The transformation of ferrihydrite into goethite or hematite, revisited

During the oxidation of iron, poorly crystallized phases are firstly formed: 2- and 6-line ferrihydrite, which presents for the last phase, a similarity with wustite FeO but also with hematite α-Fe2O3. Crystallization increases with time and the solid phase obtained is dependent on temperature and pH. Obviously, high temperature favours the formation of the oxide hematite α-Fe2O3. As for the pH factor, it is more complicated. Low and high values of pH (2–5 and 10–14) favour the formation of goethite α-FeOOH, while obtaining hematite is favoured at neutral pH (values around 7). Goethite or hematite are obtained either through a dissolution–crystallization process or in the solid state, through a topotactic transformation. Given the structural relationships observed between ferrihydrite and wustite and hematite, it is allowed to think that a structural continuity could exist between wustite Fe(1−x)O and hematite via ferrihydrite.