Hydrothermal phase transformation of hematite to magnetite

Different phases of iron oxide were obtained by hydrothermal treatment of ferric solution at 200 degrees C with the addition of either KOH, ethylenediamine (EDA), or KOH and EDA into the reaction system. As usually observed, the alpha-Fe2O3 hexagonal plates and hexagonal bipyramids were obtained for reaction with KOH and EDA, respectively. When both KOH and EDA were added into the reaction system, we observed an interesting phase transformation from alpha-Fe2O3 to Fe3O4 at low-temperature hydrothermal conditions. The phase transformation involves the formation of alpha-Fe2O3 hexagonal plates, the dissolution of the alpha-Fe2O3 hexagonal plates, the reduction of Fe3+ to Fe2+, and the nucleation and growth of new Fe3O4 polyhedral particles.