The magnesiowüstite: Iron equilibrium and its implications for the activity-composition relations of (Mg,Fe)₂SiO₄ olivine solid solutions

The chemical potential of oxygen (μO₂) in equilibrium with magnesiowüstite solid solution (Mg, Fe)O and metallic Fe has been determined by gas-mixing experiments at 1,473 K supplemented by solid-cell EMF experiments at lower temperatures. The results give: μO₂(kJ/mol) = μO₂(IW) + 2 (X_MgO ²[(5.76 + 3.47 × 10^-3T) + 0.41(3 - 4X_MgO)] where IW refers to the Fe-"FeO" equilibrium. The previous work of Srecec et al. (1987) and Wiser and Wood (1991) agree well with this equation, as does that of Hahn and Muan (1962) when their reported compositions are corrected to a new calibration curve for lattice parameter vs. composition. The amount of Fe³⁺ in the magnesiowüstite solid solution in equilibrium with Fe metal was determined by Mössbauer spectroscopy on selected samples. These data were combined with literature data from gravimetric studies and fitted to a semi-empirical equation: X_Fe³⁺_O1.5 = (1 - X_MgO ^3/2[0.103 - 0.203X_MgO + 0.213 (X_MgO ²] These results were then used to reassess the activity-composition relations in (Mg, Fe)₂SiO₄ olivine solid solutions at 1,400 K, from the partitioning of Mg and Fe²⁺ between olivine and magnesiowüstite in equilibrium with metallic Fe experimentally determined by Wiser and Wood (1991). The olivine solid solution is constrained to be nearly symmetric with W^ol_Mg-Fe = 2.6 kJ/mol, with a probable uncertainty of less than ± 0.5 kJ/mol (one standard deviation). The results also provide a useful constraint on the free energy of formation of Mg₂SiO₄.