Weathering of Permian sedimentary rocks and soil clay minerals transformations under subtropical climate, southern Brazil (Paraná State)

Mineral evolution in weathering zones is usually a complex process. Mixed-layered minerals (MLMs) can form as a result of progressive weathering advancement, promoting important geochemical changes in soils. In this contribution, four soil profiles located in Paraná State (southern Brazil) and formed from different fine-grained Permian sedimentary rocks (one siltstone, two Al-rich shales and one organic black shale) were characterized on mineralogical and geochemical bases. General physico-chemical/morphological characterization, XRD profile modeling of clay fractions, geochemical mass balance and weathering index calculations were performed to relate clay minerals transformation with the geochemical transformations in soils. MLMs dominate clay mineral suite, in progressive compositional ranges. The three soils developed from Al-rich shales (P1, P2 and P4) are dominated by several R0 K-I (kaolinite-illite) phases, and exhibit a trend of progressive kaolinization towards surface soil horizons. The soil developed from Mg-rich organic black shale (P3), however, has R0 I-S (illite-smectite) as the dominant phase, and a progressive dominance of smectite in I-S phase prevails towards surface. These results suggest the prevalence of topotactic transformations (solid-state reactions) during the transition of one clay into another as weathering advances. In the three soils where kaolinization of K-I phases prevails, geochemical mass balance calculations indicate massive losses of SiO2, Al2O3 and alkalis, in addition to high values for weathering indices. Conversely, SiO2 and MgO accumulate, and values for weathering indices are lower in the P3 soil profile. The agreement between the pathways of clay mineral transformations and the mass balance calculations and weathering indices suggests that transformations involving MLMs drive geochemical transformation in soils. The nature of clay suites, in turn, is highly dependent on the chemical compositions of fresh rocks, which determine how transformations involving clays proceed.