CRYSTAL-STRUCTURE OF THE DENSE HYDROUS MAGNESIUM-SILICATE, PHASE-D

Liu (1986, 1987) reported the diffraction pattern of a hydrous magnesi um silicate resulting from the breakdown of serpentine at 22 GPa and 1 000 degrees C and named this material ''phase D.'' Since that time the re have been several reports of the synthesis of phase D, but its conf irmed 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 Mg1.11Si1.89H2.22O6 (ideal formu la: MgSi2H2O6), cell parameters a = 4.7453(4), c = 4.3450(5) Angstrom, and V = 84.74(2) Angstrom(3), and space group P (3) over bar 1m. The crystal structure is relatively simple with all the Si occupying octah edral sites in a layer similar to that of brucite, but with one of eve ry three octahedra vacant. The MgO6 octahedra are located above and be low each vacant octahedra vacant. All O-H bonding occurs between SiO6 octahedral layers. This is the only high-pressure hydrous magnesium si licate structure reported to date that contains all octahedrally coord inated Si. The calculated density of phase D (d(cal) = 3.50 g/cm(3)) i s substantially greater than any other high-pressure hydrous magnesium silicate phase.