EXPERIMENTAL EVIDENCE OF NON-REDOX TRANSFORMATION BETWEEN MAGNETITE AND HEMATITE UNDER H2-RICH HYDROTHERMAL CONDITIONS

Magnetite (Fe₃O₄) and hematite (Fe₂O₃) are used to constrain the redox conditions of fluid (or melt) from their presence or absence in rocks. Magnetite-rich Banded Iron Formations (BIFs) and secondary hematite-rich iron ore formations have been linked to the oxygenation of the ocean and atmosphere, assuming redox transformation of magnetite – hematite. However, magnetite – hematite transformation may proceed though a non-redox (i.e., acid-base) reaction(1): Fe₃O₄ + 2H⁺ ↔ Fe₂O₃ + Fe²⁺ + H₂O. Magnetite is transformed to hematite as a result of leaching Fe²⁺ from magnetite while hematite is transformed to magnetite as a result of incorporating Fe²⁺ into hematite. We have examined the reactions (forward and reverse) using a hydrothermal cell with hydrogen electrodes for pH measurements and a hydrogen-permeable membrane for pH₂ measurements The experiments were conducted at 100 - 250°C under highly reducing conditions (pH₂ = 0.5 – 50 bar) and mildly acid conditions (pH = 4 – 6). After the system reached a steady state, iron concentration in the withdrawn sample solutions was measured. The residual solid was analyzed using XRD, SEM and HRTEM.

Our experiments have demonstrated that euhedral crystals of hexagonal dipyramidal hematite grow rapidly by reaction between magnetite and acid solutions at T ≤ 200 ºC, suggesting dissolution and reprecipitation of Fe³⁺ under highly reducing conditions. Chemical composition of the experimental solution at the given temperatures are independent of the H₂ pressure and remain constant over weeks, controlled by reaction (1). Reaction (1) is reversible, although the reverse reaction is more sluggish than the forward reaction. However, at T = 250ºC, reductive dissolution of magnetite (2) (i.e., redox reaction) controls the chemical compositions of the solution after 4 days: 1/3Fe₃O₄ + 2H⁺ + 1/3H₂ → Fe²⁺ + 4/3H₂O. No hematite was found in the residual solid from experiments at the temperature. The results of our studies suggest magnetite and hematite act as an acid-base buffer, rather than a redox buffer in low temperature environments. Because magnetite – hematite transformation does not require a redox reaction, secondary hematite-rich iron ore developed from BIFs may have formed by subsurface reaction between magnetite and acidic hydrothermal solutions at T ≤ 200 ºC.