Chromium solubility in anhydrous Phase B

• Published in: Physics and chemistry of minerals 2016-02, Vol.43 (2), p.103-110
• Main Authors: Bindi, Luca, Sirotkina, Ekaterina A., Bobrov, Andrey V., Nestola, Fabrizio, Irifune, Tetsuo
• Format: Article
• Language: English
• Subjects: Chemical composition; Chromium; Crystal structure; Crystallography and Scattering Methods; Crystals; Earth and Environmental Science; Earth mantle; Earth Sciences; Electron probe microanalysis; Electron probes; Geochemistry; Jahn-Teller effect; Lattice parameters; Mineral Resources; Mineralogy; Organic chemistry; Original Paper; Silicon; Stoichiometry; Subduction (geology); Trivalent chromium; Upper mantle
• ISSN: 0342-1791; 1432-2021
• DOI: 10.1007/s00269-015-0777-2

The crystal structure and chemical composition of a crystal of (Mg 14− x Cr x )(Si 5− x Cr x )O 24 ( x  ≈ 0.30) anhydrous Phase B (Anh-B) synthesized in the model system MgCr 2 O 4 –Mg 2 SiO 4 at 12 GPa and 1600 °C have been investigated. The compound was found to be orthorhombic, space group Pmcb , with lattice parameters a  = 5.900(1), b  = 14.218(2), c  = 10.029(2) Å, V  = 841.3(2) Å 3 and Z  = 2. The structure was refined to R 1  = 0.065 using 1492 independent reflections. Chromium was found to substitute for both Mg at the M3 site (with a mean bond distance of 2.145 Å) and Si at the octahedral Si1 site (mean bond distance: 1.856 Å), according to the reaction Mg 2+  + Si 4+  = 2Cr 3+ . Such substitutions cause a reduction in the volume of the M3 site and an increase in the volume of the Si-dominant octahedron with respect to the values typically observed for pure Anh-B and Fe 2+ -bearing Anh-B. Taking into account that Cr 3+ is not expected to be Jahn–Teller active, it appears that both the Cr 3+ –for–Mg and Cr 3+ –for–Si substitutions in the Anh-B structure decrease the distortion of the octahedra. Electron microprobe analysis gave the Mg 13.66(8) Si 4.70(6) Cr 0.62(4) O 24 stoichiometry for the studied phase. The successful synthesis of this phase provides new information for the possible mineral assemblages occurring in the Earth’s deep upper mantle and shed new light on the so-called X discontinuity that has been observed at 275–345 km depth in several subcontinental and subduction zone environments.