SINGLE-CRYSTAL STRUCTURE DETERMINATION OF (MG,FE)SIO3 POSTPEROVSKITE

The discovery of postperovskite (ppv) transition in magnesium silicate (MgSiO₃), the dominant mineral in Earth's mantle, sheds light on the origin of the D′′ layer. However, our knowledge of lower mantle phase transitions at high pressure (P) and temperature (T) conditions has been based on limited information provided by powder X-ray diffraction technique and theoretical calculations. Experimental determination of single-crystal structure of ppv was not possible because the conventional single-crystal methods require single crystals with a minimum size of 5-10 microns that are unachievable in phase transition studies at ultrahigh pressures. Here, we report the first in situ single-crystal structure determination of (Mg,Fe)SiO₃ ppv under high P in a diamond anvil cell corresponding to the condition in the Earth’s D² layer. Using a newly developed multigrain single-crystal X-ray diffraction analysis technique in a diamond anvil cell, crystallographic orientations of over 100 crystallites were simultaneously determined in a coarse-grained polycrystalline sample containing submicron ppv grains. Conventional single-crystal structural analysis and refinement methods were applied to a few selected ppv crystallites, which demonstrate the feasibility of in situ study of crystal structures of submicron crystallites in a multiphase polycrystalline sample contained within a high P device. The similarity of structural models for single-crystal Fe-bearing ppv (∼10 mol% Fe) and Fe-free ppv from previous theoretical calculations suggests that the Fe content in the mantle has a negligible effect on the crystal structure of the ppv phase.