American Mineralogist, Volume 76, pages I 153-I 164, l99l The hydrous components in garnets: Grossular-hydrogrossular Gnoncr R. RossrrlN, Rocnn D. Arxts* Division of Geological and Planetary Science,California Institute of Technology, Pasadena,California 9 I 125, U.S.A. Ansrucr Grossular garnets from 33 localities were examined for indications of OH or HrO in their infrared spectrum. All contained OH-. Classical hydrogrossular with more than 5 wto/oHrO displays systematicspectroscopic behavior consistentwith the hydrogarnet sub- stitution consisting oftwo absorption bands at 3598 and 3662 cm '. These spectroscopic characteristicswere generallynot observedin other glossular samples.Instead, 20 distinct absorption bands have been identified in the spectra of common grossular,occurring in groups of four to ten bands. Both the number and intensities of thesebands show a large variation, which does not correspond with the garnet's composition. Seven classesof spectra were identified in the OH region based on the position of the most intense ab- sorption band. The spectroscopicdata suggestthat in addition to the tetrahedral site, OH groups exist in multiple other environments. The OH content of grossulargarnets can be obtained from infrared spectra using the equation HrO wto/o: 0.0000786 x integrated absorbanceper cm in the OH region near 3600 cm '. The OH content of macroscopic grossularcrystals (expressedas weight percent HrO) rangedfrom 12.80/odown to lessthan 0.0050/0.Macroscopic grossulartypically contains less than 0.3 wto/oHrO. Grossular from rodingites and low-temperature alteration vugs contained much more than that from skarn or contact metamorphic environments. INrnooucrroN position. Becausethe hydrogarnet substitution should be prevalent grossular pyrope-almandine The hydrogamet substitution, (OH)o : SiO", has been more in than in establishedin both synthetic and natural garnets.Gros- garnets(Lager et al., 1989),we have examined a number sular analyseshave been reported with over l0o/oHrO, of grossularsamples from a variety of localities to deter- and the structure of hydrogrossularhas been determined mine if the hydrogrossularsubstitution was,indeed, com- quantify with both X-ray and neutron diffraction (Cohen-Addad monplace and to attempt to the extent of this etal.,19671,Foreman, 1968; Basso et al., 1983;Sacerdoti substitution. previously and Passaglia,1985; I-ager etal., 1987a,1989). Most hy- Infrared spectra have often been used to drogrossular with high OH contents consists of crystals characteize synthetic hydrogrossular samples. These present typically no more than a few tens of micrometers in size, spectraconfirm that OH is in hydrogrossularand often with cores of grossularor intimately admixed with should be a standard of comparison for the identification vesuvianite(Zabinski, 1966; Rinaldi and Passaglia,1989). of OH in natural samples.However, when Passagliaand (1984) Although such high HrO content garnetsare rare, previ- Rinaldi describedkatoite, a mineral in the hydro- ous infrared spectraofmacroscopic garnetshave indicat- grossularseries with lessthan one-third ofthe tetrahedral presented ed that a minor OH component is presentin a variety of sites occupiedby Si, they an infrared spectrum garnets(Wilkins and Sabine, 1973). We have previously that bearslittle resemblanceto previously published gros- reported the results of a survey of the infrared spectros- sular spectraand differs in detail from the spectraofsyn- copy of a number of pyrope-almandine garnets (Aines thetic hydrogrossular.Consequently, it seemedappropri- properties and Rossman, 1985a, 1985b) in which nearly all con- ate first to examine carefully the spectroscopic tained OH. The position of the bands in the spectrum of of garnetswith known hydrogrossularsubstitution in or- these garnets varied systematicallywith the garnet com- der to compare the spectroscopicproperties of natural position, but the amount of OH varied for garnetsfrom hydrogrossular samplesto their synthetic counterparts and different localities. to the common low OH-content grossularsamples. Such previously per- This paper is primarily concernedwith the OH content a comparison has not been critically of common, macroscopic garnets of the grossular com- formed. Expnnrprnx.rAl, DETAILS * Presentaddress: Department ofEarth Science,Lawrence Liv- Samples were obtained primarily from museums and ermore National l,aboratories, Livermore, California 94550, private collections.Details of samples,localities, and ma- U.S.A. jor element compositions are presentedin Tables I and 0003-004x/91/0708-1 I 53$02.00 I 153 1154 ROSSMAN AND AINES: HYDROUS COMPONENTS IN GARNETS TABLE1, Grossulargarnet samples Variety Locality Class 42 orange-brown [McFallmine?], calcite