X-ray diffraction and Mössbauer spectroscopy of Fe³⁺- bearing Mg-silicate post-perovskite at 128-138 GPa

The effect of ferric iron on the properties of Mg-silicate post-perovskite (PPv) were studied up to 138 GPa using synchrotron X-ray diffraction and Mössbauer spectroscopy. Our diffraction measurements revealed that the incorporation of Fe³⁺ has virtually no effect on the volume of PPv, in contrast to Fe²⁺, which increases the volume. Therefore, incorporation of Fe3+ increases the density of PPv much more effectively than Fe²⁺. Mössbauer spectroscopy suggests that Fe³⁺ enters PPv through charge-coupled substitution and is high spin in the bipolar prismatic site and low spin in the octahedral site (i.e., mixed spin state). Our results may have important implications for the gravitational stability of lower-mantle heterogeneities.