Abstract
Sufficient high-temperature data delineating the equilibrium compositions of coexisting phases in the Mg-Fe-O-SiO2 system have become available to permit calculation of activity-composition relations of the important spinels, olivines, and pyroxenes. Three methods of calculation involving (1) the thermodynamic properties of equilibrium magnesiowüstites, (2) spinel-hematite equilibria, and (3) magnesiowüstite defect data are employed. Within estimated uncertainties, the spinels generally exhibit negative deviations from ideality at 850°C and 1300°C, whereas the activity-composition relations at 1160°C are less well defined due to greater uncertainties in phase composition data. Spinel data calculated by methods 1 and 3, and 2 and 3, agree within the uncertainties inherent in the calculations at 1300°C. These uncertainties in the calculated data preclude meaningful correlations with temperature. At 1160°C, olivines and pyroxenes in equilibrium with spinels exhibit positive deviations from ideality, with olivines showing a more significantly pronounced deviation and pyroxenes showing near-ideality. This is similar to previous determinations at 1200-1250°C at lower oxygen fugacities where the silicate phases are in equilibrium with metallic iron. At 1300°C. olivines and pyroxenes in equilibrium with spinels show nearly ideal or slightly negative deviations from ideality in their activity-composition relations. Internal consistency of the data calculated from different equilibrium phase assemblages is demonstrated for the olivines.
