Spin transition of ferric iron in Al-bearing Mg–perovskite up to 200 GPa and its implication for the lower mantle

Abstract

There are still large discrepancies among the previous reports on the spin transition of iron in Mg–perovskite (Pv). To alleviate this problem, we examined the spin state of Fe3+ in Mg0.85Fe3+0.15Al0.15Si0.85O3 Pv up to 200GPa by X-ray emission spectroscopy (XES) and X-ray diffraction (XRD). The gradual decrease of the high spin (HS) ratio of Fe3+ by low temperature annealing of the samples above ~60GPa in XES and the change of the trend of unit cell volumes with pressure by annealing at 50–60GPa in XRD indicate that Fe3+ occupies the A-site and is HS below ~50GPa but above 50–60GPa it gradually replaces Al at the B-site through cation exchange reaction by annealing and becomes low spin (LS), while Fe3+ remaining at the A-site is HS up to 200GPa. This means that the spin state of Fe3+ depends on Fe3+ occupancies between the A- and B-sites and these Fe3+ occupancies are strongly controlled by the synthesis condition and annealing temperature of the samples through the cation exchange reaction. The present results combined with the previous reports indicate that in Al-bearing Mg–Pv in the lower mantle Fe2+ occupies the A-site and remains HS for the whole lower mantle, while Fe3+ occupies the A-site and is HS below ~50GPa but above 50–60GPa it replaces Al at the B-site and becomes LS, on the assumption that spin transition pressure of Fe2+ at the A-site is higher than that of Fe3+ at the same site.