Weathering trends and parent material characteristics of polygenetic oxisols from Minas Gerais, Brazil: I. Mineralogy

In geologically stable areas in the tropics, climatic changes and geomorphic cycles give origin to polygenetic soils. Polygenesis involves new soil formation phases taking place on preweathered materials from previous phases, resulting in soils with rather similar chemical and mineralogical properties. Polygenetic soils from Minas Gerais, Brazil, were investigated with the purpose of distinguishing mineralogical aspects in relation to polygenesis. The soil materials were studied by micromorphology and electron microscope and analyzed by XRD. All soils are strongly weathered and even show weathered grains of ilmenite and quartz. The soils show two phases of kaolinite formation (pseudomorphs after mica and precipitation from solution) and of accumulation of iron compounds. Desilication causes gibbsite formation from kaolinite, occasionally even in the deep saprolite. Superposed processes can be due to progressively deeper weathering or to changes of the external environment