THE ROLE OF NANO-STRUCTURES AND DOMAINS IN ENHANCING MAGNETIC COERCIVITY OF LODESTONE

Many studies have revealed that exsolution lamellae are an important contributor to the unusual remanent magnetization in slow cooling igneous and metamorphic rocks [1]. However, the role of exsolution lamellae for the coercivity and remanent magnetization is not well understood. To understand these unusual magnetic properties, we examined lodestone using X-ray diffraction (XRD), transmission electron microscopy (TEM), and superconducting quantum interference device (SQUID) magnetometer. The lodestone is a natural permanent magnet showing strong remanent magnetization and coercivity, which has partially oxidized magnetite intergrown with hematite and maghemite. Rietveld refinement analysis of a lodestone shows 64.3(4) wt % of magnetite with 26.9(3) wt % of maghemite and 8.7(4) wt % of hematite. To test the interface’s effect on the magnetism of lodestone, we prepared three lodestone samples with different hematite concentrations. Magnetic hysteresis loops from the samples suggest that the magnetic coercivity is proportional to the concentration of hematite. TEM images of lodestone show aligned nanoscale exsolution lamellae of hematite and magnetite corresponding to the interfaces {111}Mgt//(0001)Hem. The interface is followed by oxygen packing direction of cubic magnetite and rhombohedral hematite. TEM images of host magnetite often show stacking faults and twin boundaries along the [111] direction of magnetite. Interestingly, the interface and twinning can produce ABAC packing sequences that are associated with the structure of luogufengite (ε-Fe₂O₃). Luogufengite is a polymorph of maghemtite and hematite with a large magnetic coercive field at room temperature [2]. We suggest that the structure of luogufengite-like nano-domains at the interface or within a mineral could enhance the magnetic coercive field of lodestone. These observations can provide an explanation for strong coercivity in slow cooling igneous and metamorphic rocks [3]. We believe that this is a good example of using the nanostructure to describe distinctive mineral properties in natural systems. The observation of nano-minerals and nano-domains certainly helps us to better understand anomalous magnetic properties found on Earth and other planetary systems.

[1] Lee, S., & Xu, H. (2018). Minerals, 8(3), 97. [2] Xu, H., Lee, S., & Xu, H. (2017) American Mineralogist, 102(4), 711-719. [3] Lee, S., Xu, H., Xu, H., Jacobs, R., & Morgan, D. (2019) American Mineralogist, 104(9), 1238-1245.