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Science 8 September 1967:
Vol. 157. no. 3793, pp. 1177 - 1180
DOI: 10.1126/science.157.3793.1177
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Articles

Hydrous Sodium Silicates from Lake Magadi, Kenya: Precursors of Bedded Chert

Hans P. Eugster 1

1 Department of Geology, Johns Hopkins University, Baltimore, Maryland 21218

Two new hydrous sodium silicates, NaSi7O13(OH)3·3H2O (magadiite) and NaSi11O20.5(OH)4·3H2O (kenyaite), were found in lake beds at Lake Magadi, Kenya. Both are well-crystallized layered silicates with large basal spacings. Concretions within the magadiite bed consist of kenyaite or quartz (chert) in the center, surrounded by kenyaite. In dilute acids magadiite and kenyaite are converted to 6SiO2·H2O (SH), the first known crystalline hydrate of silica. The magadiite bed probably represents a chemical precipitate from alkaline brines. Percolating waters convert magadiite to kenyaite and eventually to chert. Thus a mechanism has been outlined for the formation of bedded chert deposits through inorganic precipitation. Alternations between silica-rich and iron-rich bands of iron formations may be due to concentration cycles in alkaline lakes.


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