: o tr E )€ a 20 m a_ 400 600 800 1000 Tempenatune(.c) Fig. 5. Stepwiseheating equipment (sample28) showrngcu- mulative weight loss and cumulative lossof intensity of the 3329 NRVENUMBEF cm-' OH band. Much weight lossoccurred at lower temperatures before the OH bands beganto decrease. Fig.4. Infraredspectra of sillimanite(sample 28) aftera se- riesof stepwiseheating steps showing the lossof OH groupsat elevatedtemperature. Unpolarized spectra presented for a 1.0- the remaining weight loss occurs from molecular water mm-thick(010) plate. The spectrawere taken after the sample on the edges of the crystals, probably associated with sur- washeated to the indicatedtemperature and returnedto room face damage and incipient cleavage. temperature. DrscussroN The major OH absorption bands in the sillimanite Absolute OH content spectra do not correspond to any of the hydrous phases The weight loss that accompaniesthe heating stepsbe- commonly associatedwith sillimanite. Specifically, OH tween 700 and 900 "C was used to calibrate the silliman- groups in amphiboles, boehmite, chlorites, diaspore, ite infrared spectmm under the assumption that the high- gibbsite, , micas, and pyrophyllite do not absorb temperatureweight loss was solely due to the dehydration in the region of the prominent sillimanite bands. Inten- ofbound OH ions. The total weight loss between700 and sities of these absorption features do not correlate with 900'C (0.012 wt0/o)was associatedwith the absorbance the abundanceof microscopic inclusions that are present changein the 3329 cm-' band over the sametemperature in several of the samples.We conclude that many of the range (0.160 absorbanceunits per millimeter of thick- hydroxyl absorptionfeatures, notably thoseat 3556,3329, ness).For the purposesof this calibration, the intensity 3300, 3248, and 3205 cm-r, are characteristic of silli- of the band at 3329 cm-r was used becauseit is the most manite itself and are not due to either hydrous phases prominent band in most sillimanite spectra. Further- included in the sillimanite or hydrous alteration prod- more, its intensity follows the intensities of the other ucts. componentsof the triplet of bands that representthe ma- The OH features appear unrelated to the various col- jority of OH intensity in the sillimanite spectrum. ored varieties of sillimanite describedby Rossman et al. The total mass of water associatedwith the decreasein (1982). In particular, they are not correlatedwith Fe con- the 3329 cm-' band between ll0 and 900 "C was then tent; both Fe-rich and Fe-poor varieties show OH ab- determined from this calibration. This calculation indi- sorption features.As regards B, our data aro limited to cates that only 0.020/oOH (expressedas HrO) is associ- threesamples (1,29, and 30;Grew and Rossman,1985). ated with the triplet of bands (Table 2). Although this The spectrum for the sample with the most B (sample calibration may underestimate the total OH concentra- tion becauseit doesnot account for changesin the minor OH bands, it representsthe majority of the OH intensity Tlele 2. Thedetermination of theOH content of two fragments in the sillimanite spectrum. of sillimanitesample 28 Our value of 0.02o/oHrO in sample 28 appearsto be Mass (mg) the maximum for any sillimanite analyzedto date. We 16.368 5.219 doubt that the much higher OH contents previously re- ported representbound OH in sillimanite. Stepwise heating trom 1 10 to 900 C More likely, Weight loss 0.073% these contents include adsorbed water that contributes Absorbance loss 0.210mm-1 most of the loss on ignition, as shown by our heating Stepwi3e heating lrom 700 to 900 t experiments.There is evidencefor some water in the IR Weight loss 0.016% ' spectrum of sample 28, possibly as microscopic fluid in- Absorbance loss 0.160mm clusions, but it is enough to account for only 100/oof the Note-'TotalOH loss (expressedas HrO)from 3329 cm-' band: (0.210 + x :0.021%. low-temperature weight loss. We propose that most of 0.160) 0.016% 816 BERAN ET AL.: THE HYDROUS COMPONENT OF SILLIMANITE

