A detailed study of the transformation of ferrihydrite to hematite in an aqueous medium at 92°C

Abstract

The transformation of ferrihydrite to hematite by ageing at 92°C in solution has been studied using computer-fitted 57Fe Mössbauer spectra, together with X-ray diffraction and electron microscopy. The X-ray diffraction patterns show hematite is first discernible after 10 minutes ageing and after 30 minutes the hematite peaks are sharp and definite. Mössbauer spectroscopy at room temperature shows it is discernible after 60 minutes ageing but can be detected at liquid nitrogen temperature by 30 minutes. With further ageing the ferrihydrite progressively transforms to hematite and at 116 hours hematite is the only component. The electron micrographs show the ferrihydrite particles of 3–5 nm diameter coalesce to form hexagonal hematite platelets, initially of some 20 nm diameter, which increase to 30–40 nm with ageing. The Mössbauer spectra show the broadened ferric doublet resonance of ferrihydrite and the six-line magnetic hyperfine hematite resonance. Two closely overlapping ferric doublets were computer-fitted to the ferrihydrite resonance, the widths and dips of the component peaks within each doublet being constrained initially to be equal. As these constraints were relaxed, the widths and dips became unequal. This effect is related to the progressive ordering of the ferrihydrite structure as it ages to produce a partially magnetically ordered hematite structure, with a reduced magnetic field at room temperature of initially 473 kOe, increasing to 499 kOe with time. These results suggest a direct transformation of ferrihydrite to hematite, initiated by the coalescing of the ferrihydrite particles.