Empirical derivation of activity coefficients for the magnesium-rich portion of the olivine solid solution

Activity coefficients at 1100°C for Mg²⁺ and Fe²⁺ in the olivine solid solution, in the compositional range Fo 90-100, are derived empirically from the measured Mg-Fe distribution in a group of equilibrated iron meteorites that contain silicate phases. The solution exhibits a positive nonideality in this range and the results agree qualitatively with those of Nafziger and Muan (1967). Overall activity coefficients were calculated for the orthopyroxene solid solution (En 88-100) by derivation from recent Mössbauer data of Virgo and Hafner (1969). This solution exhibits a negative deviation from ideality. It is concluded that a petrologic thermometer based on the Mg-Fe distribution between olivine and coexisting orthopyroxene probably will not work with sufficient sensitivity for those Mg-rich rocks and meteorites which are more common and of greatest interest.