[show abstract][hide abstract] ABSTRACT: X-ray absorption spectroscopy at the Mg K-edge is used to obtain information on magnesium environment in minerals, silicate and alumino-silicate glasses. First-principles
XANES calculations are performed for minerals using a plane-wave density functional formalism with core-hole effects treated
in a supercell approach. The good agreement obtained between experimental and theoretical spectra provides useful information
to interpret the spectral features. With the help of calculation, the position of the first peak of XANES spectra is related
to both coordination and polyhedron distortion changes. In alumino-silicate glasses, magnesium is found to be mainly 5-fold
coordinated to oxygen whatever the aluminum saturation index value. In silicate glasses, magnesium coordination increases
from 4 in Cs-, Rb- and K-bearing glasses to 5 in Na- and Li-bearing glasses but remains equal as the polymerization degree
of the glass varies. The variation of the C feature (position and intensity) is strongly related to the alkali type providing
information on the medium range order.
Physics and Chemistry of Minerals 05/2009; 36(5):241-257. · 1.30 Impact Factor
[show abstract][hide abstract] ABSTRACT: Experimental Mg K-edge XANES spectra were obtained for crystals and synthetic glasses. To interpret the experiments, two different ab initio XANES calculation methods were employed. The first one is based on multiple-scattering calculations and muffin-tin potentials (FEFF package). The second uses a plane-wave basis set, norm-conserving pseudo-potentials, periodic boundary conditions. The resulting calculations for reference models with three different Mg coordinations are used to derive the best way to analyze the experimental XANES spectra for the glasses.
13th International Conference on X-Ray Absorption Fine Structure (XAFS13); 07/2006
[show abstract][hide abstract] ABSTRACT: XANES spectra were collected at the Si-, Al-, and Na K-edge in hydrous silicate glasses to understand the effect of water on the local structure around these cations. Around network forming Si and Al, no drastic changes are observed. Around Na, the dissolution of water creates more ordered environments in Al-bearing glasses and less ordered environment in Al-free glasses. Ab-initio XANES calculations were undertaken to understand the structural origins for these features. Based on these results, a bond valence model was refined that considers not only the present XANES experiments and models but also NMR information. The double percolation model refined explains, among others, the explosive properties of water-bearing hydrous melts, at the origin of a number of cataclysmic eruptions in subduction zones.