The low temperature synthesis of iron silicate minerals with clay structures is possible at surface temperatures only under reducing conditions. Under oxidizing conditions clay minerals could not be synthesized. Instead quartz and quartzine were found in these X-ray amorphous Fe III hydroxide-silica precipitates after 14 days at low temperatures (20 ~ and 3~ as well as geothite or X-ray amorphous iron hydroxides. Only from solutions containing Fe-II could the different iron-containing clay minerals be built up within days at low temperatures. The presence of Fe-II enables an octahedral layer of the hrucite-gibbsite type to be formed. This is necessary for the bidimensional orientation of SiO4-tetrahedra leading to clay mineral formation. The presence of Fe ~ § and/or Mg2+-ions is necessary for the formation of the A13+-and Fea+-containing octahedral layers. The reducing conditions were obtained in the experiments by addition of dithionite. With a high content of silica (ca. 20 ppm SiO2, 7 ppm Fe) nontronite and lembergite, the di-Fe-III and tri-Fe-II octahedral, three-layer silicates, were built up in several days at low temperatures. With a lower silica content, that is, a lower Si/Fe ratio (15 ppm SiO2 and 20 ppm Fe), the two-layer silicate minerals greenalite and chamosite could be synthesized. A higher Mg content and more reducing conditions in the solutions favored the tri-as well as dioctahedral chamosite synthesis.The conditions of formation of recent naturally formed nontronite fit well with the synthesis conditions. Chamosites in sedimentary iron ores are characterized by a low content of SiO2, between 15-30% SiO2. This low content of silica cannot be the result of primary precipitation from seawater. The iron and silica ratio in seawater or in river waters would lead to a precipitation of ~60% SiO2 in the iron hydroxide precipitates. A probable origin for chamosite iron ores, which explains the low SiO2 content, is diagenesis of the lateritic weathering crust. Indeed, investigations of recent tropical shoreline sediments and in particular their trace element content confirm that cliamosite minerals have formed diagenetically from lateritic panicles in reducing sediments.

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Ahstraet-Glauconite has been synthesized at low temperature by precipitation of Fe-hydroxides from Si-, Fe-, Al-, and K-containing solutions under reducing conditions. The compositions favorable for the synthesis at 20~ and pH 8.5 are 1 ppm Fe, 0.15 ppm Al, 13 ppm SiO2, 1000 ppm KC1, and 1000 ppm dithionite. The K-content of the solutions must be sufficiently high to fix K in the precipitate. Under special early diagenetic conditions glauconite is formed in marine sediments, probably at the interface between reducing and oxidizing zones in the muddy sediments. The silica content of pore waters seems to control the formation of glauconite or chamosite rather than depth or temperatures of the bottom waters.