Charles A. Geiger,
Mineraologisch - Petrographisches Insutit. Christian-Albrechts-Universitat, Olshausenstasse 40 D-24098 Kiel, Germany.
George R. Rossman
Division of Geological and Planetary Sciences. California Institute of Technology, Pasadena. California 91125. U.S.A.
Abstact
The band positions of three partially overlapping Fe2+ spin-allowied transitions located between 4000 and 9000 cm-1 in almandine-pyrope and almandine-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 1100 cm-1 for the lower orbitals. The CFSE show a slight increase along the almandine-spessartine binary from 3730 to 3810 cm-1 and a larger increase from 3730 to 3970 cm-1 for the almandine-pyrope binary. Dodecahedral Fe2+ site distortion increases with an increase in spessartine component and decreases with increasing pyrope component, in agreement with average dodecahedral site distortions determined from diffraction experiments. The excess CFSE 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 from a symmetric mixing model. The excess CFSE are relatively small compared to the magnitudes of the excess enthalpies of mixing that have been assigned to garnet solid solutions. Moreover, they give no indication which could support the positive and asymmetric excess enthalpies of mixing that have been proposed for almandine-pyrope solid solutions.