STRUCTURE AND DYNAMICS OF MAGNESIUM IN SILICATE MELTS - A HIGH-TEMPERATURE MG-25 NMR-STUDY

Mg-25 NMR spectra for several silicate and aluminosilicate melts were obtained from 1000-1470 degrees C. The peaks are initially very broad, but narrow with increasing temperature to near 500 Hz at the highest temperatures. The peak positions for most of the melts do not shift no ticeably with temperature in the range studied, except for a sodium ma gnesium silicate composition that was previously studied by Fiske and Stebbins (1994). This material showed a decrease in frequency of the p eak position by about 4 ppm between 1150-1360 degrees C in both this a nd the previous study, consistent with an increase in the average size of the site. The chemical shifts vary with composition as well, rangi ng from 31 ppm for a potassium sodium magnesium silicate melt to 22 pp m for diopside melt (CaMgSi2O6) at 1400 degrees C. Compositions with h igher field strength cations have lower frequency chemical shifts, whi ch correspond to larger coordination numbers and bond lengths for Mg2. All of the peak positions obtained fall to slightly higher frequency than the range for sixfold-coordinated Mg in crystals and well below the fourfold-coordinated range, indicating that the Mg is in fivefold to sixfold coordination in the melts. Spin-lattice relaxation times sh ow that measurements are on the high-temperature side of the T-1 minim a, and a simple expression for quadrupolar relaxation can be used to o btain correlation times for the motion responsible for the relaxation. The correlation times obtained in this manner are very similar to the correlation time tau(shear) obtained from viscosity measurements, imp lying that the Mg motion is strongly coupled to the network motion at these temperatures. Line widths also scale with T-1 in this temperatur e range, leading to the conclusion that the viscosity is the fundament al limit to observing the Mg-25 Signal in the melt.