抄録
Re-examination of the run products in the Mg2SiO4–H2O system by Ohtani et al. (1995) revealed stability of dense hydrous magnesium silicate, phase G, and brucite at 20 GPa and 800°C, and stability of the assemblage with superhydrous phase B + phase G + fluid at 20 GPa and 1000°C under the water saturated conditions. We reported the Raman spectrum of phase G, which suggests existence of silicon ions in the six coordination sites. Brucite, superhydrous phase B, and phase G are stable at least up to 22.5 GPa and around 1000°C, and are the candidates for the hydrous phases in a cold slab descending into the base of the transition zone and the uppermost part of the lower mantle. The X-ray diffraction and the Raman spectra data revealed that the dense hydrous magnesium silicate, phase G, and phase D re-defined by Yang et al. (1997) and Kuroda and Irifune (1998) are identical. Phase F by Kanzaki (1991) and Gasparik (1993) is likely to be identical to phase G.
