Iron-rich spinels from Brazilian soils

Tropical soils often contain high amounts of iron oxides. Hematite (αFe₂O₃) and goethite (αFeOOH) are the most widespread iron oxides, but magnetite (Fe₃O₄) and maghemite (γFe₂O₃) occur in magnetic pedons. A wide range of spinel compositions in the Fe₃O₄-γFe₂O₃ 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. This revised version was published online in August 2006 with corrections to the Cover Date.