Near-surface Structural Change in Magnetite: An EPR Study

Magnetite as a lithogenic mineral is widespread in soils. Its bulk magnetic properties have been studied in detail and the grain size distribution has been used as proxy to reconstruct weathering environments. During weathering, oxidation can lead to the formation of ferric oxides at the magnetite surface. Such changes are difficult to detect by classical rock magnetic analysis, because the magnetic data are generally dominated by the bulk properties. In this research we test the use of electron paramagnetic resonance (EPR) spectroscopy in order to get an insight into near-surface properties of magnetite in a soil. The experiments were carried out on untreated and chemically treated soil samples. Chemical treatments included oxalate extraction which dissolves poorly crystallized ferric oxides, as well as citrate-bicarbonate- dithionite (CBD) treatment, which removes ferric oxides (e.g. maghemite, hematite) but also SD magnetite by reductive dissolution. Under the SEM, the magnetite particles showed a particle size of generally more than 10 microns. A minor Ti for Fe substitution was found by EDX analysis. Hysteresis loops of the untreated (KA), oxalate treated (K_ox), and CBD treated (KA_dith) samples at room temperature showed a coercive force (Hc) of 10 ±1 mT and a hysteresis closure between 250-300 mT which is indicative of magnetite. The FORC diagrams of KA, KA_ox and KA_dith were similar and revealed two features : an Hc distribution with maximum at 10 mT and a vertical spread (bias field) with Hc around 2mT. These data suggest that the different chemical treatments had no significant effect on the magnetite in the soil samples. Since CBD treatment is known to dissolve SD magnetite, an assemblage of MD and PSD magnetite particles can be inferred. The EPR spectroscopic parameters (g-value and linewidth δB) showed different values for the untreated and chemically treated magnetite. For KA, g = 2.12 and δB = 120 mT were measured. Similar values (g = 2.14 and δB =120 mT ) were found for Ka_ox. With CBD treatment, both parameters increased to 2.27 and 160 mT, respectively. The significant broadening of the lineshape after CBD treatment is due to an increase of inhomogeneity (e.g. defect structures) caused by the reductive dissolution of Fe^{(III)}. The simultaneous decrease of the resonant field, indicated by higher g-values, points to an enhancement of the internal field. This can be explained by an increase of the magnetic moments due to incongruent dissolution of Fe^{(III)} probably in the vicinity of Ti^{(IV)} sites in the near-surface region of the magnetite grains. In conclusion, EPR spectroscopy is a powerful tool to detect near-surface structural defects and can, therefore, be used to complement classical rock magnetic methods.