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 &mgr;m, intensities of 95 independent reflections were collected with sin &thgr;/&lgr;1.5 &sgr; Io at a wave length of 0.6990 ¿. The unit cell parameters obtained through the refinement of 23 reflections are: Trigonal, a=4.790(3) ¿, c=4.344(3) ¿, V=86.3(2) ¿3. 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.¿ 1997 American Geophysical Union