Mineralogical alteration of thermally treated siderite in air: Mössbauer spectroscopy results

Mineralogical alternation of thermally treated siderite in air atmosphere has been systematically analyzed by Mössbauer effects. It was preliminarily estimated from the area of subspectra that 4%, 39% and 62% of magnetite were formed at 410 °C, 490 °C and 510 °C respectively. After being incrementally heated at 530 °C the spectra consist of two sextets of Fe₃O₄. Sextet of γ-Fe₂O₃ with hyperfine field of 50T was observed at 550 °C. Spectra at 580 °C consisted of two sextets with hyperfine fields caused by γ-Fe₂O₃ and α-Fe₂O₃. Fe₃O₄ disappeared and the amount of γ-Fe₂O₃ decreased, while the quantity of α-Fe₂O₃ increased to 34% and 77% at 640 °C and 690 °C, respectively. During the early stage of decomposition and oxidation, FeO was probably produced but quickly oxidized to magnetite and unidentified in our experiment. These results, in good agreement with the X-ray diffraction analyses and microscopic observation, provide an interpretation to anomalous magnetic property changes of siderite-bearing rock samples.