Presentation Information
[R2-08]Microstructure and Magnetic Flux Distribution in Strongly Remanent Silicon-Bearing Magnetite
*Makio OHKAWA1, Jun-ichi Ando1, Naotaka Tomioka2 (1. Hiroshima Univ. Adv. Sci. and Eng., 2. JAMSTEC・KOCHI)
Keywords:
Lodestone,Silicon-bearing Magnetite,Electron holography
Magnetite-rich ores with significant remanent magnetization, known as lodestones, occur naturally in nature. The pronounced remanent magnetization of lodestones is typically ascribed to the enhanced coercivity resulting from impurity elements and the formation of microstructures within the magnetite. Two principal mechanisms have been suggested: (Type-1) the formation of exsolution textures linked to maghemitization, and (Type-2) the subdivision of magnetic grains through the exsolution of ilmenite lamellae within the titanomagnetite series. This study re-assesses the mechanism responsible for the high coercivity of a lodestone from Iron County, Utah, USA, which has traditionally been categorized as Type-1.
Microstructural observations and compositional analyses were performed using analytical TEM (JEM-ARM200F). Electron holography measurements were conducted using a 1 MV ultrahigh-voltage holography electron microscope produced by Hitachi, which is accessible via the Microscopy Imaging Solution Platform.
High-resolution TEM analyses revealed the presence of fine spinel precipitates enriched with either Si or Ca-Mg-Si within a host phase primarily consisting of pure magnetite crystals. In contrast, no conclusive evidence of maghemite was observed within the sensitivity limits of XRD, EPMA, or TEM analyses. Magnetic flux mapping, performed using ultrahigh-voltage electron holography, demonstrated that the magnetic flux lines, which were relatively uniformly distributed within the magnetite matrix, became concentrated around the precipitates. Importantly, the flux lines were aligned parallel to the elongation direction of the Ca-Mg-Si-rich precipitates. These findings suggest that, although the high coercivity of Type-1 lodestones has traditionally been attributed mainly to maghemitization, the absence of detectable maghemite (γ-Fe2O3) in the current sample implies that Si-bearing spinel precipitates are likely the primary contributors to their high coercivity.
Microstructural observations and compositional analyses were performed using analytical TEM (JEM-ARM200F). Electron holography measurements were conducted using a 1 MV ultrahigh-voltage holography electron microscope produced by Hitachi, which is accessible via the Microscopy Imaging Solution Platform.
High-resolution TEM analyses revealed the presence of fine spinel precipitates enriched with either Si or Ca-Mg-Si within a host phase primarily consisting of pure magnetite crystals. In contrast, no conclusive evidence of maghemite was observed within the sensitivity limits of XRD, EPMA, or TEM analyses. Magnetic flux mapping, performed using ultrahigh-voltage electron holography, demonstrated that the magnetic flux lines, which were relatively uniformly distributed within the magnetite matrix, became concentrated around the precipitates. Importantly, the flux lines were aligned parallel to the elongation direction of the Ca-Mg-Si-rich precipitates. These findings suggest that, although the high coercivity of Type-1 lodestones has traditionally been attributed mainly to maghemitization, the absence of detectable maghemite (γ-Fe2O3) in the current sample implies that Si-bearing spinel precipitates are likely the primary contributors to their high coercivity.
