1. 1. Stebbins, J.F. Nature 330,465-467 (1987). 2. Kraceck, F.C. J. phys. Chem. 34, 2645-2665 (1930). 3. Bruckner, R., Chun, H-U. & Goretzki, H. Glastech. Ber. 49,211-213(1976). 4. de Jong, B.H.W.S., Keefer, K.D., Brown, G.E. & Taylor, C.M. Geochim. cosmochim. Acta45,1291-1309 (1981). 5. Engelhardt, G., Zeigan, D., Jancke, H., Hoebbel, D. & Weiker, W. Z. anorg. dig. Chem. 418,17-28 (1975). 6. Marsmann, H.C. Z. Naturf. B29, 495^59 (1974). 7. Lagerstrom, G. Acta chem. Scand. 33, 722-736 (1959). 8. Ingri, N. Acta chem. Scand 13, 758-775, (1959). 9. Lippmaa, E. et al. J. Am. chem. Soc. 102, 4889^893 (1980). 10. de Jong, B.H.W.S., Schramm, C.M. & Parziale, V.E. J. Am. chem. Soc. 106, 4396-4402 (1984). 11. Grimmer, A.R. et al. Phys. Chem. Glasses 25, 105-109 (1984). 12. Dupree, R., Holland, D., McMillan, P.W. & Pettifer, R.F. J. Non-Cryst. Solids 68, 399-410 (1986). 13. Gary, L.W.,de Jong,B.H.W.S.& Dibble, W.E. Geochim. cosmochim. Acta 46,1317-1320 (1982).