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 below 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) ¿, b=14.098(2) ¿, c=10.092(2) ¿, 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.