Soil genesis, mineralogy and chemical composition in a steatite-serpentinite outcrop under tropical humid climate in Bom Sucesso, Brazil

Abstract

Serpentinite and steatite are metamorphic ultrabasic rocks, consisting of variable proportions of serpentine, talc and magnetite, among other unusual minerals. Soils derived from these rocks are typically unproductive due to excess Mg, low P content and high levels of heavy metals, but in tropical countries these soils are still poorly studied. This work aimed to understand the process of formation of eight steatite-serpentinite soils in the ultramafic complex of Morro das Almas, Bom Sucesso, Brazil, and to its mineral and geochemical composition. The soils were stratified into three basic types, in increasing degree of weathering: a) Lithic Udorthents (3 profiles); b) Oxic Dystrudepts (2 profiles); c) Acrudoxes (3 pedons). All soils presented high particle density, but low soil density due to the high packing void porosity associated to the granular structure. Soil pH and exchangeable Mg were relatively high, and there were low P and Al contents. Magnetic susceptibility increased strongly with the degree of weathering due to the concentration of magnetite. X-ray fluorescence analysis showed that Lithic Udorthents and Oxic Dystrudepts have high levels of total MgO (94 to 200 g kg-1) and higher Fe2O3 contents, while Acrudoxes are even more concentrated in Fe2O3, although they also have levels of MgO (6 to 30 g kg-1). The SiO2 and Al2O3 contents are relatively low, although silica is more concentrated in the Lithic Udorthents and Oxic Dystrudepts, poor in alumina, whereas alumina predominates over silica in the other soils. This unusual composition is due to the mineralogy of the granulometric fractions, including clays, which have an important amount of talc in Lithic Udorthents and Oxic Dystrudepts, with much lower presence of kaolinite and secondary oxides of Fe, whereas the other soils have massive concentrations of hematite. Serpentine was not found in soils, although present in the rock, and quartz occurs in low quantity in the sand, and probably. The high contentes allochtone of MgO are due to fragments of steatite-serpentinite preserved by an iron oxide crust, visible macro- and microscopically. High levels of Cr2O6 (up to 50 g kg-1 soil) suggest that part of the diffraction peaks attributed to hematite are due to chromite (FeCrO4), while higher NiO (up to 6 g kg-1 soil) was due to the substitution of Mg by Ni in the talc, forming garnierite and co-precipitation in Fe oxides. Slope and aspect strongly influence the formation of the soil and its composition, and this complex interaction results in varied formations of canyon cloud forest, semi-deciduous forest and savannas in a small area of 3.1 km2.