Spin state of ferric iron in MgSiO3 perovskite and its effect on elastic properties

Abstract

Abstract Recent studies have indicated that a significant amount of iron in MgSiO 3 perovskite (Pv) is Fe 3+ (Fe 3+ /ΣFe = 10–60%) due to crystal chemistry effects at high pressure ( P ) and that Fe 3+ is more likely than Fe 2+ to undergo a high-spin (HS) to low-spin (LS) transition in Pv in the mantle. We have measured synchrotron Mossbauer spectroscopy (SMS), X-ray emission spectroscopy (XES), and X-ray diffraction (XRD) of Pv with all iron in Fe 3+ in the laser-heated diamond-anvil cell to over 100 GPa. Fe 3+ increases the anisotropy of the Pv unit cell, whereas Fe 2+ decreases it. In Pv synthesized above 50 GPa, Fe 3+ enters into both the dodecahedral (A) and octahedral (B) sites approximately equally, suggesting charge coupled substitution. Combining SMS and XES, we found that the LS population in the B site gradually increases with pressure up to 50–60 GPa where all Fe 3+ in the B site becomes LS, while Fe 3+ in the A site remains HS to at least 136 GPa. Fe 3+ makes Pv more compressible than Mg-endmember below 50 GPa because of the gradual spin transition in the B site together with lattice compression. The completion of the spin transition at 50–60 GPa increases bulk modulus with no associated change in density. This elasticity change can be a useful seismic probe for investigating compositional heterogeneities associated with Fe 3+ .