Quantification and characterization of maghemite in soils derived from volcanic rocks in southern Brazil

Many soils developed from volcanic rocks in southern Brazil exhibit spontan eous magnetization caused by the presence of fine-grained maghemite (gamma- Fe2O3), but few attempts were made to quantify or characterize this importa nt soil component. To that end, clays were separated from freely drained so ils derived from acid (greater than or equal to 63% SiO2), intermediate (54 -62% SiO2), and basic (less than or equal to 53% SiO2) igneous rocks produc ed by the Parana flood volcanism. The sample set included soils with a wide range of pedogenic development on different landscape positions. The Fe ox ide mineralogy of these samples was examined by using a combination of sele ctive dissolution, magnetic susceptibility, and X-ray diffraction (XRD) tec hniques. Hematite and maghemite were the primary Fe oxides in mature soils (Oxisols, Ultisols, and Alfisols) developed from basic rocks; whereas goeth ite was dominant in all other soils, especially those formed from acid-inte rmediate rocks. The association of maghemite with basic rock materials sugg ests that it was primarily formed by oxidation of lithogenic magnetite. A s trong, positive correlation (R-2 = 0.89) was obtained between mass specific magnetic susceptibility (chi) of the clay fractions and maghemite contents estimated by XRD. Either method could be used for quantitative analyses, b ut chi was more sensitive than XRD at low maghemite concentrations (<2 wt. LTC). The clay-sized maghemites were superparamagnetic with an estimated va lue for the mass specific magnetic susceptibility (chi(If)) value of 91,000 x 10(-8) m(3) kg(-1) and frequency dependent Variations of 10-15%. The mag hemites also had low unit cell constants, which, if attributed entirely to replacement of Fe by Al, would correlate with Al substitutions in the range of 5-16 mole %. Selective dissolution of the soil maghemites was achieved by treatment of Fe oxide concentrates with 1.8 M H2SO4 at 75 degrees C for 7 h.