A POWDER INFRARED SPECTROSCOPIC INVESTIGATION OF GARNET BINARIES IN THE SYSTEM MG3AL2SI3O12-FE3AL2O12-MN3AL2SI3O12-CA3AL2SI3O12

Abstract

The powder IR phonon spectra of synthetic garnets of the six binaries in the system Mg3Al2Si3O12 (Pyrope-Py)-Fe3Al2Si3O12 (Almandine-Al)-Mn3Al2Si3O12 (Spessartine-Sp)-Ca3AlSi3O12 (Grossular-Gr) have been recorded and analyzed. Between 14 and 17 F1u-symmetry infrared active modes were observed for the different garnet end members and can, to a first approximation, be assigned to internal SiÜ4 vibrations and external lattice vibrations of the SiO4 tetrahedron and the Al3+ and X2+ -site cations. This description is limited by mode mixing which is most pronounced for the lowest frequency modes. Two mode behavior is observed for X2+ -cation translations for most of the binaries. The site-group and factor-group splittings have been calculated for all six binaries. For the almandine/pyrope-grossular binaries, the factor-group splittings suggest that SiO4 group vibrational interactions do not change linearly between the two end-members. No evidence in the IR spectra is found for long range X 2+ -cation order and all synthetic solid solutions are long-range disordered. The IR spectra provide information on the possible lattice heat capacities and entropies of mixing for the six solid solutions. The frequency changes of the lowest frequency external modes of pyrope-grossular and almandine-grossular garnets are consistent with the proposal of excess lattice entropies of mixing at low temperatures. The almandine-spessartine binary should be thermodynamically ideal, while the almaridine-pyrope, spessartine-pyrope and spessartine-grossular binaries could show some excess lattice properties. The higher frequency modes above 400 cm-1 of the solid solution compositions can be estimated well from a linear interpolation between the two end-members. This is consistent with the proposal that no excess lattice entropies of mixing should be present in garnet solid solutions above 300 K.