Iron-rich spinels from Brazilian soils

Tropical soils often contain high amounts of iron oxides. Hematite (alpha Fe2O3) and goethite (alpha FeOOH) are the most widespread iron oxides, but magnetite (Fe3O4) and maghemite (gamma Fe2O3) occur in magnetic pedons. A wide range of spinel compositions in the Fe3O4-gamma Fe2O3 series has been identified in magnetic Brazilian soils. Isomorphic substitution of mainly Ti4+, Al3+ and Mg2+, but also of Cr3+ and Mn2+ and other minor elements for iron are related to changes in their structural stability and magnetic properties. Magnetic iron oxides of selected Brazilian pedodomains are discussed, distinguishing those produced from mafic rocks (tuffite, basalt), where primary magnetite transforms to maghemite, from those produced in non-mafic lithologies (such as steatite), where inherited magnetite may be exceptionally stable in the soil.