On the P–T–fO ₂ stability of Fe₄O₅, Fe₅O₆ and Fe₄O₅-rich solid solutions

The high-pressure phases Fe₄O₅ and Fe₅O₆ have recently been added to the list of known iron oxides. As mixed-valence phases, it has been suggested that they could form in the Earth’s mantle once the dominant minerals become saturated in ferric iron. The possibility that Fe₄O₅ could exist in the mantle is also supported by the fact that it forms extensive solid solutions with both Mg²⁺ and Cr³⁺. In this study, we present the results of high-pressure and high-temperature multi-anvil experiments performed between 5 and 24 GPa at 1000–1400 °C aimed at constraining the stability field of the Fe₄O₅ phase. We combine these results with published phase equilibria, equation of state and Fe–Mg partitioning data to estimate the thermodynamic properties of Fe₄O₅, Fe₅O₆ and the (Mg,Fe)₂Fe₂O₅ solid solution. Using our thermodynamic model, the oxygen fugacity at which the high-pressure iron oxides become stable is calculated and the redox stability of (Mg,Fe)₂Fe₂O₅ in an assemblage of olivine and pyroxene is calculated as a function of the bulk Fe/(Fe + Mg) ratio. Fe₄O₅ and (Mg,Fe)₂Fe₂O₅ are stable at oxygen fugacities higher than the diamond stability field and are, therefore, unlikely to be found as inclusions in diamonds. The stability field of Fe₅O₆, on the other hand, extends to oxygen fugacities compatible with diamond formation. Using the Mg–Fe solid solution model, we show that Fe₄O₅-structured phases would be restricted to aluminium-poor environments in the mantle such as dunites or silica–iron oxide-rich sediments transported into the mantle via subduction.