Evaluation of Temperature-Dependent Magnetic Susceptibility Curves around the Verwey Transition: Insights into the Deformation History of Magnetite

At about 120 K magnetite undergoes a monoclinic to cubic phase transition, which is called the Verwey transition (TV) (e.g. Dunlop and Özdemir 1997). The transition is sensitive to composition, oxidation state and pressure and can be detected by temperature-dependent magnetic susceptibility measurements (k-T curves, see e.g. Reznik et al., 2016 and references therein).

In this study, k-T curves between 80 K and room temperature of magnetite single crystal and magnetite-rich ore are presented. They were approximated using a new model allowing a whole curve approximation. One shape parameter describes the width of the susceptibility increase around TV, and another one the temperature dependence above TV, being derived by the Curie-Weiss law. For the latter we used an approach similar to that described by Hrouda et al. (1997) for separating ferromagnetic and paramagnetic susceptibility contributions. The procedure is applied to k-T curves of magnetite ore samples which have been experimentally shocked up to 30 GPa. A detailed structural characterization of the magnetic samples has been performed (Reznik et al. 2016). k-T curves were performed on magnetite single crystals before and after crushing in a mortar, as well as on magnetite samples deformed in cyclic mechanical loading experiments at room temperature and at elevated temperatures. ... mehrWe also evaluated a possible influence of hematitization using different magnetite-hematite mixtures.

The inverse temperature dependence of susceptibility above TV increases with the duration of the deformation history of magnetite. This behavior can be linked to defect concentration, internal strain, or broken bonds at the grain surfaces of magnetites. Our results suggest that low-temperature k-T curves allow for a qualitative description of internal strain versus domain size refinement in magnetite, if structural analyses are combined with the described model of a whole k-T curve approximation.

References

Dunlop and Özdemir (1997), Cambridge University Press, 573 p.

Hrouda, F. et al. (1997), Geophys. J. Int. 129, 715-719.

Reznik, B. et al. (2016), Geochem. Geophys. Geosyst. 17, 2374-2393.