Spatial Organization and Mineral Transformations of 2:1 Phyllosilicates in Saline–Alkaline Soil–Lake Systems of the Pantanal (Nhecolândia, Brazil)

In the saline–alkaline lake (SAL) systems of the Nhecolândia region, Brazilian Pantanal, soils exhibit complex mineralogical assemblages controlled by sediment inheritance, pedogenesis, and hydrogeochemical gradients. This study investigates the distribution and transformation of 2:1 phyllosilicates along representative SAL toposequences. Soil samples were characterized by Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD), supported by granulometry and adjustment of the FTIR spectra. Mineralogical data were integrated with geochemical (Al, K, Mg, Ca, Na) and pH data and examined using principal component analysis (PCA). Greenish loamy horizons act as key morphological controls on hydrogeochemistry, regulating solute retention along mid- to downslope transitions. Illite is more strongly associated with upslope positions, whereas downslope alkaline environments are associated with smectitic phases (e.g., montmorillonite and Mg-rich varieties such as saponite) and mixed-layer minerals structures (e.g., illite–smectite and montmorillonite–vermiculite structures). These assemblages are consistent with non-linear transformation pathways, with illite as a possible transitional phase between micas and expandable structures. The PCA results suggest a primary mineral distribution structured by fine-material content and depth, while pH and alkalinity emerge as key geochemical controls that differentiate mineral stability fields and reinforce the hydrogeochemical compartmentalization of the profiles. Geochemical data show strong associations of Al, Mg, and K with fine-fraction accumulation. The integration of these approaches highlights that a 2:1 phyllosilicate assemblage results from multiple superimposed pedogenetic pathways, offering a conceptual framework for studying complex soil–lake systems.