Pedogenic Iron Oxides in Iron-Rich Oxisols Developed from Mafic Rocks

Abstract

Despite the considerable amount of information on the mineralogical characteristics of pedogenic Fe oxides in Brazilian soils, there are few studies on Fe-rich soils developed from mafic rocks with taxonomic identities at lower categorical levels. This study evaluated the mineralogical characteristics of pedogenic Fe oxides in B horizons (Bw) of Fe-rich Oxisols developed from several mafic rocks in the state of Minas Gerais, Brazil. The Bw horizons were sampled at a 0.8-1.0 m depth in 13 Ferric and Perferric Rhodic Oxisols along with a Mesoferric Typic Oxisol originating from basalt, gabbro, tuffite, amphibolite and itabirite in Minas Gerais. The selected soils have taxonomic identities up to the fourth categorical level of the Brazilian System of Soil Classification. In the laboratory, the following analyses were made: a) powder X ray diffraction (XRD) of the clay fraction before and after selective concentration of Fe oxides by silicate alkaline dissolution (5 mol L-1 NaOH); b) selective chemical dissolution of the clay fraction by citrate-bicarbonate-dithionite (CBD), acid ammonium oxalate (AAO), and sulfuric acid (H 2 SO 4 1.8 mol L-1); c) quantitative estimation of minerals in the clay fraction through allocation of phases from the XRD patterns, magnetic susceptibility of the clay fraction, and quantification of elements after sulfuric acid digestion (H 2 SO 4 1:1) of the air-dried fine earth and treatment of the clay fraction with CBD; and d) estimation of the mean crystal size (MCS), specific surface area (SSA), and isomorphic Al-substitution (IS) of hematite, goethite, and maghemite from the XRD patterns obtained from concentrates of Fe oxides. The results showed that estimation of Fe content of maghemite by selective dissolution with 1.8 mol L-1 H 2 SO 4 may not be accurate enough to realistically reflect the maghemite contents in the soil sample. The Al content extracted may also be influenced by other minerals that are sources of this element. Hematite crystals were predominantly placoid in shape in all Rhodic Oxisols and had smaller SSA compared to goethite, which showed both isodimensional and asymmetric habit. Higher crystallinity of maghemite and the IS values generally lower than those of hematite and goethite suggest that in well-drained soils derived from mafic rocks, the IS phenomenon in maghemites seems to result from pedogenetic advancement after its formation from magnetite oxidation.

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

“…Although XRD patterns showed weak evidence in all soils for the occurrence of goethite in the clay fraction, goethite was not observed only in LV 9 after the Fe oxides concentration treatment (Camêlo et al, 2017).…”

“…The extracted Fe (Fe d and Fe H2SO4 ) was measured by atomic absorption spectrometry and numerical values were presented by Camêlo et al (2017).…”

“…Oxisols cover more than 50 % of Minas Gerais State in Brazil and soils developed on basalt, tuffite and itabirite deserve attention because they are Fe-rich with high magnetic susceptibility (Ferreira et al, 1994;Fontes et al, 2000;Oliveira et al, 2000;Costa et al, 2014;Camêlo et al, 2017). These soils have high contents of hematite, maghemite and/or titanomaghemite in the clay fraction and significant amounts of magnetite and/ or titanomagnetite in silt and sand fractions, resulting in high magnetic susceptibility in the Fe-rich Rhodic Oxisols (Soil Taxonomy -Soil Survey Staff, 2014) (Fontes and Weed, 1991;Fabris et al, 1997a, b;.…”

Low arsenic bioaccessibility by fixation in nanostructured iron (Hydr)oxides: Quantitative identification of As-bearing phases

“…1% (K06) to approx. 45% (K23 (> 2 mm and < 2 mm) and K48 (> 2 mm)) is shown in [35]. The results show that the Fe content in soils formed in ferriferous environments can be as high as 480.5 g kg -1 (or 48% wt).…”

Charge sparsity: An index to quantify cation effects on clay dispersion in soils

“…This difference is associated to the nonlinear relationship between zeta potential and responsiveness of soils to high CS cations and to the higher microstructural stability of the soil from Londrina (Reichert et al, 2009) caused by a higher Fe ox content (Table 1). Amorphous Fe forms play a crucial role in microstructure stabilization due to a higher specific superficial area (Camêlo et al, 2017). As we used the < 2 mm fraction, soil microaggregates were still stable after the methodological procedures, leading the soil from Londrina to have lower responsivity to CS than the soil from S.J.…”