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°C) 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 brucite-gibbsite type to be formed. This is necessary for the bidimensional orientation of SiO4-tetrahedra leading to clay mineral formation. The presence of Fe2+- and/or Mg2+-ions is necessary for the formation of the Al3+- and Fe3+-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 chamosite minerals have formed diagenetically from lateritic particles in reducing sediments.

The formation of 2:1 and 1:1 phyllosilicates in Fe–Si–O–H systems occurs in various geological and engineering settings; however, the identification and characterization of these minerals is very challenging due to the limited amount that is accessible, the very small particle size, and often the large degree of heterogeneity of these samples. To overcome these drawbacks, the synthesis of iron-rich phyllosilicates was attempted in this study with an initial Fe/Si molar ratio ranging from 0.50 to 2.33. The synthesis was performed at 150°C under hydrothermal conditions over a period of 7 days. Synthesis products were characterized by X-ray diffraction and fluorescence, by infrared, Raman, and Mössbauer spectroscopies, and by transmission electron microscopy. Results revealed that the stability field of the 2:1 clay mineral was wider than that of the 1:1 clay mineral. The 2:1 clay mineral was less sensitive to redox conditions compared to the 1:1 clay mineral. In addition, a heterogeneity of phases formed (iron oxides, nontronite, cronstedtite, or greenalite) was identified.