Chromium solubility in perovskite at high pressure: The structure of (Mg1-xCrx)(Si1-xCrx)O3 (with x = 0.07) synthesized at 23 GPa and 1600 °C

The crystal structure and chemical composition of a crystal of (Mg1-xCrx)(Si1-xCrx)O-3 perovskite (with x = 0.07) synthesized in the model system Mg3Cr2Si3O12-Mg4Si4O12 at 23 GPa and 1600 C have been investigated. The compound was found to be orthorhombic, space group Pbnm, with lattice parameters a = 4.8213(5). b = 4.9368(6), c = 6.9132(8) angstrom, V= 164.55(3) angstrom(3). The structure was refined to R = 0.046 using 473 independent reflections. Chromium was found to substitute for both Mg at the dodecahedral X site (with a mean bond distance of 2.187 angstrom) and Si at the octahedral Y site (mean: 1.814 angstrom), according to the reaction Mg2+ Si4+ = 2Cr(3+). Such substitutions cause a shortening of the < X-O > and a lengthening of the < Y-O > distances with respect to the values typically observed for pure MgSiO3 perovskite. Although high Cr-contents are not considered in the pyrolite model. Cr-bearing perovskite may be an important host for chromium in the lower mantle. The successful synthesis of perovskite with high-Cr content and its structural characterization are of key importance because the study of its thermodynamic constants combined with the data on phase relations in the lower-mantle systems can provide new constraints on thermobarometry of perovskite-bearing assemblages.