Abstract

The structure and the polarized infrared absorption spectrum of OH-defects in wadsleyite (β-Mg2SiO4) are studied, at 0 and 15 GPa, by first-principles calculations based on density functional theory (DFT). Four types of OH-defects are considered: fully protonated magnesium vacancies, fully protonated silicon vacancies, silicon vacancies compensated by a magnesium cation and two protons, and OH-defects associated with the migration of a silicon cation to a normally vacant site, as reported by Kudoh and Inoue (1999). The results suggest that the main absorption band constituted by a doublet (3326 and 3360 cm−1) corresponds to at least two types of OH-defects involving M3 vacancies with protonation of the O1-type O atoms along the O1···O4 edges. The main contribution of the less intense band at 3581 cm−1 is likely related to the partial protonation of a silicon vacancy (protonation of the O3-type oxygen) associated with the migration of the silicon cation to the Si2 site. This assignment is consistent with several experimental constraints: wavenumber and pleochroism of infrared OH-stretching bands, pressure-dependence of the band wavenumber, evidence from X-ray diffraction of magnesium vacancies in M3 site, and increase of the b/a axial ratio with water content. The integrated absorption coefficients of the corresponding OH-defects are also calculated and thus complement the set of data obtained previously for forsterite and ringwoodite. Absorption coefficients of wadsleyite computed at 0 and 15 GPa indicate that for a precise quantification of the hydrogen content in in situ experiments, one must consider higher absorption coefficients than those determined at 0 GPa after quench. It is also shown that a single theoretical relation can account for the three Mg2SiO4 polymorphs at 0 GPa: Kint = 278.7 ± 18.1 (3810 ± 465 − x), where Kint is the integrated molar absorption coefficient of the OH stretching modes and x is the average wavenumber in cm−1. Absorption coefficients are significantly lower than the general calibrations, the use of which would lead to an underestimation of the water concentrations.

You do not currently have access to this article.