EXPERIMENTAL-DETERMINATION OF ACTIVITIES IN FE-MG OLIVINE AT 1400-K

The exchange equilibrium MgSi0.5O2+FeO = MgO+FeSi0.5O2 olivine oxide oxide olivine has been used to measure activity-composition relations along the olivine join FeSi0.5O2 - MgSi0.5O2 at 1400 K and 1 atm pressure. Equilibrium Fe-Mg partitioning between the two phases was determined by reversing the composition of olivine coexisting with oxide and metallic iron over the composition range FO23 to FO92. A detailed study of the thermodynamic properties of the oxide phase has recently been made by Srecec et al. and we have confirmed their results in the composition range of interest. Application of the oxide data to the exchange equilibrium enables the properties of olivine to be determined. Within experimental uncertainty (Fe,Mg)Si0.5O2 olivine can, at 1400 K, be treated as a symmetric solution with W(Fe-Mg)ol of 3.7 +/- 0.8 kJ/mol. The data permit the presence of only very slight asymmetry in the series. The data do not support recent assertions that olivine is highly non-ideal (W almost-equal-to 10 kJ/mol) under these conditions.