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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, similar to 2-3 and < 2 mol.% with and without the coexistence of (Mg,Fe)O, respectively. Therefore, the presence of (Mg,Fe)O enhances the oxygen vacancy substitution for Fe3+ in bridgmanite. It is predicted that Fe3+ is predominantly substituted following the oxygen vacancy mechanism in (Mg,Fe)O-saturated Al-free bridgmanite when Fe3+ is below similar to 0.025 pfu, whereas the charge-coupled mechanism occurs when Fe3+ > 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.