Spin and valence states of iron in (Mg_0.8Fe_0.2) SiO₃ perovskite

The spin and valence states of iron in (Mg_0.8Fe _0.2)SiO₃ perovskite were measured between 0 and 65 GPa using synchrotron Mössbauer spectroscopy. Samples were synthesized in situ in die laser-heated diamond cell under reducing conditions. The dominant spin state of iron in perovskite is high spin at pressures below 50 GPa. Above 50 GPa, die spectra shows severe changes which can be explained by appearance of two distinct iron sites with similar site weightings. One site has Mössbauer parameters consistent with high spin Fe²⁺, while die other has the parameters previously interpreted as intermediate spin. The latter intermediate-spin assignment is not unique, as similar Mössbauer parameters have been reported for high spin Fe²⁺ in almandine at ambient pressure. However, our data do not rule out the existence of low-spin iron, which may exist with a smaller fraction and explain the observation of lower spin moments in the X-ray emission spectroscopy of perovskite at high pressure. From these considerations, our preferred interpretation is that iron in perovskite is mixed or high spin to at least 2000 km depths in the mantle, consistent with computational results. Our study also reveals that reducing conditions do not inhibit the formation of Fe³⁺ in perovskite at deep-mantle pressures