In northern and western landscapes of São Paulo State, plinthite and petroplinthite are features observed commonly above the cretaceous Adamantina Formation sandstones (Bauru Group). Two representative soil profiles with plinthic, petroplinthic and mottling features were selected in the local landscape to evaluate the mineralogical characteristics and to study their formation. The study was carried out at the foot of a mountainside in the Experimental Station Pindorama of the Agronomic Institute Campinas (IAC), in the north of São Paulo State, Brazil. Scanning microscopy and pontual microanalysis of selected glaebules, as well as the mineralogical analyses of the ironfree clay fraction and the iron oxides of all studied soil profiles were carried out. It was concluded that the main clay fraction minerals of the nodules and the soil are kaolinite, hematite, and goethite. Mica, gibbsite, and anatase also contribute to the mineralogy of the clay fraction of these glaebules, being similar in composition to the surrounding soil material of the profiles. The silt and sand fraction of the nodules is composed by quartz, potassium feldspars, traces of sodium feldspars, and ilmenite. The constant presence of weatherable minerals in the petroplinthic glaebules clearly indicates that the formation of these features was related to iron segregation in the saprolite. This phenomenon, associated with low levels of aluminum in the structure of the glaebules iron oxides suggests their formation under hydromorphic conditions. The hydromorphic environment is probably related to the solubilization and mobilization of ferrous iron that was removed from the upper landscape and precipitated into the vadose zone, where oxidative conditions were favorable for iron segregation and precipitation.
soil genesis; petroplinthite; plinthite; laterite; iron oxides
