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

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 λ = 0.137 Å. The new ferrihydrite model is isostructural to akdalaite (Al 10 O 14 (OH) 2 ), a mineral having the Baker-Figgis δ-isomer of the Al 13 -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 ( VI Fe 4+ ), 20% divalent tetrahedral iron ( IV Fe 2+ ), and some IV Fe–O distances equal to or larger than the VI Fe 3+ –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.