STRUCTURE AND CRYSTAL-CHEMISTRY OF PHASE G, A NEW HYDROUS MAGNESIUM-SILICATE SYNTHESIZED AT 22 GPA AND 1050 DEGREES-C

A single crystal of phase G, Mg1.24Si1.76H2.48O6 synthesized at condit ions of 1050 degrees C and 22 GPa, using a multi-anvil apparatus was s tudied at the Photon Factory BL 10A beamline at the National Laborator y for High Energy Physics, Tukuba, Japan. With a (111) Si double-cryst al monochromator and a single crystal measuring 47x35x12 mu m, intensi ties of 95 independent reflections were collected with sin theta/lambd a<0.72 Angstrom(-1) and Io >1.5 sigma Io at a wave length of 0.6990 An gstrom. The unit cell parameters obtained through the refinement of 23 reflections are: Trigonal, a=4.790(3) Angstrom, c=4.344(3) Angstrom, V=86.3(2) Angstrom(3). The result of structure analysis in space group P (3) over bar 1m (No.162) indicates that the structure of phase G ha s close structural similarity to that of stishovite,SiO2. The calculat ed density of phase G is 3.43 g/cm(3), which is larger than any other known dense hydrous magnesium silicate, suggesting that phase G might be stable even under lower mantle conditions.