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Spin crossovers in iron bearing minerals of the lower mantle

Spin crossover of iron is of central importance in solid Earth geophysics. It impacts all physical properties of minerals that altogether constitute ∼ 95 vol% of the Earth’s lower mantle: ferropericlase [(Mg,Fe)O] and Fe-bearing magnesium silicate (MgSiO3) perovskite.[1]

Using density functional theory+Hubbard U (DFT+U) calculations, we investigate how aluminum affects the spin crossover of iron in MgSiO3 perovskite (Pv) and post-perovskite (Ppv), the major mineral phases in the Earth’s lower mantle. We find that the presence of aluminum does not change the response of iron spin state to pressure: only ferric iron (Fe3+) in the octahedral (B)-site undergoes a crossover from high-spin (HS) to low-spin (LS) state, while Fe3+ in the dodecahedral (A)-site remains in the HS state, same as in Al-free cases. However, aluminum does significantly affect the placement of Fe3+ in these mineral phases. The most stable atomic configuration has all Al3+ in the B-site and all Fe3+ in the A-site (thus in the HS state). Metastable configurations with LS Fe3+ in the B-site can happen only at high pressures and high temperatures. Therefore, experimental observations of LS Fe33+ at high pressures in Al-bearing Pv require diffusion of iron from the A-site to the B-site and should be sensitive to the annealing temperature and schedule. In the Earth’s lower mantle, the elastic anomalies accompanying the B-site HS-LS crossover exhibited in Al-free Pv are likely to be considerably reduced, according to the B-site Fe3+ population.[2]

[1] N. Tosi, D. A. Yuen, N. de Koker, and R. M. Wentzcovitch, Mantle dynamics with pressure- and temperature-dependent thermal expansivity and conductivity, Phys. Earth & Planet. Int. 217, 48–58 (2013). DOI:10.1016/j.pepi.2013.02.004

[2] H. Hsu, Y. Yu, and R. M. Wentzcovitch, Spin crossover in iron in aluminous MgSiO3 persovskite and post-perovskite, Earth & Planet. Sc. Lett. 359-360, 34-39 (2012). DOI: 10.1016/j.epsl.2012.09.029