Characterisation of Synthetic Two-line Ferrihydrite by Electron Energy Loss Spectroscopy

The precise atomic structure of the nanocrystalline iron oxyhydroxide mineral, ferrihydrite is currently under intense debate concerning two proposed models. We have synthesized two and six line ferrihydrite and concentrated on the characterisation of the two line form. We show that X-ray scattering data for the two-line form are consistent with the most recent reports and that assessment of the octahedral iron content by electron energy loss spectroscopy in the transmission electron microscope suggests some 90% of the iron is ferric with octahedral co-ordination. We also identify nitrate contamination that has not apparently affected the X-ray scattering and so we infer that it is surface adsorbed.