Infrared signatures of OH-defects in wadsleyite: A first-principles study
Résumé
The structure and the polarized infrared absorption spectrum of OH-defects in wadsleyite (b-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.