Stéphanie Rossano

The oxidation state of iron in silicic melt at 500 MPa water pressure

Abstract The dependence of the ferric–ferrous ratio in silicate melts on oxygen fugacity was studied in the system SiO 2 (Qz)–NaAlSi 3 O 8 (Ab)–CaAl 2 Si 2 O 8 (An)–H 2 O using Mossbauer spectroscopy. Experiments were performed under water-saturated conditions at 500 MPa, and at temperatures of 850 and 950 °C, covering a range typical for magmatic processes. The oxygen fugacity was varied in the f O 2 range from Cu–Cu 2 O buffer to slightly more reducing conditions than the wustite–magnetite buffer. The iron redox ratio was determined by analyzing the Mossbauer parameter distribution that was modeled based on experimental spectra collected at room temperature on the quenched samples. The obtained iron redox ratios show a linear dependence on oxygen fugacity on a logarithmic scale for both temperatures. The iron redox ratio (Fe 3+ /Fe 2+ ) decreases with temperature for a given oxygen fugacity. The spectroscopic data at 850 °C are in good agreement with Fe 3+ /Fe 2+ ratios derived from element partitioning but show considerable deviations from iron redox ratios predicted by the empirical equation given by Kress and Carmichael [Contrib. Mineral. Petrol. 108 (1991) 82]. This indicates that an extrapolation of this equation to such low temperatures may have large errors. A sample quenched slowly through the temperature range near and below T g shows considerable differences in the obtained Mossbauer spectra compared to more rapidly cooled samples, indicating ordering of the iron environment at least in the mesoscopic range. The oxidation state, however, does not differ when compared to the more rapidly quenched melts.

Medium range structure of borosilicate glasses from Si K-edge XANES: a combined approach based on multiple scattering and molecular dynamics calculations.

In order to better understand the influence of noble metals precipitated in a borosilicate glass structure, X-ray absorption near-edge structure (XANES) spectra at the silicon K-edge were recorded. The presence of noble metals, although their concentration does not exceed 2%, significantly modifies the Si K-edge spectrum. A shoulder on the high-energy side of the white line disappears when noble metals are present in the glassy matrix. Analysis of the noble metal free spectrum was performed by combining molecular dynamics simulations and multiple scattering calculations. The use of both formalisms allows the determination of the atomic environment up to 4.5 A around silicon atoms. Multiple scattering calculations permit an elucidation of the origin of this peculiar XANES feature, which is a relevant signature of the intermediate range structure. The structural changes within the borosilicate network caused by the incorporation of noble metals are interpreted in terms of modification of the [3]B/[4]B ratio and of the distribution of alkali and alkaline-earth ions within the glassy network.

Bond valence in silicate glasses

In contrast with crystals, glass structure cannot be unambiguously determined by experimental techniques, and molecular dynamics (MD) simulations are often used to model such systems. Ion–ion interactions are modeled by empirical potentials, which are validated by comparison between the structural information extracted from experimental results and the simulated structure. Bond valence (BV) theory is often used to check the reliability of experimentally determined crystal structures. However, it has not been applied in a similar fashion to the results of MD simulations of disordered systems. To illustrate the use of BV theory in silicate glasses, BV calculations were performed in a 1500 atom-3D-periodic MD simulation box of a glass of composition CaO–FeO–2SiO2 (mol%) with two sets of Born–Mayer–Huggins potentials. The structural models are analyzed from the point of viewof BV theory. The BV sums around the 1500 atoms showthat the BV model is globally verified. In addition, w e showthat the BV sums are sensitive to changes in MD potential parameters and can thus be used to test the plausibility of the MD model of a silicate glass. This newstrategy opens possibilities in the derivation of more robust and constrained structural models of silicate glasses and melts. 2002 Elsevier Science B.V. All rights reserved.

Mie scattering of a partially coherent beam

The scattering of a partially coherent electromagnetic beam by a dielectric sphere is described. Two methods are presented: a coherent mode representation and a spherical harmonics expansion. In the case of a distant blackbody source, the coherent modes are plane waves and the scattered intensity is obtained by a convolution of the classical Mie results over the source surface.

Microscopic calculation of the constitutive relations

Abstract.Homogenization theory is used to calculate the macroscopic dielectric constant from the quantum microscopic dielectric function in a periodic medium. The method can be used to calculate any macroscopic constitutive relation, but it is illustrated here for the case of electrodynamics of matter. The so-called cell problem of homogenization theory is solved and an explicit expression is given for the macroscopic dielectric constant in a form akin to the Clausius-Mossotti or Lorentz-Lorenz relation. The validity of this expression is checked by showing that the standard formula is recovered for cubic materials and that the average of the microscopic energy density is the macroscopic one. Finally, the general expression is applied to Bloch eigenstates.

Browning phenomenon of medieval stained glass windows.

In this work, three pieces of historical on-site glass windows dated from the 13th to 16th century and one archeological sample (8th century) showing Mn-rich brown spots at their surface or subsurface have been characterized by optical microscopy and Scanning Electron Microscopy coupled with Energy Dispersive X-ray spectroscopy. The oxidation state of Mn as well as the Mn environment in the alteration phase have been characterized by X-ray absorption spectroscopy at the Mn K-edge. Results show that the oxidation state of Mn and therefore the nature of the alteration phase varies according to the sample considered and is correlated with the extent of the brown alteration. The larger the brown areas the more oxidized the Mn. However, by contrast with literature, the samples presenting the more extended brown areas are not similar to pyrolusite and contain Mn mainly under a (+III) oxidation state.

Mg K‐Edge XANES Spectra in Crystals and Oxide Glasses: Experimental vs. Theoretical Approaches

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.

Comment on “Femtosecond laser-induced modification of potassium-magnesium silicate glasses: An analysis of structural changes by near edge x-ray absorption spectroscopy” [Appl. Phys. Lett. 100, 224101 (2012)]

potassium-magnesium silicate glasses: An analysis of structural changes by near edge x-ray absorption spectroscopy” [Appl. Phys. Lett. 100, 224101 (2012)] Delphine Cabaret, St! ephanie Rossano, and Nicolas Trcera Institut de Min! eralogie et de Physique des Milieux Condens! es, Universit! e Pierre et Marie Curie (UPMC), UMR CNRS 7590, c. c. 115, 4 Place Jussieu, 75252 Paris Cedex 05, France Laboratoire G! eomat! eriaux et Environnement, Universit! e Paris EST, EA 4508, 5 Bd Descartes, Champs sur Marne, 77454 Noisy-Champs Cedex 2, France Synchrotron SOLEIL, BP 48, 91192 Gif sur Yvette, France

Local Structures around Si, Al and Na in Hydrated Silicate Glasses

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.