CRYSTAL-FIELD STABILIZATION ENERGIES OF ALMANDINE-PYROPE AND ALMANDINE-SPESSARTINE GARNETS DETERMINED BY FTIR NEAR-INFRARED MEASUREMENTS

The band positions of three partially overlapping Fe2+ spin-allowed tr ansitions located between 4000 and 9000 cm-1 in almandine-pyrope and a lmandine-spessartine garnets solid solutions were measured using near- infrared (NIR) spectroscopy. The crystal field stabilization energies (CFSE) along both binaries were calculated assuming a splitting of 110 0 cm-1 for the lower orbitals. The CFSE show a slight increase along t he almandine-spessartine binary from 3730 to 3810 cm-1 and a larger in crease from 3730 to 3970 cm-1 for the almandine-pyrope binary. Dodecah edral Fe2+-site distortion increases with an increasing pyrope compone nt, in agreement with average dodecahedral site distortions determined from diffraction experiments. The excess CFSE's along both joins are negative. For the almandine-spessartine binary they are small, but are about 3.5 times larger in magnitude along the join almandine-pyrope, where an interaction parameter of W = -2.9 KJ/mole has been derived fr om a symmetric mixing model. The excess CFSE are relatively small comp ared to the magnitudes of the excess enthalpies of mixing that have be en assigned to garnet solid solutions. Moreover, they give no indicati on which could support the positive and asymmetric excess enthalpies o f mixing that have been proposed for almandine-pyrope solid solutions.