Raman spectroscopy of iron oxides and (oxy)hydroxides at low laser power and possible applications in environmental magnetic studies

Raman spectroscopy uses the inelastic scattering of electromagnetic radiation by molecules. Monochromatic light of a laser interacts with phonons, the vibrational modes in the crystal lattice. The energy of the scattered light is shifted by the scattering. The shifts in energy yield the Raman spectrum that is specific for each mineral because the phonons are specific for each mineral. In this study, Raman spectroscopy of synthetic and natural iron (oxy)hydroxides and iron oxides was performed to test its potential in environmental magnetic studies and soil science. The main aim was to distinguish between the different iron oxides occurring in soils. Most of them can be identified by magnetic methods, but there are some minerals that are not easy to differentiate from each other. In these cases, the magnetic methods can be complemented by Raman spectroscopy. A major challenge is the fast transformation of many iron minerals if laser power is applied, especially if the material is poorly crystallized as often is the case in environmental material. In this study, very low laser powers were applied. Nevertheless, the investigated iron minerals could be distinguished from each other. Thus, a magnetic method to discern lepidocrocite and ferrihydrite in soil samples could be corroborated. It is also shown that Raman spectroscopy is an easy method to distinguish magnetite and maghemite. Due to the low laser powers applied, a wuestite band at about 595 cm−1 could be established enabling a non-ambiguous identification of this mineral by its Raman spectrum. Furthermore, the potential of the method to investigate magnetic material produced by soil bacteria is demonstrated.