Difratometria de raios-X e análise térmica das frações granulométricas de caulins

Abstract

Kaolins are common geological materials and have high concentrations of kaolinite as well as the clay fraction of tropical and subtropical soils of Brazil. The characterization of kaolin is a proxy of assessing the contribution of kaolinite to important soil chemical and mineralogical attributes. This study evaluated four kaolins (commercial kaolin A (CCA), commercial kaolin B (CCB), pink sandy kaolin A (CRA) and green sandy kaolin A (CVA)) in the original form and after particle size separation into: sand (200-53 mm), coarse silt (53-20 ?m), fine silt (20-2 ?m), large clay (2-1??m), medium clay (1-0.5 ?m) and fine clay (<0.5??m) fractions. The minerals were identified by X-ray diffraction (XRD) and evaluated for crystallinity (kaolinite and halloysite) through indexes Hughes and Brown, Amigó, Bramão and the dehydroxylation temperature. The physical fractionation was efficient to concentrate minerals in specific size fractions which were not identified in the original material. In CCA kaolin was concentrated one mineral which remains unidentified in fine fractions, in kaolin CRA, zircon was concentrated in the coarse silt and different silicates in the fine fractions, in kaolin CCB were concentrated kaolinite and a silicate  in the medium and coarse clay fractions. The estimate by X-ray diffraction overestimated the amount of kaolinite and halloysite underestimated when compared to quantification by thermal gravimetric analysis The crystallinity index exhibit different behaviors depending on the mineralogy of each material, thus the correlation between the crystallinity of kaolinite and / or halloysite and other variables, may be compromised, especially in materials with distinct geological origins.