SUMMARY
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.
SUMMARY
The interpretation of soil susceptibility measurements depends on knowledge of a reference value. This reference value will be influenced by the type of soil. In the present study, data of various soil types, which formed on diverse parent material, are analysed. As the same data set has been used in a soil pollution study, we are able to estimate and exclude the influence of pollution. Susceptibility values of unpolluted soils turn out to significantly depend on the soil type. Chernozem samples yield the highest susceptibility values of the analysed soil types (50–90 × 10−8 m3 kg−1). Cambisol displays intermediate values (20–60 × 10−8 m3 kg−1) and waterlogged soils have the lowest values (5–20 × 10−8 m3 kg−1). The enrichment of topsoil susceptibility (0–20 cm depth) compared to subsoil susceptibility (40–50 cm depth) depends more on the parent material than on the soil type. Given the comparability of soil type and parent material, the values of this study can be used to identify soils with unusual susceptibility values, which may be further analysed in order to pinpoint soil pollution.
S U M M A R YThermomagnetic measurements, particularly measurements of the Curie or Néel temperature, are often used to identify magnetic minerals in rocks or sediments. In many samples it is impossible to determine the Curie or Néel temperature as mineral changes occur during heating. Especially in soils or sediments, iron (hydr)oxides may be transformed to ferrimagnetic iron oxides. In this study we analysed the thermomagnetic behaviour of some natural and synthetic iron minerals: goethite, haematite, ferrihydrite, lepidocrocite and siderite. The change of magnetization M with temperature was determined with a magnetic translation balance. The sample was heated in air to a maximum temperature of 700 • C and subsequently cooled back to room temperature. By adding organic carbon to synthetic samples and by destroying it in natural samples, the influence of organic substance on the reactions was determined. Goethite, ferrihydrite and haematite transform to a strongly magnetic phase only if organic carbon is present. Lepidocrocite and siderite transform without organic carbon, the reaction of siderite is even weakened if organic matter is added. The transformation starts below 400 • C for ferrihydrite and lepidocrocite and around 450 • C for goethite. We can conclude that, though these reactions impede the identification of the ferrimagnetic oxides in many soil and sediment samples, they can be used to distinguish between the most common soil iron hydroxides in these environments.
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