Shock‐induced deformation phenomena in magnetite and their consequences on magnetic properties

Abstract

This study investigates the effects of shock waves on magnetic and microstructural behavior of multidomain magnetite from a magnetite-bearing ore, experimentally shocked to pressures of 5, 10, 20, and 30 GPa. Changes in apparent crystallite size and lattice parameter were determined by X-ray diffraction, and grain fragmentation and defect accumulation were studied by scanning and transmission electron microscopy. Magnetic properties were characterized by low-temperature saturation isothermal remanent magnetization (SIRM), susceptibility measurements around the Verwey transition as well as by hysteresis parameters at room temperature. It is established that the shock-induced refinement of magnetic domains from MD to SD-PSD range is a result of cooperative processes including brittle fragmentation of magnetite grains, plastic deformation with shear bands and twins as well as structural disordering in form of molten grains and amorphous nanoclusters. Up to 10 GPa, a decrease of coherent crystallite size, lattice parameter, saturation magnetization (Ms), and magnetic susceptibility and an increase in coercivity, SIRM, and width of Verwey transition are mostly associated with brittle grain fragmentation. Starting from 20 GPa, a slight recovery is documented in all magnetic and nonmagnetic parameters. In particular, the recovery in SIRM is correlated with an increase of the lattice constant. The recovery effect is associated with the increasing influence of shock heating/annealing at high shock pressures. The strong decrease of Ms at 30 GPa is interpreted as a result of strong lattice damage and distortion. Our results unravel the microstructural mechanisms behind the loss of magnetization and the modification of magnetic properties of magnetite and contribute to our understanding of shock-induced magnetic phenomena in impacted rocks on earth and in meteorites.

Effect of moderate shock waves on magnetic susceptibility and microstructure of a magnetite‐bearing ore

“…All specific magnetic susceptibility data of these experiments are published by Reznik et al. (). It is well known that for magnetite the magnetic susceptibility is a function of grain size, and that these variations are closely related to the size of the magnetic domains (e.g., Gilder et al.…”

“…A detailed crystallographic analysis of the twinned lamellae and amorphous defects in shocked magnetite is presented by Reznik et al. (). As it was already mentioned before, the occurrence of twin lamellae indicates intense high‐rate plastic deformation (Armstrong and Worthington ).…”

“…Other parameters such as coercive forces or saturation remanence would probably be more useful (Reznik et al. ).…”

The effects of 10 to >160 GPa shock on the magnetic properties of basalt and diabase

“…Reznik et al . [] reported a shock‐induced increase in Verwey transition temperature T v* (determined from the maximum in the first derivative of temperature dependence of low‐field magnetic susceptibility acquired in 83–300 K range) by 6 K for all shocked samples of magnetite with regard to their unshocked analogue; however, T v* remained essentially constant within the samples shocked to 5, 10, 20, and 30 GPa. The authors attribute such changes in T v* to a combination of grain fracturing, plastic deformation mechanisms, and amorphization but argue that in the 5–30 GPa dynamic pressure range T v* cannot be used as a geobarometer because strain memory saturation occurs by 5 GPa.…”

Variation in Magnetic Fabrics at Low Shock Pressure Due to Experimental Impact Cratering

“…The effect of low shock pressure on magnetic minerals that cause variation in the magnetic properties has not yet been systematically investigated. In recent years there have been few studies, but only at higher shock pressures, >5 GPa (e.g., Mang et al, ; Reznik et al, , ). In summary, the present study fills a gap in our knowledge on the variation of magnetic properties at low shock pressure.…”