Synthesis of Iron Layer Silicate Minerals under Natural Conditions

Abstract

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 SiO 4-tetrahedra leading to clay mineral formation. The presence of Fe 2+- and/or Mg 2+-ions is necessary for the formation of the Al 3+- and Fe 3+-containing octahedral layers. The reducing conditions were obtained in the experiments by addition of dithionite. With a high content of silica (ca. 20 ppm SiO 2,7 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 SiO 2 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 SiO 2, between 15–30% SiO 2. 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% SiO 2 in the iron hydroxide precipitates. A probable origin for chamosite iron ores, which explains the low SiO 2 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.