Proton environments and hydrogen-bonding in hydrous silicate glasses from proton NMR

Abstract

The dissolution of water in magmas has profound effects on their physical and chemical properties, and an understanding of this process continues to be of major importance in igneous petrology and geochemistry. Systematic studies of the physical properties of hydrous melts, and infrared^1,2, Raman^2,3 and ²⁹Si NMR⁴ spectro-scopic studies of glasses quenched from melts, have provided valuable information, but many questions remain unanswered. Previous proton NMR studies of glasses have generally concentrated on lineshape, to distinguish between the proton environments in Si–OH groups and in molecular H₂O (refs 5, 6). Here we present proton NMR results in glasses quenched from hydrous melts, which allow us to resolve different proton sites on the basis of chemical-shift differences. Four separate proton resonances are resolved in a silica glass containing 8.7 wt% H₂O, and a strongly hydrogen-bonded Si–OH group is identified in hydrous alkali and alkaline-earth disilicate glasses. Quantitative estimates of water speciation in the silica glasses agree with those from ²⁹Si NMR⁴. Future proton NMR measurements which resolve different chemical environments will play an important role in elucidating the structures of hydrogen-bearing disordered solids.