Activity-composition data have been used to obtain excess entropies and enthalpies of mixing for almandine-grossnlar and Mg-rich pyrope-grossular solid solutions. Excess free energies of Fe-rich almandine-grossular garnets are negative and imply the formation of a stable garnet compound with a different structure from the end members in this series.
X-ray investigations of natural and synthetic calcium-poor aluminosilicate garnet s indicate a lower space group symmetry than Ia3d, that of the end members. The most probable space group is I213 in which there are two crystallographically distinct 8-coordinate sites present in equal numbers in the garnet structure.
Excess entropies of mixing for ahnandine-grossular garnets are asymmetric and can be explained by Fe-Ca ordering in calcium-poor garnets together with Fe positional disorder on sub-sites within the 8-coordinate polyhedra. In the middle of the series and towards the Ca-rich end, a high degree of sub-site disorder with little or no Fe-Ca ordering may be responsible for the high positive entropies. Excess entropies of calcium-poor pyrope-grossular garnets show a similar trend, but are slightly more positive. Excess enthalpies of mixing for Mg-rich pyropegrossular garnets are in good agreement with the high4emperature calorimetric measurements of other workers.