Synthesis and crystal structure of chromium-bearing anhydrous wadsleyite

A chromium-bearing wadsleyite (Cr-Wad) was synthesized in the model system Mg2SiO4-MgCr2O4 at 14 GPa and 1600 A degrees C and studied from the chemical and structural point of views. Microprobe data gave the formula Mg1.930Cr0.120Si0.945O4, on the basis of 4 oxygen atoms. The crystal structure has been studied by single-crystal X-ray diffraction. The orthorhombic unit-cell parameters are: a = 5.6909(5) , b = 11.4640(10) , c = 8.2406(9) , V = 537.62(9) (3), Z = 8. The structure, space group Imma, was refined to R (1) = 5.99% in anisotropic approximation using 1135 reflections with F (o) > 4 sigma(F (o)) and 43 parameters. Chromium was found to substitute for both Mg at the octahedral sites and Si at the tetrahedral site, according to the reaction Mg-VI(2+) + Si-IV(4+) = Cr-VI(3+) + Cr-IV(3+). On the whole, the structural topology is nearly identical to that of pure wadsleyite. The successful synthesis of Cr-Wad may be important for the thermobarometry of mantle phase associations.