The mineralogy of the silt fraction of a 1.5 m-deep, well developed, intensely weathered, greyish soil profile from a toposequence on a tableland covered by agricultural crops in the upper valley of the Jequitinhonha river basin, Minas Gerais, Brazil, has been studied by X-ray powder diffraction, 57Fe Mössbauer spectroscopy at 298 K and 80 K, and vibrating sample magnetometry. Mössbauer data collected at room temperature indicated about 17 atom% of the iron content of the sample to be structural Fe2+ in phyllosilicates, which X-ray diffraction showed to be mainly halloysite and kaolinite. The magnetization curve also indicates the presence of a ferrimagnetic phase, tentatively identified by Mössbauer spectroscopy as maghemite (γ-Fe2O3).
These findings support a pedogenetic model for this soil profile, by which the remaining Fe2+-bearing minerals were first formed under relatively anoxic palaeo-conditions of an intense hydric regime. Subsequent drier local conditions, due to a much improved drainage, favoured intensive weathering, leading to the presently developed Oxisol. Even under the more oxidative conditions, part of the Fe2+-containing phyllosilicates still remained in the profile, which is believed to impart the rather unusual greyish colour (Munsell 10YR 3/2) to this deep tropical soil.