Maghemite quantification and magnetic signature of Brazilian soils with contrasting parent materials

Abstract

Maghemite (Mh) is a ferrimagnetic mineral that governs magnetism in the clay fraction, especially in well weathered soils from tropical and subtropical regions. In this work, we assessed different methods for Mh quantification [1: area of the reflections obtained by X-ray diffraction (XRD) (standard method); 2: Rietveld refinement; 3: selective dissolution with H2SO4 and; 4: magnetic susceptibility (χ)] by application to Brazilian soils with contrasting parent materials (itabirite, basalt, gabbro, tuffite, gneiss and amphibolite). The methods based on H2SO4 dissolution were found to overestimate the Mh content with respect to the standard method based on X-ray diffraction. This was particularly so with samples from gneiss and amphibolite and samples with higher contents in poorly crystalline Fe oxides. Mh contents based on magnetic susceptibility measurements or the XRD-Rietveld refinement were closer to those provided by the standard method. Mh was found to accumulate in well weathered soils and soils with a high Fe2O3 content. Exception were soils from tuffite, which had high Fe2O3 contents (~330 g kg−1), exhibited low Mh levels (~5.5 g kg−1). Irrespective of the parent material, Mh particles typically fall in a single domain magnetic behavior; however, they can also acquire multidomain characteristics, probably as a result of isomorphic substitution and aggregation with hematite.