Structural evolution of FeCO3 through decarbonation at elevated temperatures

The structural evolution of siderite (s), FeCO3, through decarbonation at elevated temperatures has been investigated by the in-situ single-crystal X-ray diffraction technique using an area detector. When the crystal was heated above 255 °C, the transparent crystal turned colour in faint black from surface, indicating that the decarbonation commenced. The spinel-type magnetite (m), Fe3O4, first appeared in coexistence with the FeCO3 parent crystal. The orientation relationship between the rhombohedral FeCO3 and the cubic Fe3O4 can be described as [111]m // [001]s, and [2-1-1]m // [120]s. On further heating, additional diffraction spots appeared at 411 °C. They were indexed on the basis of the corundum-type hematite (h), (α-Fe2O3). The rhombohedral α-Fe2O3 unit cell had the same orientation relationship with the parent rhombohedral FeCO3 one, i.e., [001]h // [001]s, and [100]h // [100]s. On further heating the parent FeCO3 phase disappeared completely at 464 °C. The formation of iron oxides in FeCO3 depended on not only temperature but also the holding time. The structural relationships among FeCO3, Fe3O4, and α-Fe2O3 are discussed.