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

A revised scheme for the reactivity of iron (oxyhydr)oxide minerals towards dissolved sulfide

The reaction between dissolved sulfide and synthetic iron (oxyhydr)oxide minerals was studied in artificial seawater and 0.1 M NaCl at pH 7.5 and 25°C. Electron transfer between surface-complexed sulfide and solid phase Fe(III) results in the oxidation of dissolved sulfide to elemental sulfur, and the subsequent dissolution of the surface-reduced Fe. Sulfide oxidation and Fe(II) dissolution kinetics were evaluated for freshly precipitated hydrous ferric oxide (HFO), lepidocrocite, goethite, magnetite, hematite, and Al-substituted lepidocrocite. Reaction kinetics were expressed in terms of an empirical rate equation of the form: R-i = k(i)(H2S)(t=0)(0.5)A where Ri is the rate of Fe(II) dissolution (RFe) or the rate of sulfide oxidation (RS), ki is the appropriate rate constant (kFe or kS), (H2S)t=0 is the initial dissolved sulfide concentration, and A is the initial mineral surface area. The rate constants derived from the above equation suggest that the reactivity of Fe (oxyhydr)oxide minerals varies over two orders of magnitude, with increasing reactivity in the order, goethite < hematite < magnetite << lepidocrocite ≈ HFO. Competitive adsorption of major seawater solutes has little effect on reaction kinetics for the most reactive minerals, but results in rates which are reduced by 65-80% for goethite, magnetite, and hematite. This decrease in reaction rates likely arises from the blocking of surface sites for sulfide complexation by the adsorption of seawater solutes during the later, slower stages of adsorption (possibly attributable to diffusion into micropores or aggregates). The derivation of half lives for the sulfide-promoted reductive dissolution of Fe (oxyhydr)oxides in seawater, suggests that mineral reactivity can broadly be considered in terms of two mineral groups. Minerals with a lower degree of crystal order (hydrous ferric oxides and lepidocrocite) are reactive on a time-scale of minutes to hours. The more ordered minerals (goethite, magnetite, and hematite) are reactive on a time-scale of tens of days. Substitution of impurities within the mineral structure (as is likely in nature) has an effect on mineral reactivity. However, these effects are unlikely to have a significant impact on the relative reactivities of the two mineral groups

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

Identification of Mackinawite and Constraints on Its Electronic Configuration Using Mössbauer Spectroscopy

The Fe(II) monosulfide mineral mackinawite (FeS) is an important phase in low temperature iron and sulfur cycles, yet it is challenging to characterize since it often occurs in X-ray amorphous or nanoparticulate forms and is extremely sensitive to oxidation. Moreover, the electronic configuration of iron in mackinawite is still under debate. Mössbauer spectroscopy has the potential to distinguish mackinawite from other FeS phases and provide clarity on the electronic configuration, but conflicting results have been reported. We therefore conducted a Mössbauer study at 5 K of five samples of mackinawite synthesized through different pathways. Samples show two different Mössbauer patterns: a singlet that remains unsplit at all temperatures studied, a sextet with hyperfine magnetic field of 27(1) T at 5 K, or both. Our results suggest that the singlet corresponds to stoichiometric mackinawite (FeS), while the sextet corresponds to mackinawite with excess S (FeS1+x). Both phases show center shifts near 0.5 mm/s at 5 K. Coupled with observations from the literature, our data support non-zero magnetic moments on iron atoms in both phases, with strong itinerant spin fluctuations in stoichiometric FeS. Our results provide a clear approach for the identification of mackinawite in both laboratory and natural environments

Ewald methods for polarizable surfaces with application to hydroxylation and hydrogen bonding on the (012) and (001) surfaces of alpha-Fe2O3

We present a clear and rigorous derivation of the Ewald-like method for calculation of the electrostatic energy of the systems infinitely periodic in two-dimensions and of finite size in the third dimension (slabs) which is significantly faster than existing methods. Molecular dynamics simulations using the transferable/polarizable model by Rustad et al. were applied to study the surface relaxation of the nonhydroxylated, hydroxylated, and solvated surfaces of alpha-Fe2O3 (hematite). We find that our nonhydroxylated structures and energies are in good agreement with previous LDA calculations on alpha-alumina by Manassidis et al. [Surf. Sci. Lett. 285, L517, 1993]. Using the results of molecular dynamics simulations of solvated interfaces, we define end-member hydroxylated-hydrated states for the surfaces which are used in energy minimization calculations. We find that hydration has a small effect on the surface structure, but that hydroxylation has a significant effect. Our calculations, both for gas-phase and solution-phase adsorption, predict a greater amount of hydroxylation for the (012) surface than for the (001) surface. Our simulations also indicate the presence of four-fold coordinated iron ions on the (001) surface.Comment: 23 pages, REVTeX (LaTeX), 8 figures not included, e-mail to [email protected], paper accepted in Surface Scienc

