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

Two new hydrous sodium silicates, NaSi(7)O(13)(OH)(3).3H(2)O (magadiite) and NaSi(11)O(20.5)(OH)(4).3H(2)O (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 6SiO(2).H(2)O (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.