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The Adam and Gibbs theory depicts the viscous flow of silicate melts as governed by the cooperative re-arrangement of molecular sub-systems. Considering that such subsystems involve the silicate Q n units ( n = number of bridging oxygens), this study presents a model that links the Q n unit fractions to the melt configurational entropy at the glass transition temperature T g , S conf (T g ) , and finally, to its viscosity η . With 13 adjustable parameters, the model reproduces η and T g of melts in the Na 2 O-K 2 O-SiO 2 system (60 ≤ [SiO 2 ] ≤ 100 mol%) with 1σ standard deviations of 0.18 log unit and 10.6°, respectively. The model helps understanding the links between the melt chemical composition, structure, S conf and η . For instance, small compositional changes in highly polymerized melts generate important changes in their S conf (T g ) because of an excess of entropy generated by mixing Si between Q 4 and Q 3 units. Changing the melt silica concentration affects the Q n unit distribution, this resulting in non-linear changes in the topological contribution to S conf (T g ) . The model also indicates that, at [SiO 2 ] ≥ 60 mol%, the mixed alkali effect has negligible impact on the silicate glass Q n unit distribution, as corroborated by Raman spectroscopy data on mixed Na-K tri- and tetrasilicate glasses. Such model may be critical to link the melt structure to its physical and thermodynamic properties, but its refinement requires further high-quality quantitative structural data on silicate and aluminosilicate melts. [ABSTRACT FROM AUTHOR]

The solubility and solution behavior of F and Cl in peralkaline aluminosilicate compositions in the systems Na2O-Al2O3-SiO2 and K2O-Al2O3-SiO2 have been determined for glasses quenched from melts equilibrated at 1400 and 1600 °C in the 1.0-2.5 GPa pressure range. With Al/(Al+Si) increasing from 0 to 0.33 in sodium aluminosilicate melts, F solubility (saturation concentration) increases from 3.3 to 7.4 mol%, whereas Cl solubility decreases from 5.7 to 2.5 mol%. There is no difference in F solubility in sodium or potassium aluminosilicate melts. However, the Cl solubility in potassic aluminosilicate melts is 40-60% lower than in sodic melts with the same Al/(Al+Si) and Na or K mole fraction. Fluorine depolymerizes the silicate melt structure and forms Si-F, Al-F, and Na-F complexes, the proportion of which depends on the melt Al/(Al+Si) ratio. Dissolution of Cl results in a small degree of depolymerization of Al-free silicate melt, whereas Cl has a polymerizing effect in aluminosilicate melts. In both cases, formation of Na-Cl complexes appears to be the driving solution mechanism. The differences in F and Cl solution mechanisms result in contrasting depolymerizing effects that become more pronounced with increasing degree of magmatic differentiation. Through such modifications of melt structure, F and Cl have significant effects on magma properties such as viscosity, compressibility, and element partitioning. [ABSTRACT FROM AUTHOR]

Previous studies of hydrous glasses and melts with infrared spectroscopy have led to the conclusion that the IR combination peaks near 4500 and 5200 cm-1 reflect the existence of OH- (hydroxyl) groups and H2Omol water molecules in those materials. Here, we show that the glass chemical composition can impact profoundly the intensities and frequencies of the fundamental O-H stretching signal and, therefore, potentially those of the 4500 and 5200 cm-1 combination peaks. In alkali silicate glasses, compositional effects can give rise to peaks assigned to fundamental O-H stretching at frequencies as low as 2300 cm-1. This expanded range of Raman intensity assigned to O-H stretch is increasingly important as the ionic radius of the alkali metal increases. As a result, the combination of the fundamental O-H stretch in OH- groups with the Si-O-H stretch located near 910 cm-1 gives rise to a complex combination signal that can extend to frequencies much lower than 4200 cm-1. This combination signal then becomes unresolvable from the high-frequency limb of the band assigned to fundamental O-H stretch vibration in the infrared spectra. It follows that, when O-H stretch signals from OH- groups extend to below 3000 cm-1, the 4500 cm-1 peak does not represent the total OH- signal. Under such circumstances, this infrared peak may not be a good proxy for determining the concentration of OH- hydroxyl groups for glassy silicate materials. [ABSTRACT FROM AUTHOR]