Evaluation of al for fe substitution in soil hematites

Hematitas de nove solos e uma de itabirito foram tratadas com NaOH 5 mol L-1 e analisadas por difratometria de raios X (DRX), usando varredura escalonada (0,02°2q/20 s). Determinaram-se o espaçamento d e as dimensões a0 e c0 da cela unitária. As amostras foram dissolvidas com ditionito-citrato-bicarbonato de sódio (DCB), para determinar os teores de Fe (Fed) e Al (Ald). A substituição em Al foi estimada por DRX a partir de a0, usando-se regressões correntemente em uso, baseadas em hematitas sintetizadas a 25°C (Al25) e a 70°C (Al70). Ao parâmetro a0, variando de 0,50380 a 0,50200 nm, correspondeu uma amplitude de zero a 0,125 mol mol-1 Ald. Os valores estimados pela regressão entre a0 e Ald desviaramse da regressão Al70 por -0,003 a +0,009 mol mol-1 Al, enquanto a regressão baseada em Al25 superestimou a substituição, em média, por 0,03 mol mol-1 Al. Os resultados indicaram que, para a estimativa da substituição de Fe por Al em hematitas de solos por DRX, a regressão estabelecida com hematitas sintetizadas a 70°C (Al mol mol-1 = 31,09 – 61,714a0) é a mais adequada.Hematites from nine soils and one from itabirite rock were concentrated with 5 mol L-1 NaOH, and analyzed by XRD (step scanning 0.02o2q/20 s), to determine the unit cell parameters. The samples were dissolved by extraction with DCB to determine the Fe (Fed) and Al (Ald) content. Al substitution was estimated from a0 using established regressions currently in use based on hematites synthesized at 25°C (Al25) and 70°C (Al70). The a0 values ranged from 0.50380 to 0.50200 nm, corresponding to values of zero to 0.125 mol mol-1 Ald. The estimated values by the regression between a0 and Ald deviated from the Al70 regression by -0.003 to + 0.009 mol mol-1 Al, whereas the Al25 regression overestimated the substitution by an average of 0.03 mol mol-1 Al. Thus, for soil hematites, Al substitution is best estimated by the regression based on hematites synthesized at 70°C (Al mol mol-1 = 31.09 – 61.714a0)

Estimating specific surface area of fine stream bed sediments from geochemistry

Specific surface area (SSA) of headwater stream bed sediments is a fundamental property which determines the nature of sediment surface reactions and influences ecosystem-level, biological processes. Measurements of SSA – commonly undertaken by BET nitrogen adsorption – are relatively costly in terms of instrumentation and operator time. A novel approach is presented for estimating fine (2.5 mg kg−1), four elements were identified as significant predictors of SSA (ordered by decreasing predictive power): V > Ca > Al > Rb. The optimum model from these four elements accounted for 73% of the variation in bed sediment SSA (range 6–46 m2 g−1) with a root mean squared error of prediction – based on leave-one-out cross-validation – of 6.3 m2 g−1. It is believed that V is the most significant predictor because its concentration is strongly correlated both with the quantity of Fe-oxides and clay minerals in the stream bed sediments, which dominate sediment SSA. Sample heterogeneity in SSA – based on triplicate measurements of sub-samples – was a substantial source of variation (standard error = 2.2 m2 g−1) which cannot be accounted for in the regression model. The model was used to estimate bed sediment SSA at the other 1792 sites and at 30 duplicate sites where an extra sediment sample had been collected, 25 m from the original site. By delineating sub-catchments for the headwater sediment sites only those sub-catchments were selected with a dominant (>50% of the sub-catchment area) bedrock formation and land use type; the bedrock and land use classes accounted for 39% and 7% of the variation in bed sediment SSA, respectively. Variation in estimated, fine bed sediment SSA from the paired, duplicate sediment sites was small (2.7 m2 g−1), showing that local variation in SSA at stream sites is modest when compared to that between catchments. How the approach might be applied in other environments and its potential limitations are discussed