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Infrared signatures of OH-defects in wadsleyite; a first-principles study

Marc Blanchard, Mathilde Roberge, Etienne Balan, Guillaume Fiquet and Helene Bureau
Infrared signatures of OH-defects in wadsleyite; a first-principles study
American Mineralogist (December 2013) 98 (11-12): 2132-2143


The structure and the polarized infrared absorption spectrum of OH-defects in wadsleyite (beta -Mg (sub 2) SiO (sub 4) ) 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 (super -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 (super -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 Mg (sub 2) SiO (sub 4) polymorphs at 0 GPa: K (sub int) = 278.7 + or - 18.1 (3810 + or - 465 - X), where K (sub int) is the integrated molar absorption coefficient of the OH stretching modes and x is the average wavenumber in cm (super -1) . Absorption coefficients are significantly lower than the general calibrations, the use of which would lead to an underestimation of the water concentrations.

ISSN: 0003-004X
EISSN: 1945-3027
Serial Title: American Mineralogist
Serial Volume: 98
Serial Issue: 11-12
Title: Infrared signatures of OH-defects in wadsleyite; a first-principles study
Affiliation: Universite Pierre et Marie Curie, Institut de Mineralogie et de Physique des Milieux Condenses, Paris, France
Pages: 2132-2143
Published: 201312
Text Language: English
Publisher: Mineralogical Society of America, Washington, DC, United States
References: 44
Accession Number: 2014-000613
Categories: Geophysics of minerals and rocks
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 4 tables
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Abstract, copyright, Mineralogical Society of America. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 201401
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