Abstract: Dissociation of water into protons and hydroxyl ions is a fundamental feature of aqueous solutions. Although it exerts a profound influence on properties of magmas, this autoprotolysis reaction has been hitherto neglected for water dissolved in silicate melts. As made here with an acid–base model, in fact one has to deal with molecular water (H2Omol) and two kinds of hydroxyl groups, bonded or not to network-forming cations (OH and OH−, respectively) in hydrous silicate melts. By mixing cations and anions on distinct sublattices and quantifying the disproportionation of water dissolved in silicate melts into its ionic products, H+ and OH−, we reconcile spectroscopic determinations of water speciation, and highlight the main compositional features involving chemical exchanges between H2Omol, T OH and M OH groups (T and M being a former and a modifier, respectively). In particular, water addition to depolymerized systems, such as basalts, determine a relative predominance of OH− with respect to silicic systems, such that the increase of water concentration tends to immediately limit depolymerization rather than enhance it. This opens new perspectives to the understanding of the chemical control of hydrous magmas and their physical properties, as well as the attainment of saturation in hydrous minerals such as amphiboles or micas, particularly in depolymerized melts. [Copyright &y& Elsevier]

Abstract: Viscosity and structure of Na/K tectosilicate glasses containing 75 and 83mol% of SiO2 have been investigated. Viscosity increases non-linearly when K+ ions substitute Na+ ions in these melts. The viscosity variations depending on chemical composition cannot be reproduced using an ideal mixing model of the configurational entropy. Consequently, it appears that Na and K elements do not mix randomly in the studied aluminosilicate melts. Raman spectra of glasses show that, during the Na/K substitution, evolutions of both the ring distribution and the T-O-T mean angle, or force constant, occur. The proportion of small-membered (three, four) rings, compared to large-membered rings, is higher in K-rich glasses than in Na-rich glasses. Moreover, Raman spectra features suggest that two different TO2 environments exist and cohabit into the glass. They could represent two populations of six-membered rings differing by their force constant or their T-O-T angles. One of these environments evolves when K substitutes for Na, showing an increase of its mean T-O-T angle and force constant. The other environment remains unchanged. From the observations, we propose that Na/K mixed tectosilicate glasses contained two sub-networks: one composed of the Si, Al, O and K atoms, and another of the Si, Al, O and Na. We suggest that the different size of alkali elements combined to the charge balancing needs of Al3+ ions can be the source of the different clustering of alkali cations into different sub-networks. Furthermore, and as previously inferred by older studies, potassic tectosilicate glasses could present silica-like and leucite-rich regions, explaining notably the incongruent crystallization of the orthoclase liquid. [Copyright &y& Elsevier]

Raman spectroscopy can measure water concentrations of hydrous silicate glasses with several advantages such as: (1) high-spatial resolution of 1-2 µm²; (2) non-destructive character; and (3) easy access, without any specific sample preparation or mounting techniques. The latter reasons render Ra- man highly suitable for studying natural products, such as volcanic pumice and scoriae fragments. Two spectral regions can be distinguished in Raman spectra of hydrated silicate glasses: a low-wavenumber -1 region (15-1500 cm-1), which corresponds to vibrations of the silicate network, and a high-wavenumber -1 region (3100-3750 cm ), corresponding to the OH stretching vibrations of H2O molecules and OH groups. Behrens et al. (2006) have published empirical equations relating the area ratio between these two regions and the water content. However, the proposed internal calibrations depend on chemical composition of the glasses. In this paper, we reinvestigated the previous procedures to improve the background subtraction. Our results allow us to present a more general and linear calibration. Water concentrations up to 13 wt% can be measured for a broad range of natural silicate melts, from basalts to rhyolite (40 up to 80 wt% SiO2), using a single calibration curve with an absolute error of 0.2 wt%. [ABSTRACT FROM AUTHOR]