Navegando por Autor "Gilkes, Robert J."

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Microporosity of BIF hosted massive hematite ore, Iron Quadrangle, Brazil.
(2002) Varajão, César Augusto Chicarino; Bruand, Ary; Ramanaidou, Erick R.; Gilkes, Robert J.
O minério de hematita compacta (MHC) é um tipo de minério de ferro de alto grau usado como minério granulado na obtenção do ferro via redução direta (DRI). A influência da porosidade sobre a redutibilidade do MHC da Mina de Capitão do Mato (Quadrilátero Ferrífero, Brasil), foi investigada em amostras de furos de sonda e de afloramentos da mina, usando-se microscópio óptico e eletrônico de varredura. Hematita é o principal componente mineralógico e ocorre sob diferentes formas: granular (10 μm), microtabular (1 μm) e euédrico (10 a 30 μm). Quartzo maghemita, kenomagnetita e goethita são componetes menores. Microporos primários (Å to 1 μm) associam-se a cristais de hematita microtabular, que preenchem espaços entre cristais de hematita granular. Microporos secundários (Å to 5μm), relacionados com os cristais de martita euédrica, são os mais importantes. A porosidade total das amostras do MHC, medida através dos métodos de adsorção de nitrogênio e injeção de mercúrio, atingiu valores de até 11% para amostras intemperisadas. Amostras não alteradas de MHC têm porosidade menor que 2,5%. Verificou-se que a porosidade incrementa a redutibilidade, enquanto que a estrutura (bandamento) tem uma influência negativa na redutibilidade do MHC durante o DRI.
The relationships between kaolinite crystal properties and the origin of materials for a Brazilian kaolin deposit.
(2001) Varajão, Angélica Fortes Drummond Chicarino; Gilkes, Robert J.; Hart, Robert D.
The clay particles in a kaolin deposit from Brazil were investigated by X-ray diffraction (XRD), differential thermal analysis (DTA), analytical transmission electron microscopy (ATEM), and electron paramagnetic resonance (EPR) to examine the relationships between morphological and chemical properties of the crystals and to relate these properties to formation conditions. The XRD patterns show the dominant presence of kaolinite with minor amounts of gibbsite, illite, quartz, goethite, hematite, and anatase. ATEM observations show two discontinuities in the deposit as indicated by changes in morphology and size of the kaolinite crystals. At the base of the deposit, hexagonal platy and lath-shaped particles (mean area of 001 face = 0.26 p,m 2) maintain the original fabric of the parent rock which characterizes an in situ evolution. In the middle of the deposit a bimodal population of large (mean area of 001 face > 0.05 ixm:) and small (mean area of 001 face < 0.05 p.m 2) sub-hexagonal platy kaolinite crystals occurs. This zone defines the boundary between the saprolitic kaolinite and the pedogenic kaolinite. Near the top of the profile, laths and irregular plates of kaolinite, together with sub-hexagonal particles, define two different depositional sources in the history of formation of the deposit. Crystal thickness as derived from the width of basal reflections and the Hinckley index are compatible with the morphological results, but show only one discontinuity. At the base of the deposit, kaolinite has a lowdefect density whereas in the middle and at the top of the profile, kaolinite has a high-defect density. Likewise, EPR spectroscopy shows typical spectra of low-defect kaolinite for the bottom of the deposit and typical spectra of high-defect kaolinite for the other portions of the deposit. Despite the morphological changes observed through the profile, the elemental composition of individual kaolinite crystals did not show systematic variations. These results are consistent with the deposit consisting of a transported pedogenic kaolinite over saprolite consisting of in situ kaolinized phyllite.