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