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 (Mg 1- x Cr x )(Si 1- x Cr x )O 3 perovskite (with x = 0.07) synthesized in the model system Mg 3 Cr 2 Si 3 O 12 -Mg 4 Si 4 O 12 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) 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 Mg 2+ + Si 4+ = 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 MgSiO 3 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.