Electronic spin state of Fe,Al-containing MgSiO3 perovskite at lower mantle conditions

Abstract

We have investigated silicate perovskite with composition Mg0.83Fe0.21Al0.06Si0.91O3 relevant for the lower mantle at pressures up to 81GPa and temperatures up to 2000K using conventional Mössbauer spectroscopy and synchrotron Nuclear Forward Scattering (NFS) combined with double-sided laser heating in a diamond anvil cell. Room temperature Mössbauer and NFS spectra at low pressure are dominated by high-spin Fe2+, with minor amounts of Fe3+ and a component assigned to a metastable position of high-spin Fe2+ in the A-site predicted by computational studies. NFS data show a sharp transition (<20GPa) from high-spin Fe2+ to a new component with extremely high quadrupole splitting, similar to previous studies. Mössbauer data show the same transition, but over a broader pressure range likely due to the higher pressure gradient. The new Fe2+ component is assigned to intermediate-spin Fe2+, consistent with previous X-ray emission studies. NFS data at high temperatures and high pressures comparable to those in the lower mantle are consistent with the presence of Fe2+ only in the intermediate-spin state and Fe3+ only in the high-spin state. Our results are therefore consistent with the occurrence of spin crossover only in Fe2+ in Fe-, Al-containing perovskite within the lower mantle.