Extent of intermixing among framework units in silicate glasses and melts

One aspect of the inherent disorder in silicate glasses and melts is the intermixing among framework units, which has important implications to the macroscopic properties of silicate magmas. We present experimental evidence of extensive mixing of these framework units in silicate glasses including aluminosilicate and borosilicate glasses from oxygen-17 nuclear magnetic resonance (NMR), which shows remarkable similarity to predictions from ab initio molecular orbital calculations. We quantify the extent of framework disorder by introducing the degree of interdispersion (P) ranging from complete phase separation to random distribution for borosilicate glasses and the degree of Al avoidance (Q) for aluminosilicate glasses. Boron-II triple quantum magic angle spinning NMR results show that the fraction of the boroxol ring group increases with increasing boron content in binary borosilicate glasses, affecting the corresponding configurational enthalpy. We demonstrate that a more complete description of the macroscopic thermodynamic properties of silicate glasses can be successfully derived from detailed information on the degree of framework disorder. Copyright (C) 2002 Elsevier Science Ltd.