Mineralogical and (sub)microscopic aspects of iron oxides in polygenetic Oxisols from Minas Gerais, Brazil

Iron oxides are known to be useful paleo-environmental indicators, and their properties may give information about past and present circumstances of weathering and soil formation. Iron oxides from polygenetic Oxisols developed on saprolites of metamorphic rocks (soil-saprolite sequences) and on sediments (soil-sediment sequences) from Minas Gerais, Brazil, were studied by X-ray diffraction, Mössbauer spectroscopy, micromorphology, and electron microscopy to establish the connection between their properties and circumstances of formation. Hematite and goethite are the only iron oxides found in the clay and silt fractions. Micromorphology and mineralogy indicated the presence of three genetically different forms of hematite and four forms of goethite. XRD and Mössbauer spectroscopy indicate that these forms have differences in mean coherence length (MCL) and Al substitution. In soil-saprolite sequences, hematite is mainly formed during saprolite formation, while goethite formed mainly during soil formation. Microscopic hematite formed in the saprolite has low Al substitution and uniform MCL. In response to soil formation, it diminishes to submicroscopic size and goethite is formed towards toposoils. Goethite formed in yellow surface soils reflects recent soil formation and has high Al substitution. Hematites and goethites related to gley phenomena have low Al substitution and large MCLs. Different coexisting forms of either goethite or hematite point to varying pedo-environments, either in time, corroborating the polygenetic nature of the studied soils, or at micron-scale.