The crystal structure of the dense hydrous magnesium silicate phase H, MgSiH2O4, synthesized at 45 GPa and 1000 °C, was investigated by single-crystal X-ray diffraction. Although showing a deterioration process under the X-ray beam, the compound was found to be orthorhombic, space group Pnnm (CaCl2-type structure), with lattice parameters a = 4.733(2), b = 4.3250(10), c = 2.8420(10) Å, V = 58.18(3) Å3, and Z = 1. The structure was refined to R1 = 0.0387 using 53 observed reflections [2σ(I) level]. Magnesium and silicon were found to be disordered at the same octahedral site (with a mean bond distance of 1.957 Å). Hydrogen was not located in the difference Fourier maps, but it is very likely disordered at a half-occupied 4g position. The centrosymmetric nature of the structure of phase H is examined in relation to that reported for pure δ-AlOOH at ambient conditions (non-centrosymmetric, P21nm), and the possibility that these two compounds can form a solid solution at least at high pressure is discussed.

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