Chromium solubility in anhydrous Phase B
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The crystal structure and chemical composition of a crystal of (Mg14−x Cr x )(Si5−x Cr x )O24 (x ≈ 0.30) anhydrous Phase B (Anh-B) synthesized in the model system MgCr2O4–Mg2SiO4 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) Å3 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 Mg2+ + Si4+ = 2Cr3+. 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 Fe2+-bearing Anh-B. Taking into account that Cr3+ is not expected to be Jahn–Teller active, it appears that both the Cr3+–for–Mg and Cr3+–for–Si substitutions in the Anh-B structure decrease the distortion of the octahedra. Electron microprobe analysis gave the Mg13.66(8)Si4.70(6)Cr0.62(4)O24 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.
KeywordsAnhydrous Phase B Chromium Upper mantle X discontinuity Crystal structure Microprobe analysis Synthesis
The manuscript took advantage from the revision of Kenneth Collerson and Sabrina Nazzareni. The research was supported by “progetto di Ateneo 2013, University of Firenze” to LB, by C.N.R., Istituto di Geoscienze e Georisorse sezione di Firenze, Italy, the Russian Foundation for Basic Research (Project No. 15-55-50033 YaF) to ES and AB. ES thanks Geodynamics Research Center, Ehime University, Matsuyama, Japan, for support of her visit in 2014. FN thanks the ERC Starting Grant 2012 (Grant agreement No. 307322).
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