MN3AL2SI3O12 SPESSARTINE AND CA3AL2SI3O12 GROSSULAR GARNET - STRUCTURAL DYNAMIC AND THERMODYNAMIC PROPERTIES

The structures of synthetic Mn3Al2Si3O12 spessartine and Ca3Al2Si3O12 grossular garnet have been refined using single-crystal X-ray diffract ion methods at 100 K, 293 K, and 500-550 K. The divalent X-site cation s, located in large dodecahedral sites, sinew measurable anisotropic d ynamic disorder in contrast to the rigid vibrational behavior of the S iO4 tetrahedra and AIO(6) octahedra. The amplitudes of vibration of Mn 2+ in spessartine are similar to those of Fe2+ of almandine, in the pl ane of the longer X-O(4) bonds, and both are about twice that of Ca2in grossular, despite the lighter mass of the latter. Heat capacities measured between 300 and 1000 K on synthetic polycrystalline spessarti ne 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 consid erably distinct from the two transition metal-containing garnets as su ggested by their different low frequency IR active modes, reflecting t he different bonding properties for Mg and Ca in garnet. The large pre ssure-temperature stability field of spessartine, relative to the othe r aluminosilicate garnets, does not appear to be due to any sort of in trinsic entropy stabilization.