Evidence for the compensated continuous random network model for spodumene glass and analogues

We have combined X-ray and neutron scattering with solid state nuclear magnetic resonance (NMR) measurements on glasses of the spodumene and analogous compositions. These glasses LiAlSi2O6, NaAlSiO6 and KAlSi2O6 are fully compensated i.e. contain equal quantities of alkali and aluminium. Q-space and R-space representations for each probe suggest considerable similarity with the structure of silica and point to the crystalline analogue of P-spodumene, which has a stuffed quartz structure. Preliminary Reverse Monte Carlo (RMC) simulations have been made starting from a fully polymerised silica network model. Solid state Magic Angle Spinning NMR reveals that all aluminium retains tetrahedral coordination. This is indicative of fully compensated continuous random network (CCRN) structures for each glass. The chemical shift of Na-23 in NaAlSi2O6 glass indicates that sodium is associated with (AlO4)(-) tetrahedra. At the same time nearest neighbour oxygen distances from neutron scattering increase with size of alkali, suggesting reduction in charge transfer with alkali size together with the possibility of alkali clustering.