1. R. Mannkopff undCl. Peters, Z. Physik70, 444 (1931).
2. E. Preu�, Z. angew. Mineral.3, 8 (1940).
3. K. H. Wedepohl, Geochim. Cosmochim. Acta3, 93 (1953).
4. E. Preuss, Fortsohr. Mineral.28, 63 (1949).
5. R. L. Mitchell, The Spectrographic Analysis of Soils, Plants and Relatad Materials. Commonw. Bur Soil Science, Techn. Comm. No. 44 (1948),