Abstract

The crystal structure and chemical composition of a crystal of (Mg1–xCrx)(Si1–xCrx)O3 perovskite (with x = 0.07) synthesized in the model system Mg3Cr2Si3O12–Mg4Si4O12 at 23 GPa and 1600 ºC 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) Å, V = 164.55(3) Å3. 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 Å) and Si at the octahedral Y site (mean: 1.814 Å), according to the reaction Mg2+ + Si4+ = 2Cr3+. 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.

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