Spin transition and substitution of Fe3+ in Al-bearing post-Mg-perovskite

The spin transition and substitution of Fe3+ in Mg0.85Fe3+0.15Al0.15Si0.85O3 post-Mg-perovskite (PPv) synthesized at 165–170 GPa, 2100 K were examined by X-ray emission spectroscopy (XES) and X-ray diffraction (XRD) during decompression to 37 GPa without annealing. XRD showed that samples were single phase PPv down to 49 GPa, although the samples may have partly become amorphous. The result of XES measurement indicates that Fe3+ is fully low spin (LS) for 165–100 GPa and Fe3+ largely changes from LS to high spin (HS) between 80 and ∼40 GPa. These results combined with previous reports indicate that Al and LS Fe3+ may occupy the A-(dodecahedral) and B-(octahedral) sites, respectively, as the favorable cation sites for 165–100 GPa in Al-bearing PPv. Based on the present results on Fe3+ and recent theoretical reports on Fe2+, the spin states of iron in lower mantle PPv are discussed.

► Spin state of Fe3+ in Al-bearing post-Mg-perovskite was first examined up to 165 GPa. ► Fe3+ is fully low spin and occupies the B-site at pressures above 100 GPa. ► Fe3+–Al coupled substitution is the possible mechanism for Fe3+ substitution. ► High spin-low spin transition of Fe3+ at the B-site occurs below 80 GPa.