High Temperature Behaviour of Hercynite
About half of the original hercynite was preserved after the longest annealing treatment of 36 h. The remained iron-aluminium spinel underwent decomposition into a few iron and aluminium oxide phases, among which non-stoichiometric magnetite and corundum were the most significant. Mössbauer spectroscopy showed that hercynite is a normal spinel with a slight inversion parameter of 11 %, which was increased up to 34 % after the 36 h thermal annealing. An overlapping of reflexes for spinel-like phases γ-Fe2O3 and γ-Al2O3 with Fe–Al spinel interfered with their detection by X-ray diffractometry. The Mössbauer spectroscopy measurements confirmed the presence of magnetite in the after-annealed samples. The electron probe microanalysis of the hercynite oxidised surfaces showed well-developed, scalenohedral morphology of the crystals, and well-defined compositions corresponding to Al-containing Fe2O3 measured by wavelength-dispersive X-ray spectroscopy. The results of XRD, MS and SEM/EPMA examinations lead to establishing a general oxidation mechanism of hercynite at high temperatures.