1. Burns, J.H., and Bredig, M.A., J. Chem. Phys., 25 (1956) 1281
2. Schräder, R., and Hoffman, Br., Z. Chem., 6 (1966) 388
3. Dandurand, J-L., C.R. Acad. Sci. Paris, Ser. D (1970) 881
4. Gregg, S.J., Chem. Ind., 11 (1968) 611
5. Northwood, D.O., and Lewis, D., Amer. Miner., 53 (1968) 2089