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 multi-anvil 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, i.e., ~2-3 mol% and <2 mol% with and without 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 ~0.025 pfu, whereas the charge-coupled mechanism occurs when Fe3+ > 0.025 pfu.
Mg (open symbols) and Si (filled symbols) pdf in bridgmanite (O = 3) as a function of the Fe3+ content in MgO- and Fe2O3-rich samples. Small orange and dark blue symbols are from Hummer and Fei (2012). Thin solid and dashed lines indicate theoretical concentrations of Mg, Si pfu by FeFeO3 and MgFeO2.5 substitutions, respectively. Crossovers of Mg, Si pfu in MgO-rich samples (red lines) and theoretical calculations of pure MgFeO2.5 substitution (thin dashed lines) occur at Fe3+ ≈ 0.025 pfu.
Fei, H., Liu, Z., McCammon, C., Katsura, T., Oxygen vacancy substitution linked to ferric iron in bridgmanite at 27 GPa. Geophysical Research Letters, 2020. doi.org/10.1029/2019GL086296