1. Huffman, D. R., Nature, 225, 833 (1970).
2. Manning, P. G., Nature, 226, 829 (1970).
3. Berkes, J. S., Monograph No. 2, Materials Research Laboratory (Pennsylvania State University).
4. Cotton, F. A., and Wilkinson, G., Advanced Inorganic Chemistry, 841 (Interscience Publishers, New York, 1964).
5. Keester, K. L., and White, W. B., Proc. Fifth Intern. Mineralogical Assoc. Meet., Cambridge, 22 (1966).