1. S. P. Zhdanov and E. V. Koromal'di, Bull. Acad. Sci. USSR, Div. Chem. Sci. 811 (1959).
2. N. A. Tudorovskaya, Coll. Structure of Glass, pub. Acad. Sci. USSR, (Moscow, 1955), p. 190.
3. V. A. Florinskaya, Coll. Structure of Glass, pub. Acad. Sci. USSR (Moscow, 1955), p. 70.
4. E. A. Porai-Koshits, Coll. Structure of Glass, pub. Acad. Sci. USSR, (Moscow, 1955), p. 30
5. J. Biscoe, C. S. Robinson, and B. E. Warren, J. Am. Ceram, Soc. 22, 180 (1939).