Iron Oxide-clay Mineral Association in Brazilian Oxisols: a Magnetic Separation Study

-Selected Brazilian Oxisols were sampled and submitted to high-gradient magnetic separation (HGMS) to study the iron oxide-clay mineral associations. The soils, derived from four different parent materials, have mineralogy dominated mostly by hematite, goethite, and kaolinite. Gibbsite appears in most soil samples. The high-gradient magnetic separation showed good separation for some soils, as indicated by color differentiation and iron oxide segregation between magnetic and nonmagnetic fractions. Soils that showed a somewhat low surface area for the iron oxides associated with high phosphate adsorption were poorly separated by HGMS. This suggests a strong interaction between kaolinite and iron oxides, which would indicate a low estimation of their surface areas obtained by the difference method using BET-N2 data. A relative concentration of anatase and futile in the magnetic portion of some of the samples was attributed to the presence of Fe, either as coatings on the crystals or within the structure of these two minerals. Key Words--Brazil, Goethite, Hematite, Magnetic separation, Oxisols. I N T R O D U C T I O N In general, Brazilian Oxisols have very good physical characteristics, including high porosity, low bulk density, and a very stable granular structure. This is true even for the clayey Oxisols which may contain 70-80% clay. As they usually have their mineralogy dominated by hematite, goethite, kaolinite and gibbsite, it is believed that the association of the Fe oxides with kaolinite, and sometimes gibbsite, is responsible for such structure where the clay-size particles are so highly aggregated that they behave as sand particles. The association of silicate clays and Fe oxides for stabilizing soil aggregates has yet to be proven. Greenland and Oades (1968) and Greenland (1975) showed that kaolinite-iron oxide complexes formed under alkaline conditions were merely mechanical mixtures, and the kaolinite surface had not become coated by the Ire oxide. However, where Fe oxyhydroxide was precipitated on kaolinite surfaces at pH 3.0, sorption of the positively-charged particles caused reduction of both the specific surface area and the permanent negative charges of the complex. Golden and Dixon (1985) studied the association of kaolinite and several synthetic Fe oxides and concluded that the presence of silicate as an adsorbed anion on kaolinite inhibited Fe oxide crystallization from "ferric hydroxide" gel and facilitated an intimate coulombic association between fine, positively-charged Fe oxide particles and kaolin. On the other hand, phosphate did not affect the crystallization of Fe oxides or the kaolinite-iron oxide association. Magnetic separations, which take advantage of the magnetic properties of minerals, have had industrial applications for many years. Schulze and Dixon (1979) adapted high-gradient magnetic separation (HGMS) to Copyright 9 1992, The Clay Minerals Society the study of soil clays with the main purpose of concentrating Fe oxides. They placed a filter made of fine, stainless steel wool in a strong magnetic field, to provide the strong gradient necessary to trap weakly magnetic clay-size particles from a flowing suspension. Subsequently, HGMS has been successfully used in soil mineralogy to concentrate Fe oxides in different ways for different purposes (Hughes, 1982; Hughes and Le Mare, 1982; Russell et al., 1984; Golden and Dixon, 1985; Ghabru et al., 1988). The objective of this research was to use high-gradient magnetic separation on selected clay samples from Brazilian Oxisols to evaluate the degree of association between the soil iron oxides and the other day-size minerals, particularly kaolinite. MATERIALS AND METHODS Soil samples from the B horizons of Oxisols were collected at 1 m depth. The soils are all from the Tri~ngulo Mineiro region and are representative of important Brazilian Oxisols. The main characteristics of these soils are presented in Table 1. Organic matter was destroyed with sodium hypochlorite (Anderson, 1963) and the total clay fractions (<2 #m) were separated by gravity settling (Jackson, 1979) after clay dispersion. In total clay samples, and/ or clays separated by HGMS, the following analyses were performed: (1) color by Munsell Soil Chart on dry samples in diffuse daylight; (2) citrate-dithionite (CD) extraction of Fe oxides (Coffin, 1963); (3) surface area by N2 gas adsorption; (4) X-ray diffraction using Ni-filtered C u K a radiation and a diffracted-beam monochromator; and (5) phosphate adsorption using the Langmuir equation to estimate adsorption parameters.