Liu (1986, 1987) reported the diffraction pattern of a hydrous magnesium silicate resulting from the breakdown of serpentine at 22 GPa and 1000 degrees C and named this material "phase D." Since that time there have been several reports of the synthesis of phase D, but its confirmed composition and crystal structure have not yet been reported. We synthesized a new dense hydrous magnesium silicate at 20 GPa and 1200 degrees C and solved its crystal structure (R w = 0.015 and R = 0.014). The single crystal has composition Mg (sub 1.11) Si (sub 1.89) H (sub 2.22) O 6 (ideal formula: MgSi 2 H 2 O 6 ), cell parameters a = 4.7453(4), c = 4.3450(5) Aa, and V = 84.74(2) Aa 3 , and space group P31m. The crystal structure is relatively simple with all the Si occupying octahedral sites in a layer similar to that of brucite, but with one of every three octahedra vacant. The MgO 6 octahedra are located above and below each vacant octahedral site. All O-H bonding occurs between SiO 6 octahedral layers. This is the only high-pressure hydrous magnesium silicate structure reported to date that contains all octahedrally coordinated Si. The calculated density of phase D (d cal = 3.50 g/cm 3 ) is substantially greater than any other high-pressure hydrous magnesium silicate phase.