Mössbauer Spectroscopic Characterization of Iron in Ashes Made from the Ancient Woods Excavated in the Foothills of Mt. Chokai

Abstract

The chemical states of iron in ashes made from umoregi woods excavated in the foothills of Mt. Chokai in 2015, were characterized by using ⁵⁷Fe-Mössbauer spectroscopy and complementary analytical techniques. Four kinds of ancient trees, Japanese oak, Japanese chestnut, Japanese cedar, and Japanese zelkova were burned at 650-710°C. The ashes showed brown, reddish brown, or yellowish brown colors. Elements (> Na) in the ashes were determined by X-ray fluorescence spectrometry using a compact spectrometer. ⁵⁷Fe-Mössbauer spectra of the ashes and soil from the excavation site were recorded at 293 and 78 K. Raman scattering of the ashes was measured in the 1200-900 cm^-1 range. It was confirmed that calcium ferrite, Ca₂Fe₂O₅, was formed in the three ashes and its formation rate depended on the Fe/Ca ratio. Mössbauer spectra suggested that CaFe₂O₄ and/or similar iron oxides were contained in the two ashes. In the case of Fe/Ca > 2, sextet components assigned to α-Fe₂O₃ were also detected in the Mössbauer spectra. It was proposed that the color tone of the ashes was affected by Fe oxides. Mössbauer spectra suggested that some parts of Ca₂Fe₂O₅ formed in the ashes were such a fine particle that superparamagnetic phenomena were observable.