Water solution mechanism in calcium aluminosilicate glasses and melts: insights from in and ex situ Raman and ${}^{29}\protect \text{Si}$ NMR spectroscopy

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.