Gênese de minerais em planossolos sob clima semiárido, Nordeste do Brasil

Abstract

With a global extension of 130 million hectares, Planosols have been recognized and studied around the world. In Brazil, more specifically in the northeastern semi-arid region, these soils have been the subject of large studies. Planosols are characterized by having a plain B horizon, a striking feature of these soils is the enrichment of clay in the subsurface horizons. Soil clays are represented by clay minerals which are abundant components in fractions < 2mm. They play a significant role in soil functioning, as they control geochemical and soil formation processes. In the northeastern semi-arid region, studies of clay genesis and interstratified phases in soils that allow for evidence of formation/transformation processes are incipient. Therefore, knowing the representativeness of Planosols formed on an expressive lithology in the semi-arid region of Brazil, the aim of this research was to advance in the understanding of the clays minerals genesis of these soils formed on a granitic/gnessic suite under a semi-arid climate. For this, the crystallochemical characterization of fractions 2-0.2mm and < 0.2mm was carried out based on XRD techniques, XRD modeling, FTIR, EDX, Mössbauer spectroscopy and thermal analysis. Two profiles were selected for the study in the municipality of Lagoa do Ouro, one located in the lowest part (P1) and the other in the highest part of the landscape (P2). The results showed that the climatic conditions associated with the drainage of the study site favored the formation of interstratified mineral phases, indicating an incipient weathering. The main route of formation of phyllosilicates end members in these soils goes through the following route: Micas (biotite and muscovite) – Interstratified (Ilite-Vermiculite, Ilite-Smectite, Kaolinite-Ilite, Kaolinite-Ilite) - end members: Ilite + Smectite + Kaolinite /Halloysite. The halloysite present is probably formed from the alteration of feldspars and the presence of gibbsite in both profiles seems to be formed by neoformation in microsites of greater drainage in the soil structure. Through this study, it was possible to detail the mineral phases present in Planosols formed on a granitic/gneissic suite, expanding the understanding of soil pedogenesis in a semi-arid environment, as well as deepening the knowledge of the diversity of minerals, which may support future fertility studies, management and use of these soils.