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

Abstract

Wadsleyite [b-(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 Fo 94.6 hydrous wadsleyite (Mg1.730Fe0.098 Al0.008Si0.991H0.355O4) synthesized at 1400 8C and 17 GPa was refined in space groupI2/m from 1784 measured intensities of which 830 were unique, and, of these, 650 were of intensity greater than 4s. Unit cell parameters are a 5 5.6715(7), b 5 11.582(2), c 5 8.258(1) A ˚, and b5 90.397(9)8. The finalR(F) for all reflections was 0.026 (Rw 5 0.024); goodness of fit was 1.68. An H position was located on the nonsilicate O atom (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 nonequivalent M3 sites.