Evidence of Kaolinite Alteration in Soils of Hypersaline Tidal Flats on the Northeastern Brazilian Coastline.

Hypersaline tidal flats (HTF) are very common on the Brazilian coastline. The formation of HTF soils is a natural process in which the mangrove ecosystem is replaced by a new hypersaline environment. These places are flooded frequently, leading to a variation in the soil physio-chemical parameters in a short period of time. The characteristics of clay minerals in HTF soils were studied using XRD and TEM-EDX of the fine and coarse clay fractions. Results from one soil profile are reported here. XRD patterns of oriented clays were modeled using the software NEWMOD 3.2.1. The XRD modelling results indicate the presence of R0 mixed-layered minerals, like kaolinite-smectite (K-S), kaolinite-illite (K-I) and illite-smectite (I-S) in the fine clay fraction. K-S is the most abundant phase and occurs in a range of kaolinite layers (100 - 95 %, 88 - 80 %, 60 %, 37 %, 21 %), which was confirmed by chemical data from TEM-EDX. To achieve a better fit during the XRD modelling, K-I minerals with different layer proportions were included. The presence of K-I was also confirmed by TEM-EDX. The XRD data indicate the presence of end-members and mixed-clay minerals with higher structural order in the coarse clay fraction, composed by kaolinite, illite, smectite, illite-vermiculite (I-V), K-I and smectite-vermiculite (S-V). The data suggest the alteration of clay minerals through intermediary mixed-layer phases in the HTF soils. Furthermore, the high activity of Mg and K and the predominance of amorphous Si in the readily soluble Si pool create an environment favorable for clay transformations and formation of mixed layered minerals. Since mangroves and HTFs are genetically related, we believe that this process is similar to the process that occurs in mangrove soils, leading to the transformation of kaolinite into K-S and K-I via mixed-layer minerals.