Effects of valence and spin of Fe in MgSiO3 melts: Structural insights from first-principles molecular dynamics simulations

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Iron (Fe) is present in terrestrial melts and at all depths inside the Earth. How Fe in its varying oxidation and spin states influences the properties of silicate melts is of critical importance to the understanding of the chemical evolution of our planet. Here, we report the results of first-principles molecular dynamics simulations of molten Fe-bearing MgSiO3 over a wide pressure range covering the entire mantle. Our results suggest that the structural properties of the host melt, such as the average bond length and coordination in Mg–O and Si–O do not differ much when compared with the pure melt. More importantly, they show that the local Fe–O structure is more sensitive to the spin state (high-spin, HS and low-spin, LS) of iron than to its valence state (Fe2+ and Fe3+). For iso-valence configurations, the average Fe–O bond length and coordination number differ by more than 10% and ∼30%, respectively, between the HS and LS states. In comparison, the corresponding differences between Fe2+ and Fe3+ for iso-spin configurations are within 5 and 15%, respectively. Ferrous iron shows lower average oxygen coordination numbers of ∼3.8 for HS and ∼3.3 for LS compared to the corresponding numbers of ∼4.1 and ∼3.7 for ferric iron at 0 GPa and 3000 K. As pressure increases, the coordination gap between the ferrous and ferric iron closes for HS but persists for LS. Our analysis of the proportions of non-bridging and bridging oxygens and the rates of bond breaking/formation events suggests an equivalent role of the ferrous and ferric iron in terms of their network forming ability. The predicted structural behavior of iron in its different oxidation states is generally consistent with the experimental inferences for MgO–FeO–SiO2 melts. Unlike other ferrosilicate compositions for which the experimental data suggest that Fe3+ increases and Fe2+ decreases the viscosity of the melt, the ferrous and ferric iron, due to their structural equivalence, are likely to have a similar influence on the dynamical behavior of deep mantle iron-bearing MgSiO3 melts.