Experimental evidence for non-redox transformations between magnetite and hematite under H2-rich hydrothermal conditions

Transformations of magnetite (FeIIFe2IIIO4) to hematite (Fe2IIIO3) (and vice versa) have been thought by many scientists and engineers to require molecular O2 and/or H2. Thus, the presence of magnetite and/or hematite in rocks has been linked to a specific oxidation environment. However, the availability of reductants or oxidants in many geologic and industrial environments appears to have been too low to account for the transformations of iron oxides through redox reactions. Here, we report the results of hydrothermal experiments in mildly acidic and H2-rich aqueous solutions at 150 °C, which demonstrate that transformations of magnetite to hematite, and hematite to magnetite, occur rapidly without involving molecular O2 or H2:Fe3O4(Mt) + 2H(aq)+ ↔ Fe2O3(Hm) + Fe(aq)2+ + H2OThe transformation products are chemically and structurally homogeneous, and typically occur as euhedral single crystals much larger than the precursor minerals. This suggests that, in addition to the expected release of aqueous ferrous species to solution, the transformations involve release of aqueous ferric species from the precursor oxides to the solution, which reprecipitate without being reduced by H2. These redox-independent transformations may have been responsible for the formation of some iron oxides in natural systems, such as high-grade hematite ores that developed from Banded Iron Formations (BIFs), hematite-rich deposits formed on Mars, corrosion products in power plants and other industrial systems.