CRYSTAL-STRUCTURE OF MONOCLINIC HYDROUS WADSLEYITE [BETA-(MG,FE)(2)SIO4]

Wadsleyite [beta-(Mg,Fe)(2)SiO4] is known to accept variable amounts o f H and may be an important reservoir of H in the transition zone of t he mantle. The crystal structure of Fo(94.6) hydrous wadsleyite (Mg1.7 30Fe0.098 Al0.008Si0.991H0.355O4) synthesized at 1400 degrees C and 17 GPa was refined in space group I2/m from 1784 measured intensities of which 830 were unique, and, of these, 650 were of intensity greater t han 4 sigma. Unit cell parameters are a=5.6715(7), b=11.582(2), c=8.25 8(1) Angstrom, and beta=90.397(9)degrees. The final R(F) for all refle ctions was 0.026 (R-W=0.024); goodness of fit was 1.68. An Il position was located on the nonsilicate O atom (01). Partial occupancy of a no rmally vacant tetrahedral site adjacent to M3 was observed. This is po stulated to be the result of Si moving from M3 to the adjacent tetrahe dral void on decompression. Deviation from orthorhombic symmetry appea rs to result from ordering of H, (Mg, Fe), and possibly Si, within thr two nonequivalent M3 sites.