Navegando por Autor "Meunier, Alain"

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    Iron-bearing phases in a peat-derived duricrust from Brazil.
    (2007) Varajão, Angélica Fortes Drummond Chicarino; Costa, Geraldo Magela da; Varajão, César Augusto Chicarino; Meunier, Alain; Colin, Fabrice
    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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    Particle morphological evolution during the conversion of I/S to Illite in lower cretaceous shales from Sergipe-Alagoas basin, Brazil.
    (1995) Varajão, Angélica Fortes Drummond Chicarino; Meunier, Alain
    The illitic end of mixed-layer illite-smectite series (I/S) in shales from Lower Cretaceous Barra de Itifiba Formation, Sergipe-Alagoas basin, was examined with X-ray powder diffraction (XRD) and transmission electron microscopy (TEM). A mathematical decomposition of XRD patterns shows different I/S and illite populations. All the samples contain ordered (R = 1) I/S, poorly crystallized illite (PCI) and well crystallized illite (WCI). A randomly interstratified (R = 0) I/S was also identified in a fractured zone at 1020 m. The percentage of expandable layers in ordered I/S decrease progressively from 20% to 10%. TEM observations show a continuous change in morphology between two basic particle shapes: elongated (lath) and isometric. The size and morphology of particles change with increasing depth. The proportion of laths decreases while isometric particles become predominant. However, both particle types continuously grow and enrich the larger size fraction. The growth process is driven by a mass transfer from the dissolving small particles of predominantly I/S (R = 1) composition to the larger (more illitic) lath and isometric ones. The proportion of lath-shaped particles decreases with depth indicating that the more stable population upon increased burial is the isometric well crystallized illite (WCI) particles. Very large laths are observed in the fault zone where conditions may favor faster growth processes.