Atributos químicos e mineralógicos de caulins em diferentes frações granulométricas

Abstract

Kaolins are common geological materials and have high concentrations of kaolinite and halloysite, as the clay fraction of tropical and subtropical soils from the humid areas. The characterization of kaolins is a way to evaluate the contribution of these minerals to important chemical and mineralogical soil attributes. In this study, it was evaluated four kaolins and the C horizon of a Litholic Neosoil in the following fractions: sand (200-53 :m), coarse silt (53- 20 :m), fine silt (20-2 :m), coarse clay (2-1 :m), average clay (1-0.5 :m) and fine clay fractions (< 0.5 :m), which had the mineralogy identified by X- Rays diffraction (XRD), scanning electron microscopy (SEM) and differential thermal analysis (DTA), determined the values of effective cation exchange capacity (CECe), before (CECCa) and after potassium fixation (CECK), external (SSAe) and total (SSAt) specific surface area and the crystallinity of kaolinite and halloysite mensured by the following indexes: Hughes and Brown (1979), Bramão et al. (1952), Amigó et al. (1987), Aparício-Galán-Ferrell (2006) and by the dehydroxylation temperature of these minerals. The fractionation was efficient in concentrating minerals in specific fractions and contributed to clarify the origin of values assigned to CECCa. The crystallinity indexes exhibited distinct behaviors according to the mineralogy present in each material and the most appropriate index to the set of samples analyzed was the Bramão et al. (1952) index due to best correlation presented with SSAe, CECCa and CECK, indicating a proportional increase in values attributed to these variables as crystallinity decreases. SSAe values are related to particle diameter by a potential function, which also applies to CECCa and CECK values due to the close relationship between these variables and SSAe. A large part of the values assigned to CECCa of kaolin was from permanent charge, due to the presence of expansive minerals in the materials studied, which after accounted indicate CEC values between 1.4 e 5.77 cmolc Kg-1 e 2.02 e 5.11 cmolc Kg-1, respectively, for kaolinite and halloysite,according to the particle size.