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Structure and crystal chemistry of hydrous wadsleyite, Mg1.75SiH0.5O4: possible hydrous magnesium silicate in the mantle transition zone

Abstract

The crystal structure of hydrous wadsleyite, Mg1.75SiH0.5O4 synthesized in an MA 8-type apparatus at conditions of 1300°C and 15.5 GPa, has been analyzed and refined in space group Imma, using the X-ray intensities measured on a 60X60X10 μm single crystal. The composition (Z=8) and unit cell are Mg1.74Si0.97H0.65O4 by E.P.M.A. analysis and a=5.663(1) Å, b= 11.546(2) Å, c=8.247(4) Å, V=539.2(5) Å3. The partial M-site occupancies were determined; vacancies associated with the incorporation of water are strongly concentrated on the Mg 3 site. The OH in the structure was confirmed by Raman and FTIR spectroscopies. The result of valence sum calculation based on the refined bond lengths indicates that O1 is a hydroxyl. The formula of hydrous wadsleyite can be expressed as Mg2-xSiH2xO4, where 0≤x≤0.25. When x=0.25, all of the O1 site is hydroxyl and the maximum solubility of 3.3 wt% H2O is realized. Structural relations to other dense hydrous phases are discussed.

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Kudoh, Y., Inoue, T. & Arashi, H. Structure and crystal chemistry of hydrous wadsleyite, Mg1.75SiH0.5O4: possible hydrous magnesium silicate in the mantle transition zone. Phys Chem Minerals 23, 461–469 (1996). https://doi.org/10.1007/BF00202032

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Keywords

  • Magnesium
  • Hydroxyl
  • Silicate
  • FTIR Spectroscopy
  • Bond Length