5:15 PM - 6:45 PM
[SIT15-P03] Water solution mechanism in calcium aluminosilicate glasses and melts: insights from in and ex situ Raman and 29Si NMR spectroscopy
Keywords:water, magma, speciation, volcanism, magmatism
Water profoundly affects the properties of aluminosilicate melts and glasses, including viscosity, glass transition temperature and liquidus temperature. Because of this, it has been the topic of active research for now several decades. Data mostly come from the analysis of water-bearing glasses, obtained by rapid quench of melts from high-pressure and high-temperature conditions. In situ data also were acquired on alkali aluminosilicate melts in equilibrium with aqueous fluids at high temperature and high pressure in hydrothermal diamond anvil cells (HDACs). However, such data are scarce, and there is, furthermore, lack of data for alkaline-earth bearing compositions due to their high liquidus temperatures. This is unfortunate because most industrial and geological glasses contain alkaline-earth elements in abundant quantities.
To help address the data scarcity, we present new data on calcium aluminosilicate glasses of ambient-pressure eutectic quartz-anorthite-wollastonite composition. Water-bearing glasses were synthesized in a piston-cylinder apparatus. The glass structure, water speciation and solubility were investigated using polarized Raman spectroscopy. Further insights were obtained from 29Si nuclear magnetic resonance (NMR) spectroscopy. Unpolarized Raman spectroscopy data were also acquired directly on melts in equilibrium with aqueous fluids in HDACs, at temperature and pressure conditions ranging from 600 to 1000 °C and 300 to 1500 MPa.
Water speciation comprised of molecular H2O and OH groups bonded to Si can be determined using polarized Raman spectroscopy, independently of infrared data that are subject to a bias at water contents above 5 wt%. Raman and NMR data on glasses surprisingly show only limited changes in network polymerization. However, in situ Raman data show a clear difference between the structure of a melt at 950 °C and ~1.5 GPa with ~13 wt% dissolved water and that of a glass with the same amount of dissolved water. This could explain the existing debate in literature regarding the lack of structural changes reported upon solution of water in aluminosilicate glasses. Therefore, in agreement with past studies, those results highlight that using glass data to try understanding the effect of water on high temperature processes in melts, like viscous flow or water diffusion toward bubbles during volcanic degassing, may not be very appropriate.
To help address the data scarcity, we present new data on calcium aluminosilicate glasses of ambient-pressure eutectic quartz-anorthite-wollastonite composition. Water-bearing glasses were synthesized in a piston-cylinder apparatus. The glass structure, water speciation and solubility were investigated using polarized Raman spectroscopy. Further insights were obtained from 29Si nuclear magnetic resonance (NMR) spectroscopy. Unpolarized Raman spectroscopy data were also acquired directly on melts in equilibrium with aqueous fluids in HDACs, at temperature and pressure conditions ranging from 600 to 1000 °C and 300 to 1500 MPa.
Water speciation comprised of molecular H2O and OH groups bonded to Si can be determined using polarized Raman spectroscopy, independently of infrared data that are subject to a bias at water contents above 5 wt%. Raman and NMR data on glasses surprisingly show only limited changes in network polymerization. However, in situ Raman data show a clear difference between the structure of a melt at 950 °C and ~1.5 GPa with ~13 wt% dissolved water and that of a glass with the same amount of dissolved water. This could explain the existing debate in literature regarding the lack of structural changes reported upon solution of water in aluminosilicate glasses. Therefore, in agreement with past studies, those results highlight that using glass data to try understanding the effect of water on high temperature processes in melts, like viscous flow or water diffusion toward bubbles during volcanic degassing, may not be very appropriate.