Structure and crystal chemistry of phase G, a new hydrous magnesium silicate synthesized at 22 GPa and 1050 °C

A single crystal of phase G, Mg_1.24Si_1.76H_2.48O₆ synthesized at conditions of 1050°C and 22 GPa, using a multi-anvil apparatus was studied at the Photon Factory BL-10A beamline at the National Laboratory, for High Energy Physics, Tukuba, Japan. With a (111) Si double-crystal monochromator and a single crystal measuring 47×35×12 μ m, intensities of 95 independent reflections were collected with sin θ/λ<0.72 Å^-1 and Io >1.5 σ Io at a wave length of 0.6990 Å. The unit cell parameters obtained through the refinement of 23 reflections are: Trigonal, a=4.790(3) Å, c=4.344(3) Å, V=86.3(2) ų. The result of structure analysis in space group P3 1m (No.162) indicates that the structure of phase G has close structural similarity to that of stishovite,SiO₂. The calculated density of phase G is 3.43 g/cm³, which is larger than any other known dense hydrous magnesium silicate, suggesting that phase G might be stable even under lower mantle conditions.