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 Fe 3+ . The spectra of schwertmannite are consistent with Fe(O,OH) 6 octahedra in some of which an OH − ion has been replaced by a SO 4 2− ion which acts as a bridging bidentate ligand to two Fe 3+ ions.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 SO 4 2− ions in the form of SO 3 and H 2 O entails the simultaneous loss of OH − ions and the conversion of some OH − ions to O 2− . The loss and resulting rearrangement of ligands around those Fe 3+ ions originally liganded by SO 4 2− results in a fraction of Fe 3+ achieving the tetrahedral coordination that is associated with maghemite.