STRUCTURE AND CRYSTAL-CHEMISTRY OF HYDROUS WADSLEYITE, MG1.75SIH0.5O4- POSSIBLE HYDROUS MAGNESIUM-SILICATE IN THE MANTLE TRANSITION ZONE

The crystal structure of hydrous wadsleyite, Mg1.75SiH0.5O4 synthesize d in an MA 8-type apparatus at conditions of 1300 degrees C and 15.5 G Pa, has been analyzed and refined in space group Imma, using the X-ray intensities measured on a 60x60x10 mu m single crystal. The compositi on (Z = 8) and unit cell are Mg1.71Si0.97H0.65O4 by E.P.M.A. analysis and a = 5.663 (1) Angstrom, b = 11.546 (2) Angstrom, c = 8.247(4) Angs trom, V = 539.2(5) Angstrom(3). The partial M-site occupancies were de termined; vacancies associated with the incorporation of water are str ongly concentrated on the Mg 3 site. The OH in the structure was confi rmed by Raman and FTIR spectroscopies. The result of valence sum calcu lation based on the refilled bond lengths indicates that O 1 is a hydr oxyl. The formula of hydrous wadsleyite can be expressed as Mg2-xSiH2x O4, where O less than or equal to x less than or equal to 0.25. When x = 0.25, all of the O 1 site is hydroxyl and the maximum solubility of 3.3 wt% H2O is realized. Structural relations to other dense hydrous phases are discussed.