The 13OO0C isotherm of the system MgO-FeOFe203 was studied thermogravimetrically under conditions of controlled fugacity of oxygen. The magnesioferrite solid solutions that coexist in equilibrium with magnesiowiistite solid solutions have a cation to anion ratio experimentally indistinguishable, in the present study, from 0.750. Curves are presented expressing the change in composition of coexisting magnesioferrite and magnesiowiistite phases as a function of temperature and oxygen fugacity. These curves for the system Mg0-Fe0-Fez03 can b e used (1) to predict compositional relations under isothermal conditions for temperatures where reaction rates are sluggish, (2) as additional oxygen buffer curves for experimental control of oxygen fugacity, and (3) in certain instances to distinguish equilibrium from nonequilibrium assemblages.

The quenching technique has been used to determine equilibrium relations in the system manganese oxide‐Cr2O3 in air in the temperature range 600° to 1980°C. The following isobaric invariant situations have been determined: At 910°± 5°C tetragonal Mn3O4 solid solution, cubic Mn3O4 solid solution (=spinel), Mn2O3 solid solution, and gas coexist in equilibrium. Cubic Mn3O4 solid solution, Cr2O3 solid solution, liquid, and gas are present together in equilibrium at 1970°± 20°C. The invariant situation at which cubic Mn3O4 solid solution, Mn2O3 solid solution, Cr2O3 solid solution, and gas exist together in equilibrium is below 600°C.

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