NMR evidence for excess non-bridging oxygen in an aluminosilicate glass

The most common of man-made glasses have aluminosilicate compositions, and such glasses also form from rapidly cooling magmas1. Oxygen is the most abundant element in these materials, where it occupies either ‘bridging’ (BO) or ‘non-bridging’ (NBO) sites. BOs link two AlO4 or SiO4 tetrahedra, thereby providing strong, long-lived bonds between the smallest structural units of the aluminosilicate network. NBOs provide a relatively weak connection between one tetrahedral cation (Al or Si) and one or more network modifier cations — such as Ca2+or Na+ — that are not an integral part of the tetrahedral network. The relative abundance of these weakly bonded NBOs is critical in determining the thermodynamic and dynamical properties of aluminosilicate glasses and melts1,2,3. For glasses of ‘tectosilicate’ composition, where the charge of the modifier cation equals the number of aluminium atoms (as in NaAlSi3O8 or CaAl2Si2O8), the conventional view of glass structure is that only BOs are present1,4. Here we present experimental observations that contradict this view. Our NMR measurements of CaAl2Si2O8, which determine directly the relative abundances of BO and NBO, indicate that a considerable amount of NBO can be present in a tectosilicate glass. These excess NBOs will increase the entropy and heat capacity of the corresponding liquid and decrease its viscosity, as well as modifying flow and diffusion mechanisms2,3. As the most common rhyolitic magmas and the molten precursors of glass ceramics have near-tectosilicate compositions1,4, our results require a reassessment of the high-temperature liquid properties that control many processes in the Earth and in industry.