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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...
Powdered crystals of an iron-rich spinel separated from tuffite were examined by chemical analysis, X-ray diffractometry (XRD), magnetometry, scanning electron microscopy and Mössbauer spectroscopy. They are found to contain mainly 48 mass% Fe2O3; 16.9% FeO; 14.3% TiO2; 9% MgO and 3% Al2O3. The magnetization is 29 J T-1 kg-1 at B0 ~ 0.9 T. XRD pattern reveals two cubic phases with α0 = 0.8382(5) and...