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

Abstract

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 Mo¨ ssbauer 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 molckg 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 ironbearing 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.