Mineralogy and genesis in an alkaline soil system in the southern Pantanal wetland, Brazil

Abstract

The genesis of clays in Pantanal soils is influenced by parent material, water seasonality and elevated temperatures, resulting in soils with attributes not so common in other tropical regions. This research aimed to study the genesis of minerals in the clay fraction present in three soil profiles developed from carbonate sediments in Pantanal of the State of Mato Grosso do Sul, Brazil. Three trenches were excavated, and the soils were morphologically described and classified according to WRB in Chernozem (profiles P1 and P3) and Gleysol (P2). In addition, deformed samples from all soil horizons were sampled and used in physical, chemical, and mineralogical analyses. Clay fraction obtained from the diagnostic subsurface horizons of the profiles was submitted to identification of the mineral phases using X-ray diffraction, morphological analysis in a scanning electron microscope and microchemical analysis using energy dispersive X-ray spectroscopy. In the three profiles, the presence of interlayered minerals of kaolinite-smectite and illite-smectite; quartz and calcite in profile P3, was confirmed in the clay fraction. The clays from the three profiles presented high levels of SiO2 (ranging from 35.4 to 56.7%). The clays from profiles P1 and P2 presented high levels of Al2O3 (P1 = 24.6% and P2 = 15.2%), Fe2O3 (P1 = 14.0% and P2 = 11.8%), which suggests the formation of dioctahedral clay and illite rich in Fe. The high levels of MgO (11.1%) and Fe2O3 (7.2%) in clays of the P3 profile indicated the presence of trioctahedral Mg-smectite and illite rich in Fe. In the studied soils, the condition of moderate to imperfect drainage, water from the water table rich in cations and the alkaline environments of soils, constitutes the principal factor responsible for the current dynamics of neoformation of dioctahedral illite rich in Fe and trioctahedral Mg-smectite and dissolution of kaolinite.