1. Heller, G.: Top. Curr. Chem. 15 (2), 206/80 (1970)
2. Christ, C. L.: Am. Mineralogist 45, 334/40 (1960)
3. Tennyson, Ch.: Fortschr. Mineral. 41, 64/91 (1963)
4. Edwards, J. O., Ross, V. F.: J. Inorg. Nucl. Chem. 15, 329/37 (1960)
5. Clark, J. R., Christ, C. L.: Am. Mineralogist 56, 1934/54 (1971)