The Hydrous Component in Andradite Garnet

American Mineralogist, Volume 83, pages 835±840, 1998 The hydrous component in andradite garnet GEORG AMTHAUER* AND GEORGE R. ROSSMAN² Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, U.S.A. ABSTRACT Twenty-two andradite samples from a variety of geological environments and two synthetic hydroandradite samples were studied by Fourier transform IR spectroscopy. Their 2 spectra show that H enters andradite in the form of OH . Amounts up to 6 wt% H2O occur in these samples; those from low-temperature formations contain the most OH2. Some 42 ↔ 42 features in the absorption spectra indicate the hydrogarnet substitution (SiO4) (O4H4) whereas others indicate additional types of OH2 incorporation. The complexity of the spectra due to multi-site distribution of OH2 increases with increasing complexity of the garnet composition. 42 ↔ 42 INTRODUCTION tution (O4H4) (SiO4) . This observation has been Systematic studies have shown that hydroxide is a con®rmed by XRD of a hydrous andradite with a Si de- common minor component of grossular and pyrope-al- ®ciency of about 50%, and a high OH content (Arm- mandine-spessartite garnets (Aines and Rossman 1985; bruster 1995). The structure of this particular sample with Rossman and Aines 1991). Comparable surveys of an- space group Ia3d is composed of disordered microdo- dradite garnet have not been previously presented. Sev- mains containing (SiO4) and (O4H4) tetrahedral units. eral reports indicate that appreciable amounts of OH2 can The aim of the present investigation was to perform a be incorporated in both natural and synthetic andradite- Fourier transform infrared (FTIR) study on different sam- rich garnet (Flint et al. 1941; Peters 1965; Gustafson ples of andradite (including Ti-rich melanite) from vari- 1974; Suwa et al. 1976; Onuki et al. 1982; Huckenholz ous geological environments, i.e., different pressures and and Fehr 1982; Marcke de Lummen 1986; Kobayashi and temperatures of formation, to determine (1) if various Shoji 1987; KuÈhberger et al. 1989; Wise and Moller types of structural incorporation of OH2 occurs in andra- 1990; Armbruster and Geiger 1993; Armbruster 1995). dite as in other natural garnets or if there is only the Furthermore, elevated OH2 contents have been reported hydrogarnet substitution; (2) the amount of OH that is in other Fe31-rich garnets such as melanite (Lager et al. incorporated in natural andradites; (3) if a correlation ex- 1989) and schorlomite (Locock et al. 1995), which sug- ists between the chemical composition and the complex- gests that H incorporation in Ca-Fe31 garnet may be a ity of the spectra; and (4) the pressure and temperature general phenomenon. Solid-solution relationships be- dependence of OH2 incorporation. Finally, (5) we com- 2 tween andradite, Ca3Fe2 (SiO4)3, and hydroandradite, pare spectroscopic properties of OH in natural garnets Ca3Fe2(OH)12, are now established experimentally, and to those in synthetic hydroandradite. the amount of the hydroandradite component in andradite may be used as geothermometer (Huckenholz and Fehr EXPERIMENTAL METHODS 1982). Samples were obtained mainly from museums. Details Only a few of these studies provide direct evidence of of samples, localities, and geological environments are structural OH2 in andradites by spectroscopic or diffrac- presented in Table 1. tion methods. Kobayashi and Shoji (1987) report IR spec- The IR spectra of the natural crystals were recorded tra of some synthetic andradite-hydroandradite solid so- from doubly polished single crystals with almost parallel lutions. Locock et al. (1995) report an absorption band surfaces. The thicknesses of the platelets were between due to OH2 at 3564 cm21 in the IR spectrum of the Ice 11.5 and 1185 mm. Measurements were made on clear 1 River schorlomite with 0.036 wt% H2O . In a crystallo- and inclusion-free as well as crack-free areas of the crys- graphic study, Lager et al. (1989) were unable to locate tals. In those crystals where color zonation was present, the H in an andradite (melanite variety) with up to 14% spectra were taken from a selected number of relatively of the Si sites empty. In such H-rich garnets, OH2 most homogeneous parts of the crystal. IR spectra were mea- probably occupies tetrahedral sites whereby charge bal- sured from 114 mm 3 114 mm areas using a Nicolet ance is achieved by the well-known hydrogarnet substi- 60SX-FTIR spectrometer equipped with the NicPlan IR * Permanent address: Institut fuÈr Mineralogie, UniversitaÈt microscope and an HgCdTe detector (Rossman and Aines Salzburg, Hellbrunnerstrasse 34, A-5020 Salzburg, Austria. 