skarn, Ramona, CA, U.S.A. 2 4.6 52 orange-brown lvesper Peak?], WA, U.S.A. D 2.7 53 pale orange Asbestos, Quebec, Canada 3,4 4.8 227 green tsavorite Campbell Bridges mine, Tsavo National Park, Kenya 2b 1.8 229 green tsavorite stream gravel, Kenya 2b 3.7 771 colorless Meralini Hills,Tanzania 2b 3.4 936 orange Bric Camula, Cogoleto, Liguria, ltaly 6 22.1 937 dark red-orange Passo del Faiallo,Genova Province,Liguria, ltaly b 13.7 938 dark red-brown Ossola Valley, ltaly 3 1.5 941 pink jade Butfelsfontein,Rustenberg, S. Africa 4 89.7 946 brown-red Auerbach, Germany 7 4.8 1037 orange Dos Cabezas, lmperial County, CA, U.S.A. 2b 1.4 1038 green Jeffrey Mine, Asbestos, Quebec, Canada 5 1.5 1042 orange-brown VesperPeak, WA, U.S.A. 5 2.8 1051 pare orange lBelvidereMtn?], EdenMills, W, U.S.A. 3'| 1.0 1058 synthetic Lager et al. (1989) powoer 1059 syntheticLager et al. (1987a) 1 powder 1113 brown-orange North Hill,Riverside County, CA, U.S.A. 7 1.7 1122 orange-brown lOommercialQuarry?], Crestmore,CA, U.S.A. 7 2.7 1124 colorless Chihuahua,Mexico 3 2.1 1125 colorless Lake Jaco, Mexico 3,7 0.7 1129 pale orange BelvidereMountain, VT, U.S.A. 3 3.2 1131 black zone Lake Jaco, Mexico 3 o.2 1198 green jade Transvaal,S. Africa 4 68.3 1326 orange Ala Valley, Piedmont, ltaly c 7.2 1327 orange Sciarborosca,Ligura, ltaly c 6.9 't329 massrvevern CommercialQuarry, Crestmore,CA, U.S.A. 1 160.9 1357 orange-reo Bric Canizzi, Liguria, ltaly 5 20.5 1358 colorless CommercialQuarry, Crestmore,CA, U.S.A. 1 138.5 1359 red-orange lron gabbro metarodingite,Gruppo di Voltri, ltaly 3.4 1360 orange Basaltic metarodingite,Voltri Massif, ltaly 15.9 1364 massrvegray Maungatapu Survey District, Nelson, New Zealand 4 90.0 1409 red-brown calcite,skarn, Saline Valley, Darwin, CA, U.S.A. 2,2b 3.C 1411 birefringent skarn. Munam. North Korea o 1.2 1412 brown Mul-Kummine, South Korea 1.0 1413 pale green VilyiRiver, Siberia, U.S.S.R. 0.0 1419 pale orange Minot Ledge,Minot, ME, U.S.A. 2b 3.2 1420 brown Rauris, Salzburg,Austria 2 8.3 't422 brown Wakefield,Ontario, Canada 2b 0.3 1423 red-brown MountainBeauty mine, Oak Grove,CA, U.S.A. 0.6 1424 orange GarnetQueen mine?, Santa Rosa Mtns, CA, U.S.A. 7 3.1 1427 massive yellow Mavora Lakes, Otago, South lsland, New Zealand 4 1429 red-brown EssexCounty, NY, U.S.A. 0.6 1430 oragne EdenMills, VT, U.S.A. 3 1,1 't444 katoite Pietramassa,Viterbo, ltaly 1 powder Nofe.'Question n''ark denotes additional locality information that was not part of the documentationprovided with the sample but that we added based on our inspectionof the sample.Absorbance per mm refers to the strongest band in the OH region. Integratedintensity in the OH regiongenerally closely follows this parameter. Class refers to the class of spectra in the OH region discussed in text. Multiple numbers mean that different crystals from the same locality have different behavior or that different zones of the crystal show different behavior. 2. The infrared spectra of the natural crystals were ob- both a MAC5 (Bence-Albeecorrections) and JEOL 733 tained from doubly polished single crystals. Exceptions instruments (CITZAF corrections). were the fine-grained,massive hydrogarnet samples(nos. Batches of both CarAlr(OoHo)3and Ca3Al, (Sioo)rr8 1329, 1364, ar;'d 1427), which were studied as doubly (OoHo)., were provided by G. A. Lager. CarAlr(OuDo), polished polycrystalline slabs,and katoite (no. 1444) and that contained some residual H and was used in the neu- the fine-grained powders of synthetic hydrogarnet sam- tron structural refinement ofLager et al. (1987a)was also ples,which were studied as KBr pellets. Details of sample examined. The position and full width at half height of preparation are the sameas describedin Aines and Ross- the band in the OH region of its spectrum are essentially man (1985a). Details of spectrophotometermeasure- identical to that of CarAlr(OoHo)r. ments are generally the same as described in Aines and Rossman with the exception that most spectra were ac- Rnsur,rs quired with a Nicolet 60SX FTIR with an InSb detector at 2 cm-t resolution. This instrument allowed regions in Synthetic hydrogrossular the sample as small as 25 p.m in diameter to be studied Hydrogarnet ofthe composition Ca.Alr(OH)', has been selectively. Garnet analyseswere performed by electron synthesizedby a number of authors who have also re- microprobe as described in Aines and Rossman using ported its infrared spectrum (Zabinski, 1966; Cohen-Ad- ROSSMAN AND AINES: HYDROUS COMPONENTS IN GARNETS I 155 TrsLe 2, Grossular analyses: formula proportions lnfrared vvt"/" Number Ca Mg Fe,* Mn FeP- Cr Ti si HrO 42 2.89 0.00 0.08 0.01 1.89 0.11 0.00 na 0.00 3.02 na 0.10 52 3.03 0.00 0.00 0.05 1.67 0.30 0.00 na 0.02 3.00 na 0.14 53 2.93 0.00 0.03 0.04 1.96 0.06 0.00 na 0.00 2.98 na 0.21-0.38 227 2.99