Effect of alkalis on the Fe oxidation state and local environment in peralkaline rhyolitic glasses

International audienceIron oxidation state and coordination geometry have been determined by Fe K-edge X-ray absorption near edge spectroscopy (XANES) for three sets of silicate glasses of peralkaline rhyolitic composition with different peralkalinity values. These compositions were chosen to investigate the effect of alkali content (and oxygen fugacity) on the Fe oxidation state. The samples were produced by means of hydrothermal vessels at 800 °C with oxygen fugacity conditions ranging from NNO-1.61 to NNO+2.96 log units. Comparison of the pre-edge peak data with those of Fe model compounds of known oxidation state and coordination number allowed determination of the Fe oxidation state and coordination number in all glasses analyzed. Within each group of samples, Fe tends to oxidize with increasing oxygen fugacity as expected. However, alkali content is shown to have a strong effect on the Fe3+/(Fe3++Fe2+) ratio at constant oxygen fugacity: this ratio varies from 0.25 to 0.55 (±0.05) for the least peralkaline series, and from 0.45 to 0.80 (±0.05) for the most peralkaline series. Moreover, pre-edge peak data clearly indicate that Fe3+ is in fourfold coordination in the most peralkaline glasses. Extrapolation of pre-edge peak data suggests the presence of both fourfold and fivefold coordination for trivalent Fe in the other two series. Divalent Fe is suggested to be mainly in fivefold coordination in all the three glass series. The presence of minor amounts of sixfold- and fourfold-coordinated Fe cannot be ruled out by XANES data alone. XANES data suggest that the amount of alkalis also affects the Fe3+ coordination environment resulting in a decrease in the average coordination numbers. Extended X-ray absorption fine structure (EXAFS) data of the most oxidized and peralkaline sample indicate that Fe3+ is in tetrahedral coordination with = 1.85 Å (±0.02). This value compares well with literature data for [4]Fe3+ in crystalline phases (e.g., in tetra-ferriphlogopite or rodolicoite) or in silicate glasses (e.g., phonolite glasses) supporting the XANES-determined coordination number obtained for the most peralkaline glasses. Calculated NBO/T ratios decrease slightly with Fe oxidation because of the higher fraction of network forming Fe, thus increasing the polymerization of the tetrahedral network

Non-Magmatic Glasses

International audienceOVERVIEW On Earth, natural glasses are typically produced by rapid cooling of melts, and as in the case of minerals and rocks, natural glasses can provide key information on the evolution of the Earth. However, natural glasses are products not solely terrestrial, and different formation mechanisms give rise to a variety of natural amorphous materials. In this chapter, we provide an overview of the different natural glasses of non-magmatic origin and on their formation mechanisms. We focus on natural glasses formed by mechanisms other than magmatic activity and included are metamorphic glasses and glasses produced from highly energetic events (shock metamorphism). The study of these materials has strong repercussions on planetary surface processes, paleogeography/paleoecology, and even on the origin of life

[4]Fe3+-O distance in synthetic kimzeyite garnet

A synthetic kimzeyite analogue (Ca3Zr2[Fe2SiO12]) has been analysed by powder X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) in order to determine the [4]Fe3+–O distance, for which only few data are available. The XRD-determined cell parameter (a0 = 12.625 ± 0.001 Å) is consistent with those found in the literature for synthetic samples of similar composition. Also interatomic distances (= 2.515 Å, = 2.093 Å, = 1.774 Å, as determined by Rietveld refinement) are in keeping with structural data of natural samples when taking into account chemical differences of the samples examined. Due to the large Fe content of the tetrahedral site, the distance is unusually long compared with those of garnets where tetrahedra are occupied solely by Si, it is instead consistent with structural data for natural kimzeyite. The XANES data indicate the presence of trivalent Fe in tetrahedral coordination. The EXAFS-derived distance (1.85 ± 0.01 Å) is in agreement with the few literature data available for [4]Fe3+, i.e. tetra-ferriphlogopite ( = 1.86 ± 0.01 Å), rodolicoite ( = 1.825 Å) and silicate glasses (Fe–O = 1.85 ± 0.01 Å and 1.84 ± 0.02 for phonolitic and rhyolitic glasses, respectively). In view of the large size difference between FeO4 and SiO4 tetrahedra a further Rietveld structural refinement was performed assuming a splitting of the oxygen position, resulting in two distinct oxygen sites (OA and OB with fractional oxygen occupancies of 2/3 and 1/3) at 1.845 and 1.606 Å distance, respectively, from the tetrahedral cation. Although there are still open questions on the distribution of FeO4 and SiO4 tetrahedra and on how the structure accommodates the size difference of these two tetrahedra, this study provides a direct determination of the [4]Fe3+–O distance for which only few data are available in the literature

The Influence of Glass Composition on Iodine Solubility

International audienceTwo glass series in the ternary systems K 2 O-B 2 O 3-SiO 2 (KBS) and Na 2 O-B 2 O 3-SiO 2 (NBS) were studied in order to identify the main factors influencing the solubility of iodine. We established that iodine incorporation is strongly linked to the bulk chemistry, i.e. the SiO 2 /(B 2 O 3 +SiO 2) molar ratio, and to the physical properties of the glasses, and we assessed three different solubility limits. Iodine in Si-rich glasses has a low solubility (≤1 mol% I) regardless of the alkali ion present. On the contrary, in B-rich glasses, the solubility is five times higher than in Si-rich glasses for Na-glasses, and more than six times higher for K-glasses. The strong dependence of iodine solubility on the bulk chemistry is related to the adaptability of the glass network. Furthermore, our data suggest that iodine is stable with different redox states in the glasses here analyzed

convegno "Natural Silicate Glasses"

Topics relevant to this meeting include: Natural Glasses, Silicate Melts and Glasses, Glasses in Archaeology, and Glasses for Industrial Applications. The Natural Silicate Glasses meeting will provide a forum for discussion of both experimental and computational results related to glass structure and physical-chemical properties. Abstracts have been submitted both for oral or poster contributions. Three invited speakers (one for each topic) have given keynote talks on : viscosity of silicate melts; impact glasses; physical properties of industrial soda-lime silica glasses. Details of the meeting are reported at the web-page: http://www.unicam.it/geologia/natglass2013/index.htm

Earth’s Electrodes

International audienceThe oxidation–reduction (‘redox’) state is an important intensive property of any geologic system and is typically measured (and reported) as either the redox potential (Eh) or the oxygen fugacity (fO2). These two concepts cover the whole spectrum of geologic systems: from low-temper-ature aqueous and sedimentary systems to high-temperature rock-forming environments. The redox state determines the speciation of a fluid phase and exercises fundamental controls on phase relations and geochemical evolution. Here, we review the concepts that underpin the redox state and outline a framework for describing and quantifying the concept of the oxidation state