Mineralogy, magnetic susceptibility and geochemistry of Fe-rich Oxisols developed from several parent materials

Abstract

Fe-rich Oxisols on mafic rocks in Brazil generally have high magnetic susceptibility with high contents of some trace elements. These are taxonomically similar soils; however, differences in magnetic and geochemical properties may affect agricultural or environmental usability and subsequent management. This study investigated the pedogenesis of Fe-rich Oxisols from various parent materials and evaluated the lithogenetic influence on magnetic susceptibility and trace elements contents. Soil samples were collected from Bw horizons in 13 Rhodic Oxisols and a Typic Oxisol developed from several parent materials in Minas Gerais State, Brazil. Soils were analyzed by X-ray diffractometry (XRD) and magnetometry. Soil chemical analyses consisted of sulfuric and total (tri-acid) digestions and selective Fe oxides dissolutions and statistical correlations were determined. Fe-rich Oxisols presented a typical mineralogical composition of highly weathered soils with structural stability. The results confirm the difficulty to identify accurately magnetic minerals in different grain sizes using XRD. However, coarse fractions still exert dominant influence on the magnetic properties of the Fe-rich Rhodic Oxisols. In addition, coarse fractions probably contribute to the enrichment of superparamagnetic particles for the clay fraction. Although highly weathered, Fe-rich Oxisols may have their geochemical attributes still controlled by the parent material, where trace elements seem to be correlated with the magnetic minerals in the coarse fractions. Thus, the natural replacement of some trace elements from soil-solution equilibrium reactions during plant development could be more effective in soils with higher magnetic particles concentrations in the coarse fractions.

Mineralogical Evolution of Magnetic Rhodic Oxisols under Different Lithological Influences in Brazil

“…In Brazil, these soils occur mainly in the central-western, southeastern, and southern regions and are generally derived from mafic rocks, such as basalt, gabbro, diabase, tuffite, and itabirite. Rhodic Oxisols exhibit strong magnetic attraction due to high contents of ferrimagnetic Fe oxides (maghemite and magnetite) (Costa et al, 1999(Costa et al, , 2014Camêlo et al, 2018;Poggere et al, 2018). These minerals have often been used in studies on pedogenesis (Silva et al, 2017;Camêlo et al, 2018) for stratification of environments (Ramos et al, 2017) and soil fertility (Costa et al, 2018) in Brazil and elsewhere in the world.…”

“…There are two main processes for the genesis of magnetite and maghemite in soils: the formation of maghemite through aerial oxidation of magnetite inherited from the parent material (lithogenic) (Fontes and Weed, 1991;Camêlo et al, 2018) and the neoformation of maghemite and magnetite from pedogenic Fe oxides in reducing environments under high temperatures (500-1,000 °C). This last process is common in areas under fire events, where magnetic expression is higher in soil surface horizons (Schwertmann and Fechter, 1984;Sousa et al, 2018).…”

“…In soils originating from mafic rocks, maghemite usually originates from aerial oxidation of magnetite inherited from the parent material (Fontes and Weed, 1991;Camêlo et al, 2018). Therefore, several authors have reported on magnetite predominance in the coarse fractions (sand and silt) and maghemite in the clay fraction (Costa et al, 1999;Carvalho Filho et al, 2015;Camêlo et al, 2018;Poggere et al, 2018) of Fe-rich Rhodic Oxisols in Brazil. However, it is known that both minerals can display distinct soil size fractions or occur in all of them (Causevic et al, 2004;Maxbauer et al, 2016;Silva et al, 2017;Costa et al, 2018).…”

“…Authors have also reported on the difficulty to distinguish magnetite and maghemite by conventional techniques, such as X-ray diffractometry, due to the similarity of the atomic planes of both minerals (Fontes et al, 2000;Camêlo et al, 2018), as well as Mössbauer spectroscopy, because of differing degrees of magnetite oxidation in soils (Singer et al, 1995). On the other hand, Raman spectroscopy is considered a fast and accurate tool for the identification of these minerals in soils (Hanesch, 2009;Sousa et al, 2018); nevertheless, it is still little used on magnetic Rhodic Oxisols.…”

“…As magnetite and maghemite can have different stability degrees (Sidhu et al, 1980), influencing magnetic susceptibility, and reserve of trace elements in soils (Camêlo et al, 2018), we believe that accurate differentiation of these minerals allows a better understanding of pedological, magnetic, and geochemical aspects of soils with direct implications on agricultural and environmental management. Therefore, this study aimed to characterize mineralogically Fe-rich Rhodic Oxisols originating from several mafic rocks in Brazil, and assess the implications on their magnetic and geochemical properties.…”