Raman spectra and thermal transformations of ferrihydrite and schwertmannite

Abstract

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 The spectra of schwertmannite are consistent with Fe(O,OH)6 octahedra in some of which an OH− ion has been replaced by a SO42− ion which acts as a bridging bidentate ligand to two Fe3+ ions.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 © 2002 John Wiley & Sons, Ltd.

Frutexites-like structures formed by iron oxidizing biofilms in the continental subsurface (Äspö Hard Rock Laboratory, Sweden)

“…The bands of the spectra described here are even more shifted but also are much broader. Similar shifted band positions were reported by [34] for ferrihydrite which was transformed into hematite due to thermal influence of the laser excitation. During our experiments, it is likely that the laser power caused a transformation from ferrihydrite into hematite.…”

Characterisation of the steel concrete interface submitted to chloride‐induced‐corrosion

“…First, 2 and 6 line ferrihydrite and feroxyhyte phases cannot be easily differentiated by Raman spectroscopy. Their spectra present a broad band between 540 and 810 cm −1 33, 36. Maghemite also presents a broad band between 600 and 800 cm −1 but two shoulders are visible in this massif, one at 668 and one other at 721 cm −1 .…”

Spatial distributions of secondary minerals in the Martian meteorite MIL 03346,168 determined by Raman spectroscopic imaging

“…This phase has a very broad H 2 O/OH Raman band centered near 3550–3580 cm −1 (spectra not shown), which is very different from the OH/H 2 O bands of jarosites and Ca‐sulfates. These features bear certain similarities to those of ferrihydrite, 5Fe 2 O 3 · 9H 2 O [ Das and Hendry , ; Hanesch , ; Mazzetti and Thistlethwaite , ], and maghematite γ‐Fe 2 O 3 [ De Faria et al ., ], but not to FeOOH minerals (e.g., goethite and lepidocrocite) [ Das and Hendry , ]. We tentatively assign group 1 of HSLC to ferrihydrite, on the basis of observable H 2 O/OH peak and the absence of 1320 and 1560 cm −1 bands of maghematite.…”