Glauconite Nucleation in Silica Tubular Microstructures from Low-Temperature Solution Experiments

The origin of glauconites, has been the subject of many researches, mainly because of its importance in the interpretation of the sedimentary record. However, the precise conditions and mechanisms for crystallization are still intriguing and, in particular, data about the glauconitization kinetics are very scarce. As is known, glauconite is an authigenic dioctahedral layer-silicate, which naturally occurs, in marine sediments, mainly connected with microenvironments semi-isolated from seawater. The earlier hypothesis relates the glauconite formation to cationic diffusion processes taking place on the structure of pre-existing layer silicates. Nevertheless, further works, based on both, field evidences and improved analytical studies, clearly indicates that glauconites form in a much faster process, by direct precipitation from seawater, via nucleation and growth. In addition, the small single-crystal domains observed by HREM (Ireland et al., 1983) shown that nucleation must have been favoured relative to the growth, indicating a high supersaturation degree at the formation sites. In this work, the initial stages of the glauconite formation were, firstly, investigated by performing lowtemperature syntheses experiments. Secondly, both, the structural and compositional characteristics of the obtained crystals, were compared with those of natural glauconites becoming from different source-areas and showing various crystallinity stages. Synthetic samples were obtained from solutions by using a modified procedure of the method previously reported by (Harder, 1980). These experiments were performed at room temperature, under reducing conditions and basic pH-s (from 8 to 11). Soluble salts containing Fe 2+ , Mg 2+ , K + , Al 3+ , were employed. In order to limit the extents of the Fe 2+ , oxidation, solution of Na-dithionite in water were prepared at first. The addition of the remaining solutes was performed in a second step.