Properties of iron oxides in some New Caledonian oxisols

Oxisols from two toposequences of New Caledonia formed from peridotite, consist essentially of Fe oxides (goethite, hematite, maghemite). These Fe oxides were characterized by their mineralogy, crystal size and morphology, Al substitution, thermal behaviour and dissolution kinetics in 6 M HCl at 25oC. In one toposequence the samples were free of hematite (goethite only) at > 1050 m above sea level whereas at lower altitudes hematite was also present. Al substitution was generally low due to the low Al content of the peridotite, except in gibbsitic samples on rocks somewhat higher in Al. The surface areas of goethite and hematite ranged between 50 and 150 m2 g-1. Dithionite-extractable Ni and Cr were between 0.2 and 2.2 % Ni and 0.3 and 2.3 % Cr. The hematite-containing samples tended to be higher in Cr and lower in Ni, whereas the opposite held for samples containing goethite only. Maghemites had a low unit cell size (8.31-8.32 instead of 8.34-8.35 Angstrôm) which was attributed to Al substitution. Dehydroxylation temperature of goethites was weakly correlated with Al substitution. Dissolution kinetics could be described by a linear form of a modified first-order reaction with one straight line for samples containing goethite only and two lines for samples containing goethite plus hematite. (Résumé d'auteur

Ferrihydrite–humic associations: magnetic hyperfine interactions

7 pages, 6 figures, 1 table, 18 references.Humic–iron oxide associations are believed to exist in various surface environments, such as soils and surface waters, and may add substantially to the stability of organic matter under oxidizing surface conditions. However, a nondestructive, solid-state characterization of such associations is still lacking. In this paper synthetic coprecipitates between humic material (dissolved organic matter; DOM) obtained from a Podzol and synthetic ferrihydrite are examined using X-ray diffraction (XRD) patterns and Fe-specific Mössbauer spectra at temperatures between 4.2 K and room temperature. Lepidocrocite formed in the absence of DOM. However, DOM induced the formation of a four (XRD)-line ferrihydrite that contained 96 mg C/kg. In contrast to a pure four-line ferrihydrite, which was completely magnetically ordered at 4.2 K, the synthesized DOM–ferrihydrite was not fully ordered at 4.2 K and had a magnetic hyperfine field 1 to 2 T lower than the pure ferrihydrite. Such an effect was not observed when DOM was only surface-adsorbed. We conclude that organic components of the DOM coprecipitated with the ferrihydrite. Their interaction with the Fe atoms of the oxide prevents complete spin freezing at 4.2 K. Solid-state 13C nuclear magnetic resonance (NMR) spectra suggested that O-alkyl C of the DOM was mainly responsible for the interaction with the Fe in the oxide.Deutsche Forschungsgemeinschaft financial support.Peer reviewe

Clay mineralogy and magnetic susceptibility of Oxisols in geomorphic surfaces

Studies analyzing the variability of clay minerals and magnetic susceptibility provide data for the delineation of site-specific management areas since many of their attributes are important to agronomy and the environment. This study aimed to evaluate the spatial variability of clay minerals, magnetic susceptibility, adsorbed phosphorus and physical attributes in Oxisols of sandstones in different geomorphic surfaces. For that purpose, soil samples were collected every 25 m along a transect located within the area where the geomorphic surfaces were identified and mapped. The transect occupied the central portion of 500 ha, where it was also sampled for density purposes with one sample per six hectares. Soil samples were collected at a depth of 0.0-0.2 m. The results of the physical, chemical, mineralogical and magnetic susceptibility analyses were subjected to statistical and geostatistical analyses. The nature of the clay minerals and magnetic susceptibility was dependent on the variation of the soil parent material. High values of magnetic susceptibility were associated with the presence of maghemite and magnetite of coarse size. The spatial variability of crystallinity and the content of Fe oxides, as well as magnetic susceptibility, were dependent on the age of the geomorphic surfaces. The youngest surface had greater spatial variability of these attributes. The iron (goethite and hematite) and aluminum (gibbsite) oxides in the youngest geomorphic surface influenced the low values of soil density and high values of total pore volume, micropores and P adsorption. The characterization of the spatial variability of Fe oxides and susceptibility allowed for the delineation of homogeneous areas