Crystal chemistry of dense hydrous magnesium silicates: The structure of phase H, MgSiH2O4, synthesized at 45 GPa and 1000 °C

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) ų, 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.