%0 Journal Article
%9 ACL : Articles dans des revues avec comité de lecture répertoriées par l'AERES
%A Blanchard, M.
%A Roberge, M.
%A Balan, Etienne
%A Fiquet, G.
%A Bureau, H.
%T Infrared signatures of OH-defects in wadsleyite : a first-principles study
%D 2013
%L fdi:010061314
%G ENG
%J American Mineralogist
%@ 0003-004X
%K Wadsleyite ; OH-defect ; IR spectroscopy ; ab initio calculations ; DFT
%M ISI:000327049100022
%N 11-12
%P 2132-2143
%R 10.2138/am.2013.4468
%U https://www.documentation.ird.fr/hor/fdi:010061314
%> https://www.documentation.ird.fr/intranet/publi/2013/12/010061314.pdf
%V 98
%W Horizon (IRD)
%X The structure and the polarized infrared absorption spectrum of OH-defects in wadsleyite (beta-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 center dot center dot center dot O-4 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: K-int = 278.7 +/- 18.1 (3810 +/- 465 - x), where K-int 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.
%$ 064