SILICATE SPECIES EXCHANGE, VISCOSITY, AND CRYSTALLIZATION IN A LOW-SILICA MELT - IN-SITU HIGH-TEMPERATURE MAS NMR-SPECTROSCOPY

High-resolution, magic-angle-spinning (MAS), Si-29 NMR spectra have be en obtained for a sodium silicate glass and liquid (60 mol% SiO2) to t emperatures over 600 degrees C. Crystallization was monitored in situ, and motional averaging of the peaks for distinct silicate anionic spe cies (Q(2) and Q(3) species) in the liquid was observed. Modeling of t he spectra yielded species exchange frequencies. The inverses of these exchange frequencies are slightly longer than shear relaxation times calculated from bulk viscosity using the Maxwell relation; viscosities calculated from the NMR data using an Eyring model are close to, but slightly less than, measured viscosities. These results are similar to those obtained for Q(3)-Q(4) exchange in K2Si4O9 liquid, indicating t hat local Si-O bond breaking occurs on a time scale consistent with it being the primary control on viscous flow even in lower silica liquid s.