PHASE-RELATIONS BETWEEN CA3FE23+SI3O12-FE32+FE23+SI3O12 GARNET AND CAFESI2O6-FE2SI2O6 PYROXENE SOLID-SOLUTIONS

The stability of Ca-bearing garnets on the join Ca3Fe23+Si3O12-Fe-3(2+) Si3O12 (andradite - ''skiagite'') has been investigated as a function of pressure at 1100 degrees C. The experiments were performed using a series of glasses and slags with fixed Fe3+/Fe-total in a piston cylinder apparatus and a multi-anvil press. The maximum solubility of ''skiagite'' in andradite increases from 12 mol% at 1.7 GPa to 48 and 64 mol% at 6.0 and 7.0 GPa respectively. This is greater than that found for the solubility of ''skiagite'' in almandine (Fe32+Al2Si3O12) garnet at the same pressure and temperature conditions. The equilibrium garnet solid solutions at 1100 degrees C coexist with spinel + clinopyroxene + quartz or coesite at 0.7 < P < 7.0 GPa. Above 7.0 GPa garnet may coexist with either spinel + clinopyroxene + coesite or with Fe2SiO4-rich spinel + coesite depending on the bulk Fe3+/Fe-total. The coexisting clinopyroxene becomes progressively more Fe-rich with increasing pressure until a miscibility gap is intersected at 7.0 GPa. Above this pressure, the pyroxenes are Ca-poor and monoclinic (space group P2(1)/c), but probably have the C2/c space group at the conditions of the experiments. There is a highly asymmetric miscibility gap between this Ca-poor high pressure form and the hedenbergite-clinoferrosilite solid solution, similar to that formed between Ca-poor orthopyroxene and hedenbergite solid solutions at lower pressures. Determination of the mixing properties of the garnet solid solutions and the Delta(f)G(1373K, 1 bar)(o) for ''skiagite'' can potentially be determined from the CaFe2+ exchange between garnet and pyroxene. However this analysis is hampered by problems in modelling the properties of the coexisting clinopyroxene.