Revision 2 1 The spin state of iron in Fe 3 +-bearing Mg-perovskite and its crystal chemistry at 2 high pressure 3 4

20 Valence, spin states and crystallographic sites of Fe in (Mg,Fe)SiO3 perovskite 21 were investigated using energy-domain Fe-synchrotron Mössbauer spectroscopy 22 and powder X-ray diffraction up to 86 GPa. The volumes of Fe-bearing 23 perovskite in this study are slightly smaller than those of Mg-endmember 24 perovskite. Our Mössbauer data suggest that Fe prefers A sites coupled with Mg 25 vacancies, which is consistent with previous data at ambient conditions. Fe in the 26 A site remains in a high spin state up to 86 GPa, and some fraction of the A site is 27 occupied by Fe at pressures above 30 GPa. Fe in the A sites is also in a high 28 spin state up to 86 GPa. The coupled substitution from Mg to a high spin state of 29 Fe and Mg vacancy would make the volume of perovskite smaller than that of 30 Mg-endmember perovskite. If the lower mantle is saturated in silica, perovskite 31 containing high spin Fe in A site has a higher density. Such silica oversaturated 32 regions could sink by the density difference. 33 34