^1,2Schmitz, Birger,¹Terfelt, Fredrik
Estonian Journal of Earth Sciences 72, 94-97 Open Access Link to Article [DOI 10.3176/EARTH.2023.49]
¹Astrogeobiology Laboratory, Department of Physics, Lund University, Sweden
²Robert A. Pritzker Center for Meteoritics, Polar Studies, Field Museum of Natural History, Chicago, United States

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¹Ke Wen,²Peng Yang,^1,3Mengqiang Zhu
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2023.08.028]
¹Department of Ecosystem Science and Management, University of Wyoming, Laramie, Wyoming 82071, United States
²Chemical Sciences and Engineering Division, Argonne National Laboratory, Lemont, Illinois 60439, United States
³Department of Geology, University of Maryland, College Park, Maryland 20742, United States
Copyright Elsevier

The occurrence of manganese (Mn) oxides on Mars is believed to be an indicator of an O2-rich paleoenvironment of Mars because Mn oxides often form through the oxidation of Mn(II) by O2 on the surface of Earth. An alternative formation pathway was recently proposed, in which Mn(II) is oxidized by bromate (BrO3-), a common oxidant in contemporary Martian regolith. However, the oxidation of Mn(II) by bromate in solution is kinetically controlled and slow unless using very high concentrations (100 mM) of reactants that may be irrelevant to the conditions of Mars. We conducted laboratory simulations to determine whether iron (Fe) oxides (hematite and goethite) and a phyllosilicate (montmorillonite), abundant minerals on the surface of Mars, could catalyze the oxidation of Mn(II) by bromate. Hematite and goethite, but not montmorillonite, dramatically accelerated the oxidation with a low concentration (1 mM) of Mn(II) and bromate under various solution conditions. The reaction system was autocatalytic with Fe oxides initiating the oxidation of Mn(II) at the early stage and the subsequent catalysis mainly provided by the Mn oxide products. In contrast to producing Mn(IV)O2 only during the homogeneous oxidation of Mn(II) by bromate in solutions, the heterogeneous mineral-surface catalyzed oxidation resulted in a mixture of Mn(III)OOH and Mn(IV)O2 phases. Mn(III)OOH was an intermediate product and can be further oxidized by bromate to Mn(IV)O2. The occurrence and accumulation of the intermediate product MnOOH can be attributed to its rapid formation due to surface-enhanced nucleation and growth on Fe oxide surfaces and to its higher resistance to oxidation by bromate than Mn(III) ions or clusters. Overall, mineral-surface catalyzed oxidation of Mn(II) by bromate is favorable from both thermodynamic and kinetic perspectives, and can be a major pathway for the occurrence of Mn oxides on Mars where microorganisms are lacking to catalyze the reaction. Our study further improves our understanding of the thermodynamic and kinetic controls on Mn(II) oxidation.