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

Abstract

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.

Clay mineralogy of Pleistocene Lake Tecopa, Inyo County, California

“…If the Tecopa magadiite was precipitated in the man ner described by Eugster (1967) for the waters of Lake Magadi, then the mineral may have covered a larger area in the Lake Tecopa basin than is apparent from the present-day deposit. Subsequent erosion that has scoured the clay beds and tuffs would have removed most of the magadiite, or the sodium could have been leached out leaving behind silica which converted to microcrystalline quartz (Eugster, 1969) or opaline silica.…”

“…Magadiite was first described by Eugster (1967), who reported its occurrence at Lake Magadi, Kenya, in the High Magadi beds of Late Pleistocene to Holocene age. He, and later Jones, Rettig, and Eugster (1967), stated that the magadiite was precipitated from alkaline, silica-rich brines of high pH in waters of a more dilute precursor of present-day Lake Magadi.…”

“…He, and later Jones, Rettig, and Eugster (1967), stated that the magadiite was precipitated from alkaline, silica-rich brines of high pH in waters of a more dilute precursor of present-day Lake Magadi. Ac cording to Eugster (1969), if such brines come in con tact with fresh water, a stratification of the lake may occur producing magadiite saturation at the interface. Solubility curves for magadiite (Eugster, 1969) sug gest a minimum pH value of between 8.5 and 9.0, which may be the lower limit of pH for precipitation of magadiite.…”

“…Ac cording to Eugster (1969), if such brines come in con tact with fresh water, a stratification of the lake may occur producing magadiite saturation at the interface. Solubility curves for magadiite (Eugster, 1969) sug gest a minimum pH value of between 8.5 and 9.0, which may be the lower limit of pH for precipitation of magadiite. Bricker (1969) stated that above pH 9, magadiite would be soluble because of the increased silica solubility, and below pH 9, its solubility would be increased owing to decreasing [Na+]/[H+] ratio.…”

Synthetic, layered nanoparticles for polymeric nanocomposites (PNCs)

“…(Bottom) Low voltage SEM micrographs of Topy synthetic, non-purified Na-tetrasilicic mica at  10 k. The synthetic clay forms large aggregates of high aspect ratio rectangular platelets. 59 it has been used mainly as a base for the preparation of mesoporous catalyst. 60 The silicic acids have been synthesized by hydrothermal methods.…”