Caracterizaçao mineralógica de cambissolos originados de rochas peliticas nos Patamares do Alto Rio Itajai e no Planalto de Lages (SC)

The cambisols (inceptisols) derived from shale rocks occupy about 10% of the total area of the State of Santa Catarina, but the mineralogical composition of these soils is still unknown. The mineralogical composition of four profiles of these soils collected in the cities of Rio do Sul (C-RS), Pouso Redondo (C-PR), Bom Retiro (C-BR) and Lages (C-LG) was studied in 1992 and 1993. Chemical and physical analyses in the fine earth fraction were performed by standard procedures; in the clay, sand and silt fractions, mineralogical X-ray diffraction analysis was performed. All soils were clayey, acidic, and had high values of exchangeable aluminium and very low values for sum of bases and percentage of base saturation. The mineralogy of the clay fraction was similar for all soils, consisting predominantly of kaolinite, in addition to micas, vermiculite and/or smectite with or without interlayered hydroxy-Al, interstratified clay minerals, and quartz. Quartz was the predominant mineral in the silt and sand fractions, which also had small amounts of micas. Based on the soil mineralogy, we suggest the following soil evolution sequence: C-RS - C-BR → C-PR → C-LG. The presence of 2:1 and interstratified clay minerals, associated with narrow profiles in many cambisols, are indicative of young soils, whereas the chemical properties observed are indicative of more developed soils. This apparent antagonism is explained by the intense weathering and leaching processes that occurred in these soils, and by their location on erosional geomorphic surfaces.