Effect of shock pressure on the Verwey transition in magnetite: Modelling of the magnetic susceptibility with various distributions of transition temperatures

Magnetite occurs as an accessory mineral in many rocks. It shows a strong ferrimagnetic behavior with a phase transition at about 120 K (Verwey transition temperature), which causes a strong increase in magnetic susceptibility by warming through the transition. The transition temperature shows a complex dependence on chemical composition, oxidation and internal stresses and was suggested as an indicator of shock pressures in terrestrial impact structures [e.g., 1].

Here, susceptibility curves were modelled between 80 and 200 K assuming probabilistically distributed transition temperatures representing variations in local stress fields. An improved version of a previously presented model was used [2]. The model was applied to susceptibility curves of magnetite-rich iron ore shocked at 5, 10, 20 and 30 GPa in laboratory experiments with subsequent heat treatment [3,4]. The results show a clear change from a sharp normal distribution of transition temperatures in unstressed ore towards a broad heavy-tailed Lorentz distribution in highly stressed samples. This behavior can be related to shock-induced dislocations. The distribution of transition temperatures in stressed ore indicates interactions between magnetic domains and could be useful e.g. for the distinction between stress and oxidation effects on the Verwey transition of rocks.

[1] Carporzen et al. (2006), Earth Planet Sci Lett 251, 305-317; [2] Fuchs et al. (2019), AGU Fall Meeting 2019, GP23B-0790; [3] Reznik et al. (2016), Geochem Geophys Geosyst 17, 2374-2393; [4] Kontny et al. (2018), Geochem Geophys Geosyst 19, 921-931.