Iron oxides of soils formed on gneiss of the Bacao Complex geodomain, Quadrilatero Ferrifero, Minas Gerais, Brazil

The objective of this work was to characterize iron oxides from B-horizons of three soil profiles developing on gneiss of the Bacao Complex geodomain in the Quadrilatero Ferrifero. Minas Gerais, Brazil. Samples were collected from the uppest, middle and lowest segments along the slope. The earth fine fractions (mean diameter, phi = 2 mm) were separated for all samples. The chemical composition of the major elements was determined with the X-ray fluorescence technique; the mineralogical analysis was performed with powder X-ray diffractomer and Mossbauer spectroscopy. All samples have similar mineralogical composition, with a general occurrence corresponding to the sequence quartz >> gibbsite > kaolinite > goethite. From the 4.2 K-Mossbauer results, the coexistence of goethite (major) and hematite is confirmed. The isomorphic aluminum contents, as they were deduced from the hyperfine fields, lead to the following chemical formulas for goethites: alpha Fe0.79Al0.21OOH (upslope), (alpha Fe0.75Al0.25OOH (midslope) and alpha Fe0.78Al0.22OOH (downslope). The iron oxides transformation dynamics in B horizons along the slope is a useful indicator of the paleo-climatic oscillations in this area: aluminous goethite is an indicator of humid paleo-environments, whereas aluminous-poorer goethite reveals drier pedogenic conditions.