Oxygen Vacancy Substitution Linked to Ferric Iron in Bridgmanite at 27 GPa

Ferric iron can be incorporated into the crystal structure of bridgmanite by either oxygen vacancy substitution (MgFeO2.5 component) or charge‐coupled substitution (FeFeO3 component) mechanisms. We investigated the concentrations of MgFeO2.5 and FeFeO3 in bridgmanite in the MgO‐SiO2‐Fe2O3 system at 27 GPa and 1700–2300 K using a multianvil apparatus. The FeFeO3 content increases from 1.6 to 7.6 mol.% and from 5.7 to 17.9 mol.% with and without coexistence of (Mg,Fe)O, respectively, with increasing temperature from 1700 to 2300 K. In contrast, the MgFeO2.5 content does not show clear temperature dependence, that is, ~2–3 and 0.025 pfu. Plain Language Summary Bridgmanite, the most abundant mineral of the Earth's lower mantle, can contain Fe3+ although the valance of iron is 2+ in general. An important question is how Fe3+ is substituted in the crystal structure of bridgmanite. It may form the MgFeO2.5 component, in which oxygen anions are partly missing. Or it may form the FeFeO3 component, which has no missing cations or anions. Since bridgmanite is present in the lower mantle together with (Mg,Fe)O, we investigated the MgFeO2.5 and FeFeO3 contents in Al‐free bridgmanite that coexists with and without (Mg,Fe)O under the topmost lower mantle conditions. The results show that the presence of (Mg,Fe)O enhances the formation of MgFeO2.5. The solubility of MgFeO2.5 component is about 2.5 mol.% in bridgmanite that coexists with (Mg,Fe)O, whereas it is nearly zero when (Mg,Fe)O is absent. Key Points The presence of (Mg,Fe)O enhances the formation of the MgFeO2.5 component in Al‐free bridgmanite Fe3+ substitution predominantly follows the oxygen vacancy mechanism in (Mg,Fe)O‐saturated Al‐free bridgmanite when Fe3+ content is low The solubility of the MgFeO2.5 component in Al‐free bridgmanite is about 0.025 pfu, and relatively insensitive to temperature