Crystal structure of monoclinic hydrous wadsleyite [β-(Mg,Fe)2SiO4]

Wadsleyite (β-(Mg,Fe)2SiO4) is known to accept variable amounts of H, and may be an important reservoir of H in the transition zone of the mantle. The crystal structure of Fo94.6 hydrous wadsleyite (Mg1.730Fe0.098 Al0.008Si0.991H0.355O4) synthesized at 1400 oC and 17GPa has been refined in space group I2/m from 1784 measured intensities of which 830 were unique, and of these, 650 were of intensity greater than 4σ. Unit cell parameters are a = 5.6715(7) Å; b = 11.582(2) Å; and c = 8.258(1) Å; β = 90.397(9)o. The final R(F) for all reflections was 0.026 (Rw = 0.024); and goodness of fit was 1.68. A H position was located on the non-silicate oxygen (O1). Partial occupancy of a normally vacant tetrahedral site adjacent to M3 was observed. This is postulated to be the result of Si moving from M3 to the adjacent tetrahedral void on decompression. Deviation from orthorhombic symmetry appears to result from ordering of H, Mg/Fe, and possibly Si, within the two non-equivalent M3 sites.