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New Raman and NMR spectroscopy data on hydrous Ca aluminosilicate melts and glasses, with eutectic quartz–anorthite–wollastonite composition, are presented here. The glasses were obtained by rapid quench of melts equilibrated at high P and high T in a piston–cylinder apparatus. In situ Raman observations of the structure of the melts were also performed during hydrothermal diamond cell experiments. Using the intensities of the ${\sim }860~\text{cm}^{-1}$ and ${\sim }1630~\text{cm}^{-1}$ Raman signals, respectively assigned to vibrations of T–OH and $\text{H}_2\text{O}_{\mathrm{mol}}$ species, we determined the speciation of water in the glasses. T–OH and $\text{H}_2\text{O}_{\mathrm{mol}}$ values compare well with those determined from infrared (IR) spectra, except above ${\sim }5$ wt% total water where IR determinations actually underestimate the proportion of hydroxyl groups. The analysis of the polarized Raman spectra and of the ${}^{29}\text{Si}$ MAS NMR spectra of the hydrous glasses suggests limited changes in glass polymerization with variations in dissolved water content. However, at high temperatures, in situ Raman spectroscopy observations indicate that the hydrous melt structure differs very strongly from that of a glass containing a comparable concentration of dissolved water. Because of this, this study reinforces the fact that using glass data to try understanding high temperature processes in hydrous melts, like viscous flow or water diffusion toward bubbles during volcanic degassing, may not be very appropriate.

New Raman and NMR spectroscopy data on hydrous Ca aluminosilicate melts and glasses, with eutectic quartz-anorthite-wollastonite composition, are presented here. The glasses were obtained by rapid quench of melts equilibrated at high P and high T in a piston-cylinder apparatus. In situ Raman observations of the structure of the melts were also performed during hydrothermal diamond cell experiments. Using the intensities of the ~860 cm-1 and ~1630 cm-1 Raman signals, respectively assigned to vibrations of T-OH and H2Omol species, we determined the speciation of water in the glasses. T-OH and H2Omol values compare well with those determined from infrared (IR) spectra, except above ~5 wt% total water where IR determinations actually underestimate the proportion of hydroxyl groups. The analysis of the polarized Raman spectra and of the 29Si MAS NMR spectra of the hydrous glasses suggests limited changes in glass polymerization with variations in dissolved water content. However, at high temperatures, in situ Raman spectroscopy observations indicate that the hydrous melt structure differs very strongly from that of a glass containing a comparable concentration of dissolved water. Because of this, this study reinforces the fact that using glass data to try understanding high temperature processes in hydrous melts, like viscous flow or water diffusion toward bubbles during volcanic degassing, may not be very appropriate. [ABSTRACT FROM AUTHOR]

The first version of the machine learning greybox model i-Melt was trained to predict latent and observed properties of K 2 O-Na 2 O-Al 2 O 3 -SiO 2 melts and glasses. Here, we extend the model compositional range, which now allows accurate predictions of properties for glass-forming melts in the CaO-MgO-K 2 O-Na 2 O-Al 2 O 3 -SiO 2 system, including melt viscosity (accuracy equal or better than 0.4 log 10 Pa ⋅ s in the 10−1-1015 log 10 Pa ⋅ s range), configurational entropy at glass transition (≤ 1 J mol−1 K−1), liquidus (≤ 60 K) and glass transition (≤ 12 K) temperatures, heat capacity (≤ 3 %) as well as glass density (≤ 0.02 g cm−3), optical refractive index (≤ 0.006), Abbe number (≤ 4), elastic modulus (≤ 6 GPa), coefficient of thermal expansion (≤ 1.1 10−6 K−1) and Raman spectra (≤ 25 %). Uncertainties on predictions also are now provided. The model offers new possibilities to explore how melt/glass properties change with composition and atomic structure. [Display omitted] • i-Melt predicts various properties for calc-alkaline and alkaline aluminosilicates. • Uncertainties can be estimated via MC Dropout and conformal predictions. • i-Melt predicts cationic partial molar volumes V m and liquid C p l i q u i d • The partial molar volumes and C p l i q u i d of cations change with composition. • Al coordination number correlates with the glass molar volume but not with C p l i q u i d [ABSTRACT FROM AUTHOR]

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]

• Only Ti4+ was found at high temperature in silicate melts at f O 2 between 0.21 and 10−6. • Ti-bearing melts systematically crystallize above 1500 °C under very reducing atmospheres. • Crystallisation might be preferred to reduction. Ti is usually tetravalent in silicate glasses. It may be trivalent under very reduced conditions, but this remains unclear. In this study, in situ Ti K-edge XANES experiments were performed on four different Ti-bearing melts, equilibrated at high temperatures using a micro-furnace with controlled atmosphere. No formation of Ti3+ was observed, even at fO 2 ∼ 10−15 (estimated). Instead, systematic crystallisation of the melts was observed. In experiments at fO 2 ≤ 10−12, the system favours Ti4+ crystallisation instead of reduction to Ti3+ in the studied melts. Fluxing melts with reduced gas mixtures, like Ar-2%H 2 , may be a novel way to induce surface crystallisation and produce ceramic-coated glass materials. [ABSTRACT FROM AUTHOR]

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]