Concentração de argilominerais 2:1 em solos subtropicais

Abstract

The 2:1 expandable clay minerals have high importance when present in soil, even in small amount, as in the case of soil with a high degree of secondary alteration. The study of these minerals is essential to understanding the physical and chemical behavior of soils, however, the techniques have not always permitted their suitable concentration, in order to facilitate their study. Therefore, this study aimed to analyze the concentration of phyllosilicates clay minerals with 2:1 structure through the technique of selective dissolution of oxides and kaolinite, aiming their study by X-ray diffraction and thermal analysis. The samples were collected in Rio Grande do Sul, in different weather conditions in profiles of Regosols (RRd and RRe), Ferralsol (LVd) and Vertisol (VEo). The clay fraction of A and B horizons of soil were separated and subjected to treatment with dithionite-citrate-bicarbonate and then with warm NaOH in 10 different concentrations, from 0.5 to 5 mol L-1. For mineralogical characterization we used the techniques of X-ray diffraction (XRD), thermogravimetry (TG) and differential thermal analysis (DTA). The results obtained by the analysis of X-ray diffraction showed that for most weathered soils as the Ferrasol, the NaOH solution does not contribute to the concentration of the 2:1 mineral. However, to the Vertisol the solution of NaOH 4.0 mol L-1 was more efficient in the concentration of minerals. To the Regossol RRd no concentration of NaOH solution was effective and to the RRe the concentration of 2.5 mol L-1 solution of NaOH was most suitable for concentrating the 2:1 clay. The extraction of kaolinite and gibbsite by NaOH solution was only partial for kaolinite. The thermal analysis showed predominance of kaolinite and gibbsite in relation to the 2:1 minerals in the samples free of oxides. The most effective treatment for the thermal analysis for 2:1 mineral concentration was NaOH 4 mol L-1 for samples LVd B, RRh A, RRe A and Veo A. The NaOH 3 mol L-1 treatment was more efficient for samples LVd A and VEo B. The relative concentration of 2:1 was higher in samples with dystrophic character.