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Structure and crystal chemistry of hydrous wadsleyite, Mg1.75SiH0.5O4: possible hydrous magnesium silicate in the mantle transition zone |
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| Authors: | Y. Kudoh T. Inoue H. Arashi |
| Affiliation: | 1. Institute of Mineralogy, Petrology, and Economic Geology, Faculty of Science, Tohoku University, 980, Sendai, Japan 2. Department of Earth Sciences, Faculty of Science, Ehime University, 790, Matuyama, Japan 4. Department of Machine Intelligence and Systems Engineering, Faculty of Engineering, Tohoku University, 980, Sendai, Japan |
| 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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