Quantification and Characterization of Maghemite in Soils Derived from Volcanic Rocks in Southern Brazil

Antonio Carlos S. da Costa1, Jerry M. Bigham2, Fred E. Rhoton3 and Samuel J. Traina2
1 Departamento de Agronomia, Universidade Estadual de Maringá, Maringá-PR, CEP 87020-900, Brazil
2 School of Natural Resources, The Ohio State University, Columbus, Ohio 43210, USA
3 National Sedimentation Laboratory, USDA-ARS, Oxford, Mississippi 38655, USA

Abstract: Many soils developed from volcanic rocks in southern Brazil exhibit spontaneous magnetization caused by the presence of fine-grained maghemite (γ-Fe2O3), but few attempts were made to quantify or characterize this important soil component. To that end, clays were separated from freely drained soils derived from acid (≥63% SiO2), intermediate (54–62% SiO2), and basic (≤53% SiO2) igneous rocks produced by the Paraná flood volcanism. The sample set included soils with a wide range of pedogenic development on different landscape positions. The Fe oxide mineralogy of these samples was examined by using a combination of selective dissolution, magnetic susceptibility, and X-ray diffraction (XRD) techniques. Hematite and maghemite were the primary Fe oxides in mature soils (Oxisols, Ultisols, and Alfisols) developed from basic rocks; whereas goethite was dominant in all other soils, especially those formed from acid-intermediate rocks. The association of maghemite with basic rock materials suggests that it was primarily formed by oxidation of lithogenic magnetite. A strong, positive correlation (R2 = 0.89) was obtained between mass specific magnetic susceptibility (χ) of the clay fractions and maghemite contents estimated by XRD. Either method could be used for quantitative analyses, but χ was more sensitive than XRD at low maghemite concentrations (<2 wt. %). The clay-sized maghemites were superparamagnetic with an estimated value for the mass specific magnetic susceptibility (χ1f) value of 91,000 × 10−8 m3 kg−1 and frequency dependent variations of 10–15%. The maghemites 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°C for 2 h.

Key Words: Aluminum Substitution • Ferrimagnetic • Maghemite • Magnetic Susceptibility • Magnetite • Selective Dissolution

Clays and Clay Minerals; August 1999 v. 47; no. 4; p. 466-473; DOI: 10.1346/CCMN.1999.0470408
© 1999, The Clay Minerals Society
Clay Minerals Society (www.clays.org)