Origin and properties of kaolinites from soils of a toposequence in Southern Brazil

Abstract

ABSTRACT Kaolinite is the main clay mineral in most soils around the world and has been widely used for industrial purposes. This research aimed to study chemical, morphological and crystallographic characteristics of kaolinite, and establish the origin of kaolinitic samples on Serra do Mar and kaolinitic layers on peatlands, located at Southern Brazil. Samples were collected on different geomorphological positions: two samples at Serra do Mar (kaolinitic saprolite – SAP, and kaolinitic layers - KL); and two cores at the peatland with Sapric Histosols from Quaternary sedimentary basin. Granulometry and total organic carbon (TOC) were determined in soil samples. Kaolinite in silt and clay fractions was studied by chemical extractions, X-ray diffraction (XRD), thermal analysis (DTA/TG), and scanning electron microscopy with energy dispersive spectroscopy – SEM/EDS. Chemical and mineralogical characteristics of kaolinite were divided into two groups, according to the particle size and the location of the deposit in the relief. Silt fraction: i) SAP – genesis mainly derived from mica weathering; ii) peatland, containing pseudomorph crystals smaller than those found in Serra do Mar; Clay fraction: i) Serra do Mar – there was a larger contribution of K-feldspar weathering in the genesis of kaolinite from KL in relation to SAP; ii) peatland – the stronger weathering and the hydromorphic conditions resulted in less neoformed crystalline kaolinites. For both environments, the substitution of Al 3+ by Fe 3+ into the octahedral sheet led to a reduction in the mineral thickness and also increased the occurrence of structural deformations in clay kaolinite. Kaolinite in peatland is a combination of the following genesis processes: transportation from Serra do Mar (mainly in the silt fraction) and; formation in situ through neogenesis process (dominant in the clay fraction).