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, Mg1.24Si1.76H2.48O6 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 µm, 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 P 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.