Iron-bearing phases in a peat-derived duricrust from Brazil.
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2007
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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.
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VARAJÃO, A. F. D. C. et al. Iron-bearing phases in a peat-derived duricrust from Brazil. European Journal of Soil Science, v. 58, p. 1096-1106, 2007. Disponível em: <http://onlinelibrary.wiley.com/doi/10.1111/j.1365-2389.2007.00891.x/abstract>. Acesso em: 20 jun. 2017.