American Mineralogist, Volume 82, pages 740±747, 1997 Mn3Al2Si3O12 spessartine and Ca3Al2Si3O12 grossular garnet: Structural dynamic and thermodynamic properties CHARLES A. GEIGER1 AND THOMAS ARMBRUSTER2 1Mineralogisch-Petrographisches Institut der Christian-Albrechts-UniversitaÈt zu Kiel, Olshausenstr. 40 D-24098 Kiel, Germany 2Laboratorium fuÈr chemische und mineralogische Kristallographie, UniversitaÈt Bern, Freiestrasse 3, CH-3012 Bern, Switzerland ABSTRACT The structures of synthetic Mn3Al2Si3O12 spessartine and Ca3Al2Si3O12 grossular garnet have been re®ned using single-crystal X-ray diffraction methods at 100 K, 293 K, and 500-550 K. The divalent X-site cations, located in large dodecahedral sites, show measur- able anisotropic dynamic disorder in contrast to the rigid vibrational behavior of the SiO4 21 tetrahedra and AlO6 octahedra. The amplitudes of vibration of Mn in spessartine are similar to those of Fe21 of almandine, in the plane of the longer X-O(4) bonds, and both are about twice that of Ca21 in grossular, despite the lighter mass of the latter. Heat ca- pacities measured between 300 and 1000 K on synthetic polycrystalline spessartine and two natural nearly end-member spessartine crystals are similar to those of almandine. In addition, the IR active modes of spessartine at low frequencies are very similar to those of almandine suggesting that their heat capacities are also similar at lower temperatures. The low-energy phonon spectra of pyrope and grossular are probably considerably distinct from the two transition metal-containing garnets as suggested by their different low fre- quency IR active modes, re¯ecting the different bonding properties for Mg and Ca in garnet. The large pressure-temperature stability ®eld of spessartine, relative to the other aluminosilicate garnets, does not appear to be due to any sort of intrinsic entropy stabilization. INTRODUCTION Ca3Fe2Si3O12) have been investigated by temperature- The garnet group is extremely diverse showing a wide dependent, X-ray single-crystal re®nements between 100 compositional range (e.g., Geller 1967). One class of gar- and 500 K (Geiger et al. 1992; Armbruster et al. 1992; net, the silicate garnets of space group Ia3d (Menzer Armbruster and Geiger 1993). An analysis of the atomic 1928), which occur commonly in nature, are, in addition, mean-square difference displacement parameters (BuÈrgi thermodynamically stable over very large pressure and 1989) as a function of temperature shows that the SiO4 temperature regimes. They show, in comparison with tetrahedra and the AlO6 octahedra can be considered as most silicate structures, little in the way of phase transi- rigid bodies over this temperature range, thus con®rming tions or instability as a function of pressure and temper- the static polyhedral description of the structure (Zemann ature. The aluminosilicate garnets of the general formula 1962). The divalent X-site cations show, in contrast, sub- stantial anisotropic dynamic disorder in the large irregular X3Al2Si3O12, where for the most common end-members X 5 Fe21 (almandine), Ca21 (grossular), Mn21 (spessar- dodecahedral site (Geiger et al. 1992; Armbruster et al. tine), and Mg21 (pyrope), are a good example for such 1992; Armbruster and Geiger 1993). Recognition of this phase stability. As a group, they are stable from 1 atm dynamic disorder allows new insight into the garnet struc- up to several 10s of GPa from roughly 400 8C to very ture and has enabled, for example, several unusual and high temperatures. The aluminosilicate garnets also pos- previously not understood spectroscopic features to be sess the ability to form extensive solid solutions, as elucidated (Geiger et al. 1992; Quartieri et al. 1997; Ko- shown by the mixing of different cations on the dodeca- lesov and Geiger, in press). hedral X-site, Wycoff position 24(c). In addition, thermodynamic properties such as heat ca- The static structural description of garnet, largely based pacity could depend on the dynamic properties of the on room temperature diffraction experiments, is well X-site cations. The relatively large heat capacity of py- known since its initial structure determination by Menzer rope at low temperatures (Haselton and Westrum 1980), (1928). Recently, the dynamic properties of the polyhe- for example, may be related to the large amplitude, low dral units and the X-site cations of some of the end-mem- energy vibrations of the Mg cations. The phonon density ber silicate garnets (almandine, pyrope, and andradite- of states of the silicate garnets are unknown. Only a few 0003±004X/97/0708±0740$05.00 740 GEIGER AND ARMBRUSTER: SPESSARTINE-GROSSULAR PROPERTIES 741 dispersion curves for the acoustic phonons of pyrope apparently slowly reduced to MnO during the experiment, were measured (Artioli et al. 1996) and hence, a rigorous and spessartine nucleation and subsequent growth pro- analysis of the lattice dynamics of it or of the other sili- ceeds in a slower fashion than when using MnO