Critical evaluation of the revised akdalaite model for ferrihydrite
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A. Manceau
Abstract
The defect-free akdalaite model (fhyd6) for six-line ferrihydrite (6Fh) derived from a pair distribution function (PDF) analysis of high-energy X-ray scattering (HEXS) data was revised (model ferrifh) by Michel et al (2010) using data from a sample produced by heating two-line ferrihydrite (2Fh) at 175 °C for 8 h in the presence of citrate. We show here that the scattering pattern for this sample is similar if not the same as that for hydromaghemite, which in turn is a mixture of maghemite (γ-Fe2O3), hematite and 6Fh. As in the case of fhyd6, the PDF of ferrifh was regressed using the structure of the weakly hydrated phase akdalaite [Al10O14(OH)2] after substituting Fe for Al as a starting model. We show that the ferrifh model is implausible for the following reasons. (1) It is derived from a sample, ferrifh, that appears to be hydromaghemite, not pure 6Fh. (2) It has 20% tetrahedral Fe, a coordination that had been eliminated previously using XANES, Mössbauer, and EELS spectroscopies. (3) 75% of the Fe octahedra have shared edge lengths considerably longer than the shortest unshared edges in violation of Pauling’s distortion rule. (4) Three tetrahedral Fe-O bonds are longer than three octahedral Fe-O bonds, inducing significant polyhedral distortions. And (5) the calculated composition [Fe10O14(OH)2⋅1.2H2O] disagrees with literature data on weight loss from dehydration for 6Fh.
We present an alternative interpretation of the histogram of Fe-Fe distances up to 3.7 Å obtained from the PDF of the fhyd6 ferrihydrite as a mixture of local structures of goethite/akaganeite (α/β- FeOOH) and feroxyhite/hematite (δ-FeOOH/α-Fe2O3). Within this interpretation Fe only occupies octahedra that are bonded to each other by faces, edges, or double-corners. This polyhedral connectivity is confirmed experimentally by analysis of the EXAFS spectra of six-line ferrihydrites measured at room and liquid helium temperature. The Fe-Fe pairs from EXAFS data are described reasonably well by a mixture of approximately 70% feroxyhite (containing some nanohematite) and 30% akaganeite, without resorting to other phases. This set of evidence indicates that HEXS data are consistent with the Drits model for ferrihydrite (Drits et al. 1993a).
© 2015 by Walter de Gruyter Berlin/Boston
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Articles in the same Issue
- Roebling Medal Lecture. The three partners of metamorphic petrology
- Relationship between structure, morphology, and carbon isotopic composition of graphite in marbles: Implications for calcite-graphite carbon isotope thermometry
- Crystal structure, mosaicity, and strain analysis of Hawaiian olivines using in situ X-ray diffraction
- Characterization of deep weathering and nanoporosity development in shale—A neutron study
- Structural water in ferrihydrite and constraints this provides on possible structure models
- Critical evaluation of the revised akdalaite model for ferrihydrite
- Neutron diffraction study of hydrogen in birnessite structures
- Pressless split-sphere apparatus equipped with scaled-up Kawai-cell for mineralogical studies at 10–20 GPa
- Dislocation microstructures in majorite garnet experimentally deformed in the multi-anvil apparatus
- Density of carbonated peridotite magma at high pressure using an X-ray absorption method
- Stability and bulk modulus of Ni3S, a new nickel sulfur compound, and the melting relations of the system Ni-NiS up to 10 GPa
- Far-infrared spectra of synthetic [4][(Al2-xGax)(Si2–yGey)](OH,OD,F)2-kinoshitalite: Characterization and assignment of interlayer Ba-Oinner and Ba-Oouter stretching bands
- High-temperature elasticity of polycrystalline orthoenstatite (MgSiO3)
- Anatomy of a metabentonite: Nucleation and growth of illite crystals and their coalescence into mixed-layer illite/smectite
- Zn-O tetrahedral bond length variations in normal spinel oxides
- A new thermodynamic analysis of the intergrowth of hedenbergite and magnetite with Ca-Fe-rich olivine
- Raman spectroscopic investigations of some Tl-sulfosalt minerals containing pyramidal (As,Sb)S3 groups
- A first record of strong structural relaxation of TO4 tetrahedra in a spinel solid solution
- The high-pressure behavior of orthorhombic amphiboles
- XAS determination of the Fe local environment and oxidation state in phonolite glasses
- Chemical variation and significance of micas from the Fregeneda-Almendra pegmatitic field (Central-Iberian Zone, Spain and Portugal)
- Light-induced molecular change in HgI2·As4S4: Evidence by single-crystal X-ray diffraction and Raman spectroscopy
- Low-temperature electron paramagnetic resonance studies on natural calumetite from Khetri copper mine, Rajasthan, India
- Chromatite and its Cr3+- and Cr6+-bearing precursor minerals from the Nabi Musa Mottled Zone complex, Judean Desert
- Hazenite, KNaMg2(PO4)2⋅14H2O, a new biologically related phosphate mineral, from Mono Lake, California, U.S.A.
- The fractional latent heat of crystallizing magmas
