Iron-bearing phases in a peat-derived duricrust from Brazil

The formation of aluminium‐substituted iron‐minerals is still subject to debate. We report a case study in a Humic Gleysol soil profile, developed on a sedimentary saprolite, from a basin floor on the Uberaba Plateau (Minas Gerais State, Brazil) where iron‐phases are crystallized by a ferruginization process. The mineralogical and chemical properties were investigated by X‐ray diffraction, thermogravimetric analysis, differential scanning calorimetry and Mössbauer spectroscopy. The specific surface area (BET method) and cation exchange capacity (CEC) were also determined. The soil profile is mainly composed of gibbsite, kaolinite and amorphous alumino‐silicate phases, the latter being more frequent in the H horizon where the organic carbon content is greater. The surface area and CEC values are also greater in the H horizon (58.6 m²g⁻¹ and 4.65 mol_ckg⁻¹, respectively) which indicates an increase in porosity caused by the presence of 20.8% of organic matter and amorphous materials. Goethite occurs as a secondary mineral in the H horizon and as the main mineral in the duricrust in association with haematite. The omnipresence of aluminium in the environment (24.6–46.2% of Al₂O₃) resulted in Al‐substitution in all iron‐bearing phases but the goethite from the H horizon has the greatest Al‐for‐Fe substitution, with at least 20 mole per cent of aluminium. In spite of the greater microporosity and wetness of the H horizon, the immediate contact with the richest Al‐source (gibbsite) favours the precipitation of unusually greatly Al‐substituted goethite instead of haematite in this horizon.