Tracing the reactive melting of glass-forming silicate batches by in situ Na-23 NMR

The kinetics of the reaction of batches of powdered quartz and sodium carbonate was studied by in situ 23Na nuclear magnetic resonance (NMR) spectroscopy using a laser-heated probe. We show for the first time that the technique allows one to study solid-state reactions at high temperatures with good time resolution and without the risk of quenching artifacts. The reaction is controlled by solid-state Na+ diffusion across the grain interface. Independent of the batch composition, the first reaction product is crystalline sodium metasilicate, Na2SiO3, even if the temperature is high enough for much of the composition space between silica and metasilicate to be above the equilibrium liquidus. Fast Na+ diffusion allows the reaction front to cross the grain interface and form the solid product before liquid intermediate equilibrium products can be formed. This purely solid-state reaction slows down as the thickness of the interface increases; the reaction is more deceleratory than published models suggest. If excess quartz is present, it reacts in a second step involving a liquid film wetting the excess grains. Once this reaction has started, it pulls the reaction into the thermodynamic regime, which leads to an increase even in the rate of the first step leading to intermediate solid metasilicate.