Mg-vacant structural modules and dilution of the symmetry of hydrous wadsleyite, beta-Mg2-xSiH2xO4 with 0.00 <= x <= 0.25

The crystal structures of the two hydrous wadsleyite crystals with formulae , Mg1.75SiH0.50O4 (0.5H-beta) and Mg1.86SiH0.28O4 (0.3H-beta) have been ana lyzed in this study. The single-crystal X-ray diffraction data showed that the unit cells of the 0.3H-beta and the 0.5H-beta are metrically monoclinic with a slight distortion from the orthorhombic cell but their intensity di stributions conform to the orthorhombic symmetry within the limit of experi mental errors. The Fourier and the difference Fourier syn theses were calcu lated. Small but significant Fourier peaks were found at the site, Si2, in a normally vacant tetrahedral void adjacent to Mg3 site as reported for the monoclinic hydrous wadsleyite by Smyth et al.. From the comparison of the hydrous and anhydrous wadsleyite structures, the Mg-vacant structural modul es were found to be the building units for the structure of hydrous wadsley ite. The dilution of symmetry from orthorhombic to monoclinic in the hydrou s wadsleyite structure is interpreted qualitatively due to lack of minor pe rpendicular to the a axis in the module. The mode of arrangement of the Mg- vacant structural modules interprets the symmetry and hydrogen content of t he hydrous wadsleyite and gives the structural relationship between hydrous wadsleyite and hydrous ringwoodite.