as a cate garnets is not possible. starting material. In this manner, spessartine crystals up We examine, herein, the structures of synthetic to about 200 mm can be produced. Several types of crys- Mn3Al2Si3O12 spessartine and Ca3Al2Si3O12 grossular gar- tals were observed. Large single crystals, somewhat net between 100 and 500-550 K, as determined by single rounded, showing {110} and {211} faces were found, as crystal X-ray re®nements, and analyze the behavior of the were more skeletal-like crystals having pointed projec- X-site cations with respect to the temperature dependence tions atypical of the normal garnet habit. Similar crystals of their amplitudes of vibration. The heat capacity of were documented in the work of Matthes (1961; see Fig. spessartine, which was previously unknown, is also pre- 7). The cores of most of the larger crystals are reddish sented between 300 to 1000 K and is compared with the orange in color, while the rims are generally more trans- heat capacities of the other aluminosilicate garnets. The lucent and lighter orange. The darker color is probably powder FTIR spectra of the four synthetic aluminosilicate attributable to small amounts of Mn31 substituting for garnets between 450 and 80 cm21 are also compared. Fi- Al31 in the octahedral site. A re®nement of the unit-cell nally, the large P-T stability of spessartine is addressed dimension, a0, of this material based on a powder X-ray in light of these new measurements. pattern between 208 and 1208 2u, gave 11.618(1) AÊ , with the error representing 2s statistics. EXPERIMENTAL METHODS Polycrystalline spessartine was also synthesized anhy- Inasmuch as nearly all natural garnets are composi- drously at high pressures and temperatures for heat ca- tionally complex solid solutions, it is necessary to syn- pacity measurements. First, a glass of stoichiometric spes- thesize end-member garnet compositions for the study of sartine composition was prepared by melting MnO, SiO2, dynamic properties using X-ray diffraction. Static posi- and Al2O3 in a graphite capsule at 1523 K and then tional disorder resulting from localized compositional dif- quenching in water. About 200 mg of ®nely ground glass, ferences cannot be separated easily from true dynamical without water, was welded into a platinum capsule of 5 disorder, and therefore, end-member garnet compositions mm diameter and crystallized at 1273 K and 1.6 GPa in must be synthesized for these measurements. A short de- a piston-cylinder device. The resulting synthesis product scription of the synthesis methods used for the alumino- was better than 99.5% spessartine and gave an a0 cell silicate garnets is given in Geiger et al. (1988). dimension of 11.6163 (5), AÊ based on a least-squares re- The grossular single crystals used for X-ray study were ®nement of a powder X-ray pattern, using NBS 640b Si synthesized hydrothermally at 1073 K and 0.2 GPa for a as an internal standard, in the range between 10±1358 2u. period of eight weeks from a ®ne-grained and intimately [The cell dimension of spessartine depends on OH2 con- ground oxide mixture of CaO (from CaCO3), SiO2 tent for those synthesized hydrothermally at low temper- 31 (quartz), and Al2O3 (corundum) of stoichiometric gros- atures and probably on the amount of Mn . Geiger et al. sular composition. Standard cold-seal vessels and asso- (1988) considered the value ao 5 11.615(1) the best es- ciated methods were used. It is dif®cult to synthesize timate for end-member stoichiometric spessartine.] large crystals of grossular (greater than several hundred Heat-capacity measurements on spessartine were made micrometers), and the largest crystals obtained were using an automated Perkin Elmer DSC 7 differential about 100 mm in diameter. Most of the synthesis product scanning calorimeter (Watson et al. 1964). The experi- consisted of ®ner grossular crystals about 15 mm in di- mental methods adopted are identical to those reported in ameter. Additional phases, which were not identi®ed, Bosenick et al. (1996). The step-scanning technique was were present at about 5±10 vol%. used and machine stability and accuracy was checked by It is also dif®cult to synthesize large spessartine single measuring the heat capacity of single crystal MgO. Mea- crystals. Attempts to grow large crystals hydrothermally surements were made on the synthetic polycrystalline using an oxide mixture of MnO, SiO2, and Al2O3 or a spessartine and, in addition, on two natural spessartine glass of stoichiometric spessartine composition result in single crystals obtained from G.R. Rossman (California crystals of about 10±15 mm in size or smaller. Spessar- Institute of Technology). The ®rst sample, labeled GRR tine shows the lowest temperature stability of the alumi- 43b, is from the Rutherford no. 2 Mine in Amelia, Vir- nosilicate garnets (Hsu 1968), and it appears that rapid ginia. The second, labeled GRR 1791, is from an un- nucleation at low temperatures does not allow the growth known location in Namibia.