1991). A 153 re¯ecting objective and a 103 condenser ² E-mail: [email protected] were used for the measurements. A resolution of 2 cm21 0003±004X/98/0708±0835$05.00 835 836 AMTHAUER AND ROSSMAN: HYDROUS COMPONENT IN ANDRADITE TABLE 1. Andradite samples and their chemical compositions Sample Locality Geology Variety Color GRR1669 Bombay, India cavity in basalt hydroandradite yellow GA33 Wurlitz, Bavaria, FRG serpentinite andradite yellow-green GA34 Banat, Romania serpentinite andradite yellow-green GRR48 rim Val Malenco, Italy serpentinite andradite yellow-green GRR134 Val Malenco, Italy serpentinite andradite yellow-green GRR169 Santa Rita Peak, San Benito Co., CA, U.S.A. serpentinite andradite yellow-green GRR684 San Benito Co., CA, U.S.A. serpentinite melanite black GRR1263 San Benito Co., CA, U.S.A. serpentinite melanite black GRR1328 San Benito Co., CA, U.S.A. serpentinite melanite black GRR1765 Ural Mtns, Russia serpentinite demantoid green GA32 Davib Ost, South Africa skarn andradite yellow-green GA35 Grua, Norway skarn andradite yellow-brown GRR54 Stanley Butte, Graham Co., AZ, U.S.A. skarn andradite yellow GRR149 Franklin, NJ, U.S.A. skarn andradite orange-brown GRR1015 Franklin, NJ, U.S.A. skarn andradite yellow-orange GRR1137a2 Stanley Butte, Graham Co., AZ, U.S.A. skarn andradite brown-yellow GRR1447 Humbolt Co., NV, U.S.A. skarn andradite yellow-brown GRR1448 Reeseville, NY, U.S.A. skarn andradite orange-brown GA24 Kaiserstuhl, Germany phonolite melanite black GA36 Frascati, Italy magmatic melanite black GRR1446 Frascati, Italy magmatic melanite black CITH3110 Rusinga Island, Kenya magmatic melanite black GAIV/1/4 375 8C, 1.5 kbar, HM²² synthetic powder hydroandradite pale red GAIIIa/3 375 8C, 1.5 kbar, HM²² synthetic powder hydroandradite pale green * Quantitative amounts of OH2 in andradite garnets are determined from the grossular calibration of Rossman and Aines (1991). ² From Wise and Moller (1990). ³ Rim. § From Amthauer et al. (1979). \ Inhomogenous. # Compositionally zoned. ** Described in KuÈhberger et al. (1989). ²² HM 5 Fe2O3/Fe3O4-oxygen fugacity buffer. ³³ Composition from XRD parameters. was chosen for most samples with 1000 scans suitable to sures of 1.5 and 2.0 kbar, and the oxygen fugacities of produce high-quality spectra. Reference spectra were the hematite 1 magnetite buffer (Tippelt and Amthauer, taken in air. In addition, some re¯ectance spectra from in preparation). Purity and homogeneity of the samples synthetic garnet powders were taken with the same ex- were checked by XRD of powders using CuKa-radiation. perimental setup using polished Au as a reference. Com- The chemical composition, in particular the OH2 content, parison of single-crystal spectra and re¯ectance spectra X was calculated from the lattice constant ao according of the same garnet sample revealed a similar resolution to the regression equation of Huckenholz and Fehr (1982) of the OH features in each case. a 5 12.057 1 0.4906 X 1 0.2379 X2 2 0.0824 X3. Because an independent calibration of the OH2 con- o centration in andradite does not exist, the calibration of The small grain sizes (,20 mm) and morphology of the Rossman and Aines (1991) for grossular was used to es- synthetic samples made it impractical to obtain spectra timate the H content of andradite. The integrated absor- from single crystals; therefore only powder spectra were bance under the OH2 stretching region of the IR spectrum taken of the synthetic garnet. (normalized toa1cmthick sample) is determined and the H content is obtained by reference to the grossular RESULTS calibration. The IR spectra of three andradite samples from ser- Chemical analyses of the samples were performed with pentinites are shown in Figure 1. GRR134 is representa- an automated JEOL 733 electron microprobe using a 15 tive of almost pure andradite and exhibits the simplest nA beam current. Data were corrected using the program spectrum, i.e., a dominant absorption band at 3555 cm21. CITZAF (Armstrong 1988) employing the absorption Weak higher energy absorptions and a weak shoulder on correction of Armstrong (1982), the atomic number cor- the low energy site of the main peak are also present. The rection of Love et al. (1978), and the ¯uorescence cor- IR spectrum of andradite from the Ural mountains rection of Reed (1965) as modi®ed by Armstrong (1988). (GRR1765) looks almost identical. This garnet is also al- The structural formulae of the samples normalized to 12 most pure end-member andradite with minor amounts of O atoms are reported in Table 1. Mg (0.01 wt%), Al (0.01 wt%), and Cr (0.05 wt%). Both Andradite-hydroandradite solid solutions were synthe- are clear and contain 0.10 wt% H2O. The spectrum of the sized hydrothermally from glass of an appropriate com- San Benito andradite (GRR1263) is more complex. Al- position at temperatures between 300 and 400 8C, pres- though the

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