Tetrahedral Occupancy of Ferric Iron in (Mg,Fe)O at High Pressures

(Mg,Fe)O, the second most abundant phase in the lower mantle, plays important roles in the dynamics of the deep Earth. The presence of ferric iron (Fe3+) in (Mg,Fe)O can have significant effects on its transport properties such as atomic diffusivity, electrical conductivity, and viscosity, since the distribution of Fe3+ affects the equilibrium concentration of point defects. Fe3+ is generally assumed to substitute into octahedrally-coordinated cation sites in (Mg,Fe)O. Our previous thermodynamic analysis, however, suggested that Fe3+ occupies tetrahedrally-coordinated interstitial sites at high pressures. Two different sites of Fe3+ occupancy are predicted by different pressure dependence of concentration of Fe3+. Therefore, more detailed studies on the atomistic mechanisms of Fe3+ dissolution are necessary to provide us with some insight into the transport properties of the lower mantle. In this study, we have conducted Mössbauer spectroscopy of (Mg,Fe)O single crystals synthesized at 1200-2000 °C and 5-15 GPa under different oxygen fugacity using a multianvil apparatus. The Mössbauer spectra were recorded at room temperature in the transmission mode. Each obtained Mössbauer spectrum consists of a dominant quadrupole doublet of ferrous iron (Fe2+) superimposed with a minor peak of ferric iron (Fe3+). The site occupation and concentration of Fe3+ are estimated based on the position of the isomer shift and the absorption intensity. These data suggest that 1) Fe3+ occupies octahedral sites at low pressures and temperatures while it occupies tetrahedral sites at high pressures and temperatures, 2) Fe3+ solubility decreases with temperature in the regime of octahedral occupancy, while it slightly increases in the regime of tetrahedral occupancy. Based on these results, we infer that Fe3+ in (Mg,Fe)O predominantly occupies tetrahedral sites in the lower mantle.