Revista Brasileira de Ciência do Solo (Jun 2009)
Métodos de concentração de minerais 2:1 secundários na fração argila visando sua identificação por difratometria de raios x Methods for concentrating secondary 2:1 minerals in the clay fraction for subsequent identification by x-ray difractometry
Abstract
Além dos baixos teores normalmente encontrados na fração argila dos solos sob clima tropical e subtropical, o tamanho reduzido e a baixa cristalinidade dos minerais 2:1 secundários dificultam sua identificação por difratometria de raios X (DRX). Este estudo objetivou avaliar métodos químicos e físico de concentração de minerais 2:1 secundários na fração argila para facilitar a identificação por DRX, incluindo a natureza dos minerais quanto ao local de formação de cargas permanentes (lâmina tetraedral ou octaedral). Coletaram-se amostras de dois Cambissolos originados de argilito da Formação Guabirotuba na Bacia Sedimentar de Curitiba (PR): horizontes A, Bi, C1 (1,2 a 1,5 m), C2 (2,2 a 2,5 m), C3 (3,2 a 3,5 m) e C4 (4,2 a 4,5 m). Após remoção da matéria orgânica e dispersão da terra fina seca ao ar, a fração argila foi submetida a tratamentos sequenciais com ditionito-citratobicarbonato (DCB) (amostra desferrificada - remoção de óxidos de Fe pedogenéticos) e com soluções de NaOH a quente, em diferentes concentrações (0,5; 1,0; 1,5; 2,5; 3,5; 4,0; 4,5 e 5,0 mol L-1), para extração de gibbsita e caulinita, em diferentes graus. A fração argila desferrificada também foi submetida à separação física (centrifugação) em argila grossa (0,2 a 2 m) e fina (Besides the typically low levels of secondary 2:1 minerals found in the clay fraction of soils in tropical and subtropical climates, the small mineral size and low crystallinity make their analyses by X-ray difractometry (XRD) difficult. This study aimed to evaluate physical and chemical methods for concentrating secondary 2:1 minerals in the clay fraction to facilitate their identification by XRD. Also, it was sought to analyze the nature of the minerals regarding the sites of origin of permanent charges (tetrahedral or octahedral sheets). Samples were collected in two Cambisols originated from argillite of the Guabirotuba formation in the sedimentary basin of Curitiba (PR), Brazil, in the horizons: A, Bi, C1 (1.2 to 1.5 m), C2 (2.2 to 2.5 m), C3 (3.2 to 3.5 m) and C4 (4.2 to 4.5 m). After removing the organic matter and dispersing the airdried fine earth, the clay was sequentially treated with dithionite-citrate-bicarbonate (DCB) (iron-free sample - removal of pedogenetic iron oxides) and with hot NaOH solutions at different concentrations (0.5, 1.0, 1.5, 2.5, 3.5, 4.0, 4.5, and 5.0 mol L-1) for kaolinite and gibbsite extraction at different degrees. The iron-free clay fraction was also physically separated (centrifugation) in coarse (0.2 to 2 µm) and fine clay (< 0.2 µm). Secondary treatments were performed to identify the 2:1 mineral species in the clay fraction: saturation with Mg and solvation with ethylene glycol, K saturation, air-drying and heating to 550 ºC; and Li saturation (Greene-Kelly test). Results showed that the classical method of kaolinite extraction with hot 5.0 mol L-1NaOH solution should not be used to concentrate secondary 2:1 minerals because the solution also removed a large portion of these minerals. The treatment with DCB and with 3.5 mol L-1NaOH solution was more efficient to concentrate and favored the identification of secondary 2:1 minerals by XRD in samples of the A, Bi and C1 horizons. In samples from greater soil depths (C2, C3 and C4 horizons), due to the higher levels of these minerals and the smaller crystal size (fine clay), a less concentrated NaOH solution (1.5 mol L-1) was more efficient for this purpose. In A horizon, 2:1 minerals were concentrated in the coarse clay fraction, what is compatible with the higher weathering degree of this horizon. Smectite with Al-hydroxy interlayers was identified in most surface horizons (A and Bi) and smectite was found in the C horizon samples. Li saturation made the identification of dioctaedral minerals (montmorillonite and beidellite/nontronite) possible. The modification of the standard method (5 mol L-1NaOH) favored the concentration of secondary 2:1 minerals in the soil clay fraction; the concentration of the NaOH solution must be higher for horizons with lower mineral content.
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