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Evaluation of the structural model for ferrihydrite derived from real-space modelling of high-energy X-ray diffraction data

LGIT, Maison des Géosciences, CNRS and Université Joseph Fourier, 38041 Grenoble Cedex 9, France

^* E-mail: Alain.Manceau{at}obs.ujf-grenoble.fr

(Received 31 January 2008; revised 19 June 2008)

A new structural model for ferrihydrite that challenges the standard ferrihydrite model established by X-ray diffraction and confirmed by neutron diffraction and single-crystal electron nanodiffraction was recently proposed by Michel et al. (2007a) from the simulation of the pair distribution function obtained by Fourier transformation of diffraction data measured at = 0.137 Å. The new ferrihydrite model is isostructural to akdalaite (Al₁₀O₁₄(OH)₂), a mineral having the Baker-Figgis -isomer of the Al₁₃-Keggin structure as its structural motif. The new model is unrealistic because: (1) it is completely periodic (i.e. defect-free); (2), it has 20% tetravalent octahedral iron (^VIFe⁴⁺), 20% divalent tetrahedral iron (^IVFe²⁺), and some ^IVFe–O distances equal to or larger than the ^VIFe³⁺–O distances, thus violating Pauling's 2^nd rule; (3) it does not describe X-ray diffraction and EXAFS spectroscopic data; and, (4) it is inconsistent with electron microscopy results and contradicts previous X-ray scattering studies.

KEYWORDS: ferrihydrite, feroxyhite, structure, PDF, XRD, EXAFS spectroscopy, bond valence