Phase Equilibria in the System MgO‐FeO‐Fe 2 O 3 : The 1300°C Isothermal Section and Extrapolations to Other Temperatures

The 1300°C isotherm of the system MgO–FeO–Fe 2 O 3 was studied thermogravimetrically under conditions of controlled fugacity of oxygen. The magnesioferrite solid solutions that coexist in equilibrium with magnesiowüstite 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 magnesiowüstite phases as a function of temperature and oxygen fugacity. These curves for the system MgO‐FeO‐Fe 2 O 3 can be 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.