NA-23 NMR CHEMICAL-SHIFTS AND LOCAL NA COORDINATION ENVIRONMENTS IN SILICATE CRYSTALS, MELTS AND GLASSES

In order to decipher information about the local coordination environm ents of Na in anhydrous silicates from Na-23 nuclear magnetic resonanc e spectroscopy (NMR), we have collected Na-23 magic angle spinning (MA S) NMR spectra on several sodium-bearing silicate and aluminosilicate crystals with known structures. These data, together with those from t he literature, suggest that the Na-23 isotropic chemical shift correla tes well with both the Na coordination and the degree of polymerizatio n (characterized by NBO/T) of the material. The presence of a dissimil ar network modifier also affects the Na-23 isotropic chemical shift. F rom these relations, we found that the average Na coordinations in sod ium silicate and aluminosilicate liquids of a range of compositions at 1 bar are nearly constant at around 6-7. The average Na coordinations in glasses of similar compositions also vary little with Na content ( degree of polymerization). However, limited data on ternary alkali sil icate and aluminosilicate glasses seem to suggest that the introductio n of another network-modifier, such as K or Cs, does cause variations in the average local Na coordination. Thus it appears that the average Na coordination environments in silicate glasses are more sensitive t o the presence of other network-modifiers than to the variations in th e topology of the silicate tetrahedral network. Further studies on sil icate glasses containing mixed cations are necessary to confirm this c onclusion.