THE STRUCTURAL OF MG IN SILICATE LIQUIDS - A HIGH-TEMPERATURE MG-25, NA-23, AND SI-29 NMR-STUDY

We have studied the structural role of Mg in silicate liquids as a fun ction of temperature and composition using high-temperature Mg-25, Na- 23, and Si-29 NMR. To establish the relationship between the isotropic chemical shift (delta(iso)) and the coordination number for Mg-25 in silicates, we have obtained high-resolution Mg-25 MAS NMR spectra on i sotopically enriched akermanite ([4]Mg) and diopside ([6]Mg) using spi n-echo pulse sequences and absolute-value Fourier-transform techniques . The delta(iso) for akermanite is 49 +/- 3 ppm, and that for diopside is 8 +/- 3 ppm, demonstrating that the chemical-shift range for Mg-25 in the silicates is similar to that established in oxides. The delta( iso) for Mg-25 in (CaO)0.29 (MgO)0.14 (SiO2)0.57 liquid at 1400-degree s-C is 8 ppm, suggesting that Mg resides in a structural environment s imilar to that in diopside. The delta(iso) for Mg-25 in (Na2O)0.28(MgO )0.18(SiO2)0.54 liquid shifts from 34 ppm at 1150-degrees-C to 29 ppm at 1360 degrees-C, indicating smaller coordination numbers than in the Ca liquid but a relatively large increase in the Mg-O bond length or coordination number (or both) with increasing temperature. The delta(i so) for Na-23 shows similar, but less pronounced, changes, whereas the Si-29 delta(iso) is unchanged over this range of temperatures. This s uggests that Mg undergoes the largest increases in bond distance or CN (or both) of any of the cations in the liquid. Spin-lattice relaxatio n times for Mg-25 and Na-23 were measured in liquid (Na2O)0.28-(MgO)0. 18(SiO2)0.54 over the same range of temperatures. The apparent activat ion energy of the Mg-25 relaxation (related to Mg diffusional motion) is 119 kJ/mol, intermediate between that of Na and Si. The apparent ac tivation energy for Na-23 relaxation is 85 kJ/mol, which is higher tha n that observed in sodium silicate liquids of similar polymerization b ut comparable to that in mixed alkali silicate liquids, suggesting tha t Na and Mg may show a mixed-cation effect.