Weathering and pedogenesis of mafic rock in the Brazilian Atlantic Forest

Abstract The effects of parent material on soils tend to be stronger than those of temporal and environmental conditions that interfere with weathering and pedogenesis. The main objective of the present study was to characterize and evaluate rock weathering and pedogenesis in the toposequence of gabbro-derived soils using geochemical and micromorphological data. Four profiles were sampled from a toposequence in the highlands of the Atlantic Forest region of Rio de Janeiro, Brazil. We carried out chemical and physical characterization of samples from disturbed soil horizons, followed by soil classification. Samples from undisturbed soil horizons were subjected to mineral weathering and micromorphological analyses, and scanning electron microscopy was used for microanalyses. We determined the total content of elements (SiO2, Al2O3, Fe2O3, TiO2, CaO, MgO, Na2O, K2O, Mn, P2O5, and Zr) and calculated the following weathering indices: chemical alteration index, chemical weathering index, and weathering index scale. We observed the progressive alteration of olivine into chlorite, weathering of clinopyroxenes and plagioclase, and neoformation of clay minerals such as kaolinite and iron oxyhydroxides such as goethite and hematite. The weathering of magnetite and ilmenite resulted in hematite and leucoxene, respectively. The high quantities of Si and Al in the soil matrix and the presence of iron oxyhydroxides and nodules indicated that fersialitization was the predominant pedogenetic activity associated with soil formation. Moreover, partial hydrolysis favored the formation of 2:1 clay minerals. Other processes included eluviation and illuviation, evidenced by the micromorphological analysis that indicates clay coating on pores, cavities, and aggregates.