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 lambda = 0.137 angstrom. The new ferrihydrite model is isostructural to akdalaite (Al10O14(OH)(2)) a mineral having the Baker-Figgis delta-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 (Fe-VI(4+)), 20% divalent tetrahedral iron (Fe-IV(2+)), and some Fe-IV-O distances equal to or larger than the Fe-VI(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.