High‐pressure stability of a dense hydrous magnesian silicate Mg23Si8O42H6 and some geophysical implications

Phase equilibria in the system MgO‐SiO2‐H2O have been studied in the pressure range 85–160 kbar in the temperature range 750°–1200°C. In runs for a gel with composition of 64.1 wt % MgO, 31.9 wt % SiO2, 4.0 wt % H2O (atomic ratio Mg/Si = 3.0) a hydrous magnesian silicate, denoted phase B by Ringwood and Major, was found to exist from about 100 kbar to at least 160 kbar at temperatures 750°–1200°C. At pressures be1ow about 100 kbar the assemblage of phase A(Mg7Si2O14H6) + forsterite + periclase was stable at relatively low temperatures, while the assemblage of chondrodite (Mg5Si2O10H2) + periclase + forsterite was stable at relatively high temperatures. X ray diffraction shows that phase B is monoclinic (space group, P21/c) with cell constants a = 10.600(2) Å, b = 14.098(2) Å, c = 10.092(2) Å, and β = 104.05(2)°. The chemical formula of phase B was deduced to be Mg23Si8O42H6. The observed density, 3.32±0.04 g/cm3, is the highest of the hydrous silicate phases reported in the system MgO‐SiO2‐H2O and is about 3% higher than that of forsterite. While this study indicates the stability of phase B for bulk compositions which are richer in MgO than accepted mantle models, it is regarded as a potential hydrous silicate phase in the upper mantle and transition zone in the depth range of 300 km to at least 460 km.