Abstract
Coexistence of magnetite and hematite in hydrothermal systems has often been used to constrain the redox potential of fluids, assuming that the redox equilibrium is attained among all minerals and aqueous species. However, as temperature decreases, disequilibrium mineral assemblages may occur due to the slow kinetics of reaction involving the minerals and fluids. In this study, we conducted a series of experiments in which hematite or magnetite was reacted with an acidic solution under H2-rich hydrothermal conditions (T=100-250°C, PH2=0.05-5MPa) to investigate the kinetics of redox and non-redox transformations between hematite and magnetite, and the mechanisms of iron oxide transformation under hydrothermal conditions. The formation of euhedral crystals of hematite in 150 and 200°C experiments, in which magnetite was used as the starting material, indicates that non-redox transformation of magnetite to hematite occurred within 24h. The chemical composition of the experimental solutions was controlled by the non-redox transformation between magnetite and hematite throughout the experiments. While solution compositions were controlled by the non-redox transformation in the first 3days in a 250°C experiment, reductive dissolution of magnetite became important after 5days and affected the solution chemistry. At 100°C, the presence of maghemite was indicated in the first 7days. Based on these results, equilibrium constants of non-redox transformation between magnetite and hematite and those of non-redox transformation between magnetite and maghemite were calculated. Our results suggest that the redox transformation of hematite to magnetite occurs in the following steps: (1) reductive dissolution of hematite to Fe(aq)2+ and (2) non-redox transformation of hematite and Fe(aq)2+ to magnetite.
Original language | English |
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Pages (from-to) | 6141-6156 |
Number of pages | 16 |
Journal | Geochimica et Cosmochimica Acta |
Volume | 74 |
Issue number | 21 |
DOIs | |
State | Published - Nov 2010 |
Funding
The authors acknowledge H. Barnes, S. Brantley, P. Heaney, T. Lasaga, K. Osseo-Asare, I. Johnson, and D. Bevacqua for valuable comments on the early manuscript. The authors also acknowledge J. Rosenqvist, M. Angelone, and J. Cantolina for technical assistance. Comments by K.M. Rosso and two anonymous reviewers are greatly appreciated. This project was supported by grants from NASA Astrobiology Institute ( NCC2-1057 ; NNA04CC06A ) and NSF ( EAR-0229556 ) to H.O. D.J.W.’s effort and a portion of the effort of T.O. were supported by the U.S. Department of Energy, Office of Basic Energy, Geoscience Research Program, at Oak Ridge National Laboratory, managed by UT-Battelle, LLC, for the U.S. Department of Energy (DE-AC05-00OR22725).