MAS-NMR Studies of Carbonate Retention in a Very Wide Range of Na2O-SiO2 Glasses

Glasses that contain carbon are of geological interest, and the form of that carbon can be probed by Magic-Angle Spinning Nuclear Magnetic Resonance (MAS-NMR) spectroscopy. Previous studies of the Na2O-SiO2 glass system could only reach 56 mol% Na2O. Here we reproduce and extend those studies to cover a very wide compositional range, from 20 to 70 mol% Na2O, by using a combination of conventional melt-quench and twin roller quenching technologies on natural and 99% 13C-enriched sodium silicate glasses. 13C MAS-NMR reveals that measurable levels of carbon retention occur above at least 40 mol% Na2O,and takes the form of CO32- ions incorporated in the glass structure. These CO32- anions are surrounded by Na+ cations, forming nanoscale domains in a sodium silicate glass network. 23Na MAS-NMR showed a linear decrease in mean Na—O bond length with increasing Na2O content, up to 60 mol% Na2O, above which the mean Na—O bond length increased. Elemental analysis detected significant (>5%) carbonate by mass in the 65 and 70 mol% Na2O glasses. For the 70 mol% Na2O glass, 13C and 23Na MAS-NMR detected ordered nanoscale domains composed of only Na2O and CO2. These results have shown the quantity and nature of carbon retention in the archetypal sodium silicate glass system, which will better inform structural models and carbonate solubility limits.