Raman spectra and thermal transformations of ferrihydrite and schwertmannite

Raman and infrared spectra of 2-line and 6-line ferrihydrite and schwertmannite were recorded. The spectra of the ferrihydrites are compatible with octahedrally coordinated Fe3+. The spectra of schwertniannite are consistent with Fe(O,OH)(6) octahedra in some of which an OH- ion has been replaced by a SO42- ion which acts as a bridging bidentate ligand to two Fe3+ ions. The Raman spectra show that under heating by the He-Ne laser beam 2-line ferrihydrite and schwertmannite transform to hematite via a maghemite intermediate, whereas 6-line ferrihydrite transforms directly to hematite. The different thermal transformation behaviour of the two ferrihydrites accords with recent reports that the two forms are structurally different and suggests that it is the disordered component of 2-line ferrihydrite that transforms to maghemite on heating. In the case of schwertmannite the loss on heating Of SO42- ions in the form of SO3 and H2O entails the simultaneous loss of OH- ions and the conversion of some OH- ions to O2-. The loss and resulting rearrangement of ligands around those Fe3+ ions originally liganded by SO42- results in a fraction of Fe3+ achieving the tetrahedral coordination that is associated with maghemite. Copyright (C) 2002 John Wiley Sons, Ltd.