IRON-RICH SPINELS FROM BRAZILIAN SOILS

Tropical soils often contain high amounts of iron oxides. Hematite (al pha Fe2O3) and goethite (alpha FeOOH) are the most widespread iron oxi des, but magnetite (Fe3O4) and maghemite (gamma Fe2O3) occur in magnet ic pedons. A wide range of spinel compositions in the Fe3O4-gamma Fe2O 3 series has been identified in magnetic Brazilian soils. Isomorphic s ubstitution of mainly Ti4+, Al3+ and Mg2+, but also of Cr3+ and Mn2+ a nd other minor elements for iron are related to changes in their struc tural stability and magnetic properties. Magnetic iron oxides of selec ted Brazilian pedodomains are discussed, distinguishing those produced from mafic rocks (tuffite, basalt), where primary magnetite transform s to maghemite, from those produced in non-mafic lithologies (such as steatite), where inherited magnetite may be exceptionally stable in th e soil.