30,0.4o/oBrO.) showsno OH absorptionfeatures at all. notable exceptions to the observed trends, such as the We thus feel justified in ruling out the substitution SiO'* amphibolite-faciessillimanite that gives a spectrum lack- : BOH2+as a mechanism for hydroxyl incorporation in ing absorptionfeatures in the 4000 to 2500 cm-r range sillimanite. This conclusion is re-enforced by Grew and (no. 7) and a relatively hydrated xenolithic sillimanite Hinthorne's (1983) studies, which indicate that rocks (no. l9). In this alternative interpretation, the OH ab- containing B-bearing sillimanite form at high tempera- sorption featuresare explained by incipient alteration at tures under low water activities. Thus, we must consider the unit-cell scale. For example, water could enter the that hydroxyl is incorporated in the sillimanite structure and be incorporated as submicroscopiclamellae either through a substitution involving Al or Si or by of hydrated aluminosilicate along discrete planes in the hydrous defects at the unit-cell scale. Other characteris- sillimanite structure, a situation resembling the occur- tics of the spectral data further support this interpreta- rence of humite layers in olivine (Kitamura et al., 1987) tion. and the alteration of pyroxene to amphibole in submi- The energiesof severalof the OH bands(<3350 cm-') croscopiclamellae (Veblen and Buseck, l98l). In con- are lower than the energiescommonly observedin trast to the amphibole lamellae in pyroxene,the lamellae minerals that have OH as a part of their normal stoichi- of hydrated aluminosilicate would be so small that OH ometry. The correlation of Nakamoto et al. (1955) be- vibration is controlled by the host sillimanite. The re- tween the wavenumber of the OH band and the strength sulting vibrational spectrum would thus be characteristic of the hydrogenbond as measuredby the O-H' ' 'O dis- of sillimanite and unlike known hydrous aluminosilicates tance indicates that the O-O distanceis lessthan 2.82 A', such as pyrophyllite or kaolinite. Incorporation of hy- comparableto the O-O distanceswithin the Al octahedra droxyl by formation of such a hydrous aluminosilicate and tetrahedra in the sillimanite structure (Burnham, would depend on a variety of factors, for example, infil- 1963).Furthermore, the sharpnessofthe bandssuggests tration of water-rich fluids after crystallization, fracture that OH rather than HrO is the vibrating species. development during deformation, or retrogressionof the Ifhydroxyl were incorporated in the structure through high-temperatureanhydrous mineral assemblagesto low- the substitution 3(OH) + ! : Al3+ + 3o'?- proposedby er-temperatureassemblages with hydrous phases. Beran et al. (1983), we would expect sillimanite water Secondary hydration would be expected to produce contents to correlate directly with estimated water activ- variable sillimanite water contents. For example, silli- ities during sillimanite formation. In line with studies of manite nos. 12 (hydrous) and 25 (anhydrous)both crys- regional metamorphism (e.9.,Lamb and Valley, 1988), tallized during an Archean pyroxene granulite-facies we presumethat the water activities during crystallization event, but only sillimanite no. 12 was subjectedto tec- would have been greatestfor the amphibolite-facies sil- tonic reactivation and retrogressionin the amphibolite limanites and least for the pyroxene granulite-facies sil- facies;sample 25 was not affected(Rossman et al., 1982; limanites. Moreover, low contents are predicted for the Sheratonet al.. 1987).Water could have been incorpo- xenolithic sillimanites, which were subjectedflrst to gran- rated into sillimanite no. 12 when associatedhigh-tem- ulite-facies metamorphism and then heated to high tem- perature minerals (e.g.,surinamite) were replacedby cor- peraturesin a dry environment. dierite and biotite formed either during retrograde In general,the observed intensities ofthe OH absorp- metamorphism or during a late, water-rich stageof peg- tion bands fit these trends. For example, the averagein- matite crystallization (Grew, I 9 8 I a). Relatively high water tensitiesof the most intensebands, 3556 cm-' and 3248 activities along the retrogradeP-T palh (e.g.,no. 29, Grew cm r, decreasein the following sequence:amphibolite fa- et al., 1987)or polymetamorphismmay accountfor the cies (0.174,0.266, respectively,from Table 1, excluding OH absorption features in many of the other samples. sample 6), hornblende granulite facies (0.162, 0.242), Wetter conditions at the time of crystallization need