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

H.StC. O'Neill, M.I. Pownceby and C.A. McCammon

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 1473 K supplemented by solid cell EMF experiments at lower temperatures. The results give:

µO2 (kJ/mole) = µO2 (IW) + 2 (XMgO)2 [(5.76+3.47x10-3T)+0.41(3-4XMgO)]

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:   

XFeO1.5 = (1-XMgO)3/2 [0.103 - 0.203 XMgO + 0.213 (XMgO)2

These results were then used to reassess the activity-composition relations in (Mg,Fe)2SiO4 olivine solid solutions at 1400 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 WMg-Fe(ol) = 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.