The mixing properties of Fe3A12Si3012 -Ca3AlzSi3Ola garnet solid solutions have been studied in the temperature range 850-1100 ~ C. The experimental method involves measuring the composition of garnet in equilibrium with an assemblage in which the activity of the Ca3A12Si3012 component is fixed. Experiments on the assemblage garnet solid solution, anorthite, AlzSiO s polymorph and quartz at known pressure and temperature fix the activity of the Ca3AlzSi3012 component through the equilibrium:
This equilibrium, with either sillimanite or kyanite as the aluminosilicate mineral, was used to control a~ta3A12Si3012 . The compositions of the garnet solutions produced were determined by measurement of their unit cell edges.
At 1 bar Fe3A12Si3012-Ca3A12Si3012 garnets exhibit negative deviations from ideality at the Ferich end of the series and positive deviations at the calcium end. With increasing pressure the activity coefficients for the Ca3A12Si3012 component increase because the partial molar volume of this component is greater than the molar volume of pure grossular. Previous studies indicate that the activity coefficients for the Ca3A12Si3012 component also increase with increasing (Mg/Mg+Fe) ratio of the garnet.
The region of negative deviation from ideality implies a tendency towards formation of a stable Fe-