Abstract

Premelting effects are common in silicates and have been mostly observed as significant heat capacity anomalies beginning 100 to 200 degrees C below congruent melting points. To assess the role of cation dynamics in this phenomenon, we collected nuclear magnetic resonance (NMR) spectra and relaxation time data to within 20 to 50 degrees C of the melting points of sodium metasilicate (Na 2 SiO 3 ), which displays a large premelting effect, and isostructural lithium metasilicate (Li 2 SiO 3 ), which displays little premelting. From 7 Li NMR, Li (super +) site hopping is clearly observed in Li 2 SiO 3 by a partial averaging of the 7 Li quadrupolar peak shape, requiring exchange among a few, ordered orientations of LiO 4 tetrahedra. From 23 Na NMR, Na (super +) site hopping in Na 2 SiO 3 appears to involve a more liquid-like behavior, implying exchange among many sites with different orientations in a disordered fashion. For this phase, 29 Si spectra indicate that in an oxidizing environment, no liquid phase is present at 20 degrees C below the melting point, well within the calorimetric premelting regime. However, partial averaging of the low-temperature, biaxial chemical shift powder pattern (typical of Si sites in chain silicates) occurs, suggesting some kind of extensive, librational motion of SiO 4 tetrahedra that is possibly linked to rapid Na (super +) diffusion near the melting point. In contrast to the simple Li (super +) diffusion in Li 2 SiO 3 , this process may require considerable non-vibrational energy and may thus be related to the heat capacity anomaly just prior to melting.

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