THE OXIDATION OF AIMAND|NE AND IRON CORDIERITE

Solid solutions of the two end mennbers magnetite and hercynite are common oxidation products ol alurninous iron silicates. Activity coefficients for both componenff may be derived from the solvus in the system Fe3oa-FeAl2oa. It is then poriiblu to calculate oxidation equilibria irwolving almandine, iron cordierite, sillimanite, and pure rnagnetite from experimental data. Inrnonucyronr At high temperatures, the oxidation products of aluminous silicates commonly include solid solutions of magnetite and hercynite. The exis-tence of a solvus in the system Feuon-FeAlron indicates that these solu-tions are non-ideal. consequently, thermodynamic calculations in the system Siq-AlrOa-FeO-O should involve activity coefficients. These may be derived frorn the solvus. It then becomes possible to calculate the standard free energy changes of several experimentally determined re-actions and by linear combinations obtain standard free energy changes of other reactions of interest. This information allows the calculation of phase-relations in the system SiOr-Al2Os-FeO-O. Essentially this proce-dure represents an extrapolation of relatively high temperatures experi-mental data to lower temperatures. This is of particular significance because, in the prcsence of quertz, the only stable oxide phase at lower temperatures is nearly pure magnetite. Thus it becomes possible to cal-culate oxidation reactions at metamorphic conditions involving almanding iron cordierite, sillimanite, and pure magnetite. Drnrver:ow or Acrnrrry Corrrrcrrxrs The unit cell of the spinel shucture contains 8 cations in 4-fold co-ordination (A positions) and 16 cations in 6-fo1d coordination (B posi-tions). In most spinels, rhe structure conforrns closely to either one of two types of cation distribution