Hydrothermal phase transformation of hematite to magnetite

Different phases of iron oxide were obtained by hydrothermal treatment of ferric solution at 200°C with the addition of either KOH, ethylenediamine (EDA), or KOH and EDA into the reaction system. As usually observed, the α-Fe(2)O(3) 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 α-Fe(2)O(3) to Fe(3)O(4) at low-temperature hydrothermal conditions. The phase transformation involves the formation of α-Fe(2)O(3) hexagonal plates, the dissolution of the α-Fe(2)O(3) hexagonal plates, the reduction of Fe(3+) to Fe(2+), and the nucleation and growth of new Fe(3)O(4) polyhedral particles.