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Premelting effects are common in silicates and have been mostly observed as significant heat capacity anomalies beginning 100 to 200 °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 °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 +  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 +  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 °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 +  diffusion near the melting point. In contrast to the simple Li +  diffusion in Li 2 SiO 3 , this process may require considerable nonvibrational energy and may thus be related to the heat capacity anomaly just prior to melting.

Cation dynamics and premelting in lithium metasilicate (Li 2 SiO 3 ) and sodium metasilicate (Na 2 SiO 3 ): A high-temperature NMR study