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After (Mg,Fe)SiO₃ perovskite, (Mg,Fe)O is likely the next most abundant mineral in the lower mantle. The crystal chemistry of (Mg,Fe)O, particularly with respect to Fe³⁺, is important for understanding properties of the lower mantle such as electrical conductivity. We have already measured the extent of Fe³⁺ solubility in Fe_0.2Mg_0.8O at 1 bar as a function of temperature and fO₂, (see Annual Report 1993), and developed a technique for determining accurate values for Fe³⁺/Σ Fe in (Fe,Mg)O using Mössbauer spectroscopy and X-ray diffraction. We found that the Fe³⁺ composition is a strong function of fO₂, where large amounts of Fe³⁺ (Fe³⁺/Σ Fe > 10%) can be incorporated in the structure at high fO₂.

To study the solubility of Fe³⁺ in (Mg,Fe)O at high P,T, two samples of Fe_0.2Mg_0.8O were prepared, one with a high Fe³⁺ content and one containing minimum Fe³⁺. Both were run in a multianvil press at 18 GPa and 1000 °C at two different oxygen fugacities: a) in equilibrium with Fe (reduced); and b) in equilibrium with a Re-ReO₂ buffer (oxidised). In all high pressure experiments, the amount of Fe³⁺ found in the quenched phases using both Mössbauer spectroscopy and X-ray diffraction was extremely small, including in the phase starting with a high Fe³⁺ content and run under oxidising conditions. This implies that the solubility of Fe³⁺ becomes very low in Fe_0.2Mg_0.8O at high pressure, regardless of fO₂. The difference in solubility of Fe³⁺ between high and low pressure is shown in Fig. 3.3-8, illustrating the dramatic decrease in Fe³⁺ solubility at high pressure. One explanation for this behaviour is the presence of high-pressure phase transitions in both Fe₃O₄ and MgFe₂O₄ which stabilises oxygen in the spinel phase at high pressure, hence producing a more stoichiometric (Mg,Fe)O composition.
 

Fig. 3.3-8: Solubility of Fe3+ in (Mg,Fe)O at 1000 °C as a function of composition at atmospheric pressure (left) and at 18 GPa (right). Phase boundaries in the left figure and data for FexO in the right figure are derived from data in the literature. No data are available yet for iron-rich (Mg,Fe)O at 18 GPa and high oxygen fugacity.