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 magmas(1). Oxygen is the most abundant element in these materials, where it occupies either 'bridging' (BO) or 'nonbridging' (NBO) sites, BOs link two AlO4 or Si O4 tetrahedra, thereby providing strong, long-lived bonds between the smallest structural units of the aluminosilcate 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 abundan ce of these weakly bonded NBOs is critical in determining the thermody namic and dynamical properties of aluminosilicate glasses and melts(1- 3). For glasses af 'tectosilicate' composition, where the charge of th e modifier cation equals the number of aluminium atoms (as in NaAlSi3O 8 or CaAl2Si2O8), the conventional view of glass structure is that onl y BOs are present(1,4). Here we present experimental observations that contradict this view. Our NMR measurennents of CaAl2Si2O8, which dete rmine directly the relative abundances of BO and NBO, indicate that a considerable amount of NBO can be present in a tectosilicate glass, Th ese excess NBOs will increase the entropy and heat capacity of the cor responding liquid and decrease its viscosity, as well as modifying flo w sind diffusion mechanisms(2,3). As the most common rhyolitic magmas and the molten precursors of glass ceramics have near-tectosilicate co mpositions(1,4), our results require a reassessment of the high-temper ature liquid properties that control many professes in the Earth and i n industry.