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

Mineralogical alternation of thermally treated siderite in air atmosphere has been systematically analyzed by Mossbauer effects. It was preliminarily estimated from the area of subspectra that 4%, 39% and 62% of magnetite were formed at 410 degrees C, 490 degrees C and 510 degrees C respectively. After being incrementally heated at 530 degrees C the spectra consist of two sextets of Fe3O4. Sextet of gamma-Fe2O3 with hyperfine field of 50T was observed at 550 degrees C. Spectra at 580 degrees C consisted of two sextets with hyperfine fields caused by gamma-Fe2O3 and alpha-Fe2O3. Fe3O4 disappeared and the amount of gamma-Fe2O3 decreased, while the quantity of alpha-Fe2O3 increased to 34% and 77% at 640 degrees C and 690 degrees 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.