not xenolith (0.086, 0.024), and pyroxene granulite facies be invoked as the cause.Moreover, the relatively strong (0.046,0.018). The differencesbetween successive aver- absorptionof alluvial samplescould be attributed to water agesin the sequenceare in most casesless than one stan- incorporated during residencein the alluvial deposit rath- dard deviation and are thus, strictly speaking,not statis- er than during original crystallization. tically significant. Nevertheless, the intensities of OH Clarification of the structural details of water incor- absorptionfeatures for sillimanitesformed under the driest poration in sillimanite has important implications for conditions (pyroxene granulite facies, xenoliths) are sig- metamorphic petrology. If water is incorporated in silli- nificantly less than those for sillimanite formed under manite and other AlrSiO5 phases during crystallization wetter conditions. Moreover, the decreasein intensity is through coupled substitutions involving Al and Si, then more marked for the 3248 cm I band, becausethe spectra AlrSiO, phaserelations could be significantly affectedby of the sillimanites from the pyroxene granulite faciesand fractionation of HrO among the phases,an effect analo- xenoliths are type II or lack absorption features in the gous to that ofFe3* fractionation betweenandalusite and 2500 to 4000 cm ' range.By contrast,most of the silli- sillimanite (Kerrick and Speer, 1988). However, if water manitesformed under lower-temperatureconditions have is incorporated in the AlrSiO5 minerals during postmeta- type I spectra. morphic processes,the anhydrous compositions would Alternatively, secondary hydration could explain the be appropriate for applying theoretical AlrSiOs relations BERAN ET AL.: THE HYDROUS COMPONENT OF SILLIMANITE 8r7 to experimental results and to rocks, at least as far as the -(1980) Sapphirine + quartz association from Archean rocks in role of water is concerned. Detection of thin, OH-rich Enderby Land, Antarctica. American Mineralogist, 65, 821-836. -(l98la) taaffeite,and beryllian sapphirine from peg- phases Surinamite, lamellae in sillimanite and the other AlrSiO, re- matites in granulite-faciesrocks in CaseyBay, Enderby Land, Antarc- quires careful high-resolution transmission-electron-mi- tica. American Mineralogist, 66, 1022-1033. croscopystudies that are a logical extensionofthe present -(1981b) Granulite-facies metamorphism at Molodezhnaya Sta- study. tion, East Antarctica. Joumal of Petrology,22, 297-336' -(1982) Sapphirine,kornerupine, and sillimanite + orthopyroxene AcrNowr,nocMENTS in the charnockitic region of South India. Journal of the Geological Societyof lndia, 23, 469-505. We acknowledgethe following peopleand institutions for their generos- Grew.E.S., and Hinthome,J.R. (1983)Boron in sillimanite'Science,22l, ity in providing samplesfor this study: C. Frondel, Harvard University 547-549 (samples9, 13);John S. White, Jr., U.S. National Museum (4,7, 8, 10, Grew. E.S.. and Rossman, G R. (1985) Co-ordination of boron in silli- 11);G. E Harlow, American Museum of Natural History (17); J. Fabrids, manite. MineralogrcalMagazine, 49, 132-135. Mus6um Nationale d'Histoire Naturelle Min6ralogie(19); R. L Gait, Roy- Grew, E.S.,Abraham, K., and Medenbach,O. (1987)Ti-poor hoegbomite al Ontano Museum (30); U. HAlenius (6); E. R. Padovani (14); and S. in kornerupine-cordierite-sillimaniterocks from Ellammankovilpatti, Ghose,University ofWashington (27). Portions ofthis study were funded Tamil Nadu, India. Contributions to Mineralogy and Petrology, 95, by the National ScienceFoundation (U.S.A.) grants EAR-86-18200 and z t-5 t. DPP-8613241 and the Austrian Fonds zur Forderung der wissenschaft- Hf,lenius, Ulf. (1979) State and location of iron in sillimanite. Neues lichen Forschung(proJect 3735) Financial support to A. B. by the Ful- Jahrbuch fiir Mineralogie Monatshefte, 165-174. bright Commission is also acknowledged. Kerrick, D.M., and Speer,J.A. (1988) The role of minor element solid solution on the andalusite-sillimaniteequilibrium in metapelitesand Rrrnnnxcns crrno peraluminousgranitoids. American Journal of Science,288, 152-192. Kitamura, M., Kondoh, S., Morimoto, N., Miller, G.H., Rossman,G.R., Aines,R.D, and Rossman,G.R. 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