[S-EM13] Geomagnetism, paleomagnetism and rock magnetism

[SEM13-04] Single-domain particles of ε-Fe₂O₃ in red-brown obsidians

*Koji Fukuma¹, Hiroshi Isobe², Satoko Ishimaru², Keiji Shinoda³, Tadashi Ohachi (1.Department of Environmental System Science, Faculty of Science and Engineering, Doshisha University, 2.Department of Earth and Environmental Science, Faculty of Advanced Science and Technology, Kumamoto University, 3.Department of Geoscience, School of Science, Osaka Metropolitan University)

Keywords:rock magnetism, Raman spectroscopy, X-ray diffraction, magnetic hysteresis

ε-Fe₂O₃ is a magnetic material with a huge coercivity exceeding 1 T at room temperature, and a single phase of tens of nanometers was first synthesized in the 2000s. It has also been found in archeological artifacts and scoriae and was recognized as a new mineral, luogufengite, in 2016. Although ε-Fe₂O₃ is easily identified by its low Curie temperature (< 210°C), archeological artifacts and scoriae show large but constricted magnetic hysteresis loops, indicating the coexistence of ε-Fe₂O₃ and other magnetic minerals. In this study, bulk red–brown obsidian samples from several localities were examined by X-ray diffraction and Raman microscopy, and magnetic hysteresis curves were unmixed to obtain hysteresis properties of a single phase ε-Fe₂O₃. Rietveld analysis was performed on the X-ray diffraction data of red-brown obsidians, and it was found that ε-Fe₂O₃ and α-Fe₂O₃ (hematite) were present in a volume ratio of 1:2 to 1:3. Raman spectra showed that α-Fe₂O₃ predominated in the reddish-brown obsidians, but ε-Fe₂O₃ of a few μm was identified, and the presence of Fe₃O₄ (magnetite) was also confirmed. Low-temperature magnetic hysteresis loops were obtained on reddish brown chips, and unmixing was performed to separate the loops into three components. The component with the highest coercivity has a coercivity greater than 1 T at room temperature and reaches its maximum value at about 200 K. Mr/Ms is almost constant at 0.5, independent of temperature. This hysteresis property is consistent with the synthesized ε-Fe₂O₃, indicating that red-brown obsidians contain uniaxially anisotropic single-domain particles of ε-Fe₂O₃.