Compositions of coexisting solid-solution phases in the system MgO-“FeO”-SiO2 at low oxygen partial pressures have been determined at solidus and subsolidus temperatures. The data have been used to derive activity-composition relations for the silicate solid solutions within the system, and stability data for end-member compounds. The olivine solid solution has a moderate positive deviation from ideality, whereas the pyroxene solid solution is practically ideal in its activity-composition relation.

The data obtained provide means of checking and refining some of the assumptions which have been made by previous authors in dealing with problems of the distribution of Fe2+ and Mg2+ among rock-forming minerals. Comparison of the data obtained in the present investigation with those available for meteorites and terrestrial rocks may also be used as a valuable guide to judge whether equilibrium among selected phases has been approached in some of these natural bodies.

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