Abstract
In the Southern Espinhaco Mountain Chain and in the Upper Jequitinhonha Valley, magnetic soils, in different pedogenetic stages, are found to be forming over intrusions of basic lithology. The essential chemical and mineralogical properties of samples from magnetic soil profiles from those two physiographic environments in the state of Minas Gerais, Brazil, are reported. Three of the pedons (Rhodic Kandiustox – RKox, Rhodic Haplustox – RHox, and Typic Argiustoll - TAoll) were identified as being indeed developed over basic rocks; the fourth pedon (Typic Haplustox - THox) is currently forming on an acidic rock. Particle size and routine chemical analyses were performed on samples from all horizons of the four selected soil profiles. For a deeper insight into the dominant mineralogy of each diagnostic soil horizon, the elemental contents, expressed in terms of the corresponding metal cation oxides, namely Fe2O3, Al2O3, and MnO2, were obtained from digesting the whole soil samples with sulfuric acid. A similar chemical analytical procedure was performed for the residual solid extracts obtained from attacking the whole soil materials with mixtures of (i) dithionite - citrate - bicarbonate and (ii) oxalate - oxalic acid. The soil samples were also analyzed by Mossbauer spectroscopy at room temperature (~298 °K) in an attempt to better identify the main magnetic iron oxides. Maghemite (δFe2O3) was found in all samples and magnetite (Fe3O4) was identified only for the sample from the Typic Argiustoll. The pedogenetic loss of silica and consequent accumulation of iron and aluminum oxides along the profile are found to be somehow correlated to the weathering sequence in the soils forming on basic rocks: TAoll < RKox < RHox.
Highlights
The Southern Espinhaço Mountain Chain (SdEM) occupies approximately 3.7 million ha in 53 municipalities of the state of Minas Gerais, Brazil
According to Vahl (1999), P is fixed by oxides, oxy-hydroxides, and Fe and Al hydroxides, which prevent their release into the soil solution
The higher levels of P in the BA horizon of the TAoll may result from a slightly weathered profile, with pH around 6.0 (Table 2), that originated from green schist formed by mineral-rich P rock (Chaves and Vasconcelos, 2006)
Summary
The Southern Espinhaço Mountain Chain (SdEM) occupies approximately 3.7 million ha in 53 municipalities of the state of Minas Gerais, Brazil. Iron oxides, neoformed from the release of iron ions into the soil by weathering of primary (lithogenic) or secondary (pedogenic) minerals, are a group of minerals characterized as being indicators sensitive to environmental conditions and pedogenic processes (Schwertmann and Taylor, 1989) They are widely used as pedoenvironment indicators, and the study of their properties may reveal information regarding past events and current circumstances relevant to soil formation (Kämpf and Curi, 2000; Muggler et al, 2001; Inda Junior and Kämpf, 2003; Cunha et al, 2005; Figueiredo et al, 2006). The chemical and mineralogical properties essential to characterization of four materials sampled from magnetic soil profiles are described in this study Three of these materials (Rhodic Kandiustox – RKox, Rhodic Haplustox – RHox, and Typic Argiustoll – TAoll) developed from basic and acidic rock (Typic Haplustox - THox) of the SdEM and UJV, which are both physiographic regions of the state of Minas Gerais, Brazil. The studies focus especially on the properties of the ferrimagnetic structures derived from iron oxides to identify the dominant pedogenetic processes and to determine the weathering sequence involving mineralogical changes in iron oxides in the transformation of rock to soil
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