Sigrid Griet Eeckhout

Magnetism of intermediate hydromaghemite in the transformation of 2‐line ferrihydrite into hematite and its paleoenvironmental implications

[1] A maghemite-like phase referred to here as ‘‘hydromaghemite’’ was obtained as an intermediate product in the hydrothermal transformation of phosphated 2-line ferrihydrite into hematite. In this study, we used magnetic and non-magnetic (e.g., X-ray Absorption Near Edge Structure, XANES) techniques in combination to characterize a series of the intermediate products obtained in the aging of phosphated ferrihydrite (P/Fe atomic ratio = 0.03) at 150C for 120 days. Particle size calculated from both the specific surface area and average unblocking temperature increased with time. XANES spectra revealed the presence of some tetrahedrally coordinated iron, which is consistent with the formation of hydromaghemite (the dominant magnetic phase in the intermediate products). Thus grain size in newly formed hydromaghemite particles increased with time from the initial values in the superparamagnetic region to others in the single-domain region. Further transformation of hydromaghemite into hematite, which was complete by day 120, was probably due to hydromaghemite becoming unstable relative to hematite when the surface to volume ratio fell below a given threshold. The relationships between pedogenically produced maghemite and hematite contents in various soils and paleosols suggest that the ferrihydrite ! hydromaghemite ! hematite transformation may constitute a major pathway accounting for the magnetic enhancement in many soils.

XAS determination of the Fe local environment and oxidation state in phonolite glasses

Abstract The Fe oxidation state, coordination geometry, and distances have been determined by Fe K-edge XANES and EXAFS for a set of silicate glasses of phonolite composition produced at different oxygen fugacity conditions with the aim of determining the effect of iron oxidation state and local structural environment on the viscosity of the corresponding melts. Comparison of the pre-edge peak data with those of Fe model compounds with known oxidation state and coordination number allowed for determination of the Fe oxidation state and coordination number for all the glasses analyzed. The Fe3+/(Fe3++Fe2+) ratio varies from 0.44 to 0.93 (±0.05) in the glasses studied. The determined values are in excellent agreement (within 0.03 difference) with those independently measured by the titration method. Moreover, pre-edge peak data clearly indicate that Fe3+ is in fourfold coordination, whereas Fe2+ exists both in fourfold and fivefold coordination for this phonolitic composition, although the presence of minor amounts of sixfold-coordinated Fe cannot be ruled out by XANES data alone. EXAFS data of the most oxidized sample indicate that Fe3+ is in tetrahedral coordination with = 1.85 Å (±0.01). This value compares well with literature data for [4]Fe3+ (e.g., in tetra-ferriphlogopite or rodolicoite). Calculated NBO/T ratios decrease with Fe oxidation (from 0.23 to 0.19). For phonolitic glasses of this study, going from reducing to oxidizing conditions results in a higher fraction of network-forming Fe, thus increasing the polymerization of the tetrahedral network and producing shorter (and stronger) bond distances. Both the polymerization increase and the structural variations in the Fe local environment can qualitatively explain the strong increase in melt viscosity observed at higher oxygen fugacity

Cr local environment by valence-to-core X-ray emission spectroscopy.

Valence-to-core X-ray emission spectra have been measured for a number of chromium compounds such as oxides, carbides, borides, nitrides, phosphides etc. The experimental spectra are in good agreement with the calculated ones. They form the basis for a detailed analysis of the first coordination shell of Cr. The unique application of this technique for material chemistry and environmental science is illustrated by two very different examples. In the former case the presence of Cr–C bonds in the bulk structure of electrochemically deposited amorphous chromium coatings was confirmed. In the latter one we have shown that a contaminated soil sample most probably contains Cr carbides and/or phosphides from anthropogenic origin.

X-ray Linear Dichroism in cubic compounds: the case of Cr3+ in MgAl2O4

The angular dependence (x-ray linear dichroism) of the

Effect of alkalis on the Fe oxidation state and local environment in peralkaline rhyolitic glasses

text{Cr}text{ }K

The oxidation state of vanadium in titanomagnetite from layered basic intrusions

pre-edge in

XANES study of the oxidation state of Cr in lower mantle phases: Periclase and magnesium silicate perovskite

{text{MgAl}}_{2}{text{O}}_{4}:{text{Cr}}^{3+}

Site preference and local geometry of Sc in garnets: Part I. Multifarious mechanisms in the pyrope-grossular join

spinel is measured by means of x-ray absorption near-edge structure spectroscopy and compared to calculations based on density functional theory (DFT) and ligand field multiplet (LFM) theory. We also present an efficient method, based on symmetry considerations, to compute the dichroism of the cubic crystal starting from the dichroism of a single substitutional site. DFT shows that the electric dipole transitions do not contribute to the features visible in the pre-edge and provides a clear vision of the assignment of the

Thermal stability and dehydration of anapaite

1sensuremath{rightarrow}3d

North American microtektites are more oxidized than tektites

transitions. However, DFT is unable to reproduce quantitatively the angular dependence of the pre-edge, which is, on the other side, well reproduced by LFM calculations. The most relevant factors determining the dichroism of