Clay Minerals, (2008) 43, 155–163
Vermiculite with hydroxy-aluminium interlayer and kaolinite formation in a subtropical sandy soil from south Brazil E. C. BORTOLUZZI1, B. VELDE2, M. PERNES3, J . C . D U R 3 AND D . T E S S I E R 3 , * 1
Faculdade de Agronomia e Medicina Veterina´ria, Fundac¸a˜o Universidade de Passo Fundo (FAMV-FUPF), 611, 99051-000 Passo Fundo, Rio Grande do Sul, Brazil, 2 De´partement de Ge´ologie, Ecole Normale Supe´rieure, 24 rue Lhomond, 75231 Paris Cedex 05, France, and 3 Institut National de la Recherche Agronomique (INRA), Unite´ PESSAC, Route de Saint-Cyr, 78026 Versailles Cedex, France
(Received 19 March 2007; revised 6 November 2007)
AB ST R ACT : The purpose of this study was to investigate the clay mineral phases in a Rhodic Acrisol soil and subsequently discuss their evolution in subtropical conditions. Prairie and forest soil profiles were sampled and clay fractions of the parent material and soil horizons analyzed by X-ray diffraction (XRD) at the Federal University of Santa Maria, Rio Grande do Sul-Brazil. The XRD results show the presence of interstratified kaolinite-smectite and illite-smectite as well as illite in the parent material. These minerals were also found in the soil samples but with two new phases: hydroxy-aluminium interlayered vermiculite (HIV), which showed incomplete collapse with ˚ ). Under a subtropical climate and a treatment at 550ºC, and a newly formed kaolinite (d = 7.17 A sandy lithology, HIV and kaolinite appear to be a result of a specific pedogenic clay formation, in relation with the natural vegetation. Originally under the prairie area, the intensity of the weathering processes were weak (within 2:1 clay minerals), as only small quantities of kaolinite and Fe oxides, and no evidence of gibbsite, were found.
KEYWORDS: hydroxy-Al interlayer, soil genesis, particle size distribution, sandy soils, XRD, Brazil. In tropical and subtropical areas, soil mineralogy is most often based on the presence of kaolinite, gibbsite and Fe oxide minerals such as hematite and goethite. However, there is a great variation in mineralogy and mineral-abundance, especially due to the influence of specific weathering conditions (Bhattacharyya et al., 2000). In soils, the presence of 2:1 minerals is an important factor in soil fertility because their permanent charge determines at least part of the overall cation retention and cation selectivity behaviour of the soil. In contrast, with 1:1 minerals as well as Fe oxides and organic matter, the surface
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[email protected] DOI: 10.1180/claymin.2008.043.2.03
charge varies with the pH, soil solution concentration and redox state. Despite the cation exchange capacity (CEC) always being small in sandy soils, the magnitude of the permanent charge of 2:1 minerals plays a significant part in terms of their fertility. It is thus very important to estimate the presence of 2:1 minerals, not only because of their contribution to soil chemical characteristics, but also the physical properties of the soil such as structure stability (Shepherd et al., 2001), water retention and solid/water affinity (Tessier, 1984; Bradford & Blanchar, 1999) as well as the mobility of colloidal particles in the environment (Rousseau et al., 2004). In south Brazil, most of the studies were carried out on the mineralogy of Oxisols and related rocks (Kampf & Schwertmann, 1983; Kampf et al., 1995;
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E. C. Bortoluzzi et al.
Ker & Resende, 1990). However, soils developed on a sandstone bedrock have rarely been studied in detail. These soils mainly occur in grassland areas extending over ~35000 km2 in Rio Grande do Sul state, a significant coverage, even relative to the whole of Brazil. At present, these soils are becoming increasingly cultivated and their fertility, as well as the environmental consequences of using them for agriculture, is debatable. Basic information relating to these soils concerns their mineralogical nature, which is often a determining parameter of their properties and use. This paper deals with the characterization of soil clay minerals in sandy soil developed from a sandstone formation in Rio Grande do Sul State and subsequently discusses their soil evolution.
MATERIALS AND METHODS Location and characteristics of the sampled soils The study area is located at the Federal University of Santa Maria in Rio Grande do Sul State, Brazil (29º42’52’’S and 53º42’10’’W; 90 m above sea level). The climate is subtropical, with a mean annual rainfall of 1600 mm and mean annual temperature of 19ºC. The parent material is Upper Triassic sandstone of the Santa Maria Formation (Silve´rio da Silva, 1997). The soil was classified as a ‘Rhodic Acrisol’ in world soil taxonomy and is common in this area. The soil has a strong clay gradient from top to bottom of its profile. The soil shows a brunification or reddening, mainly in the bottom horizons, due to the presence of free Fe oxides and well drained conditions. Two soil profiles were sampled: (1) under natural vegetation regeneration (Forest-F) located on a hilltop and (2) under permanent grass (Prairie-P), located on a gentle slope in a rolling landscape. The grass is the natural vegetation of this region. Samples were extracted from each horizon, as shown in Table 1, as well as the parent material (the Upper Triassic sandstone) at ~250 cm depth, i.e. the PR5 and FR5 samples. The main soil characteristics are given in Table 1.
Sample analysis On dried soil samples the effective cation exchange capacity (CECE) and the exchangeable cations (e.g. Ca2+, Mg2+, Na+ and K+) were
measured by the cobalt hexamine trichloride method (Ciesielski & Sterckeman, 1997). The Al3+ was extracted by exchange with KCl. The CEC at pH 7.0 (CEC7 ) was determined by exchange with ammonium acetate buffered at pH 7.0. The total organic carbon (TOC) was obtained using a C and N element analyzer (Fison Carlo Erba). The pHH2O was measured in a 1:1 soil/water ratio. The CaO, MgO, K2O, Na2O and total Fe2O3 were dissolved in HF and measured by atomic absorption spectroscopy (AAS). The Si, Al and Ti were dissolved in an alkaline fusion and measured by X-ray fluorescence (XRF). The free Fe was obtained following the methods of Mehra & Jackson (1960) and Afnor (1996). Clay extraction. The clay fraction was extracted from the soil samples after destruction of organic matter using H2O2 at room temperature. Clay dispersion was accomplished with mechanical treatment. The
