At about 120 K magnetite undergoes a sharp Verwey transition (TV) caused by atom coordination changes through a monoclinic to cubic phase transition (e.g. Dunlop and Özdemir 1997). TV is sensitive to composition, oxidation and pressure and can be determined by temperature-dependent magnetic susceptibility measurements (k-T curves, see e.g. Reznik et al., 2016 and references therein). The shape across the transition is rather complex in magnetite-bearing rocks compared to single crystal experiments, consisting of at least a sigmoidal increase followed by exponential decay.
In the present work a new approximation procedure is developed which considers the complete shape of the transition region for modelling, using superposition of infinitesimally small magnetite volumes similar to magnetic domains whereby different transition temperatures for different domains are assumed. Such temperature variations may be caused e.g. by internal stresses. The procedure allows a precise fitting even in the range of peak susceptibility where both, sigmoidal increase and exponential decay, have a strong influence on the measured susceptibility, leading to a quantitative determination of shape parameters correlating with deformation conditions of the examined rocks.
As a first approximation some simple assumptions on magnetic behavior are made. Verwey transition of a single volume element is modelled as an instantaneous increase of susceptibility at TV with rising temperature. Above TV susceptibility is modelled assuming an inverse temperature dependence based on Curie’s law, similar to Hrouda et al. (1997). We suggest that the paramagnetic contributions can be caused by surface defect structures in magnetite indicating strain. Gaussian distributions of transition temperatures over the magnetite volume allow modelling of the observed sigmoidal increase and identification of populations showing distinct magnetic behavior.
To clarify whether k-T curves across the Verwey transition are sensitive to strain, we subjected metamorphic quartz-magnetite ore to cyclic uniaxial loading at different frequencies and applied the approximation procedure to k-T curves. Defects in magnetite for these seismic-related fatigued samples are presented by Reznik et al. (this vol.).
Dunlop and Özdemir (1997), Rock magnetism, fundamentals and frontiers, Cambridge University Press, 573 p.
Hrouda, F. et al. (1997), Refined technique for susceptibility resolution into ferromagnetic and paramagnetic components base on susceptibility temperature-variation measurement, Geophys. J. Int. 129, 715-719.
Reznik, B. et al. (2016), Shock-induced deformation phenomena in magnetite and their consequences on magnetic properties, Geochem. Geophys. Geosyst. 17, 2374-2393.
Reznik et al. (2019), Magnetic and microstructural fatigue of a magnetite-quartz rock, Geo Münster 2019.