Complexity of clay minerals and its effects on silicon dynamics in hypersaline coastal wetland soils, Brazil

Context. Tidal dynamics and high evaporative rates in hypersaline tidal flat (HTF) soils favour a range of clay reactions and the formation of complex clay assemblages. HTFs have a geochemical environment conducive to mineral reactions, and therefore, clay mineral alteration controls essential nutrients in coastal wetlands. Aims. The crystallochemical characteristics of clay minerals from two HTFs were studied to provide insights into the complexity of fine clay assemblages in HTF soils and their effects on Si biogeochemistry. Methods. The fine clay fraction (<0.2 mu m) from these HTFs were investigated using X-ray diffraction (XRD) modelling, XRF, FTIR, TEM-EDS, and silicon sequential extractions. Key results. The results have indicated the presence of endmembers and R0 mixed-layered minerals such as kaolinite, smectite, illite, kaolinite-smectite (K-S), kaolinite-illite (K-I), illite-smectite (I-S), and illite-vermiculite (I-V). In general, K-S was the dominant mineral in the samples, occurring in a range of kaolinite layers. Alteration of detrital kaolinite to other mixed-layered minerals occurs in soils, leading to silicon removal from the soil solution because of mineral genesis. There is Mg enrichment at the expense of Al in the samples, and bio-opal appears to be the source of Si for the kaolinite alteration process. Conclusions. Our findings indicate that clay authigenesis is an important factor controlling Si dynamics in HTF soils, acting as a sink of Si during the formation of new clay phases.