Chemical weathering on Mars thermodynamic stabilities of primary minerals (and their alteration products) from mafic igneous rocks |
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| Authors: | James L Gooding |
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| Institution: | Department of Geology and Institute of Meteoritics, University of New Mexico, Albuquerque, New Mexico 87131, USA |
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| Abstract: | Chemical weathering on Mars is examined theoretically from the standpoint of heterogeneous equilibrium between solid mineral phases and gaseous O2, H2O, and CO2 in the Martian atmosphere. Thermochemical calculations are performed in order to identify important gas-solid decomposition reactions involving the major mineral constituents of mafic igneous rocks. Where unavailable in the thermochemical literature, Gibbs free energy and enthalpy of formation are estimated for certain minerals and details of these estimation procedures are given. Partial pressure stability diagrams are presented to show pertinent mineral reaction boundaries at 298 and at 240°K. In the present Martian environment, the thermodynamically stable products of gas-solid weathering of individual minerals at 240°K should be Fe2O3, as hematite or maghemite (from fayalite, magnetite, and Fe-bearing pyroxenes), quartz (from all silicates), calcite (from Ca-bearing pyroxenes and plagioclase), magnesite (from forsterite and Mg-bearing pyroxenes), corundum (from all Al-bearing silicates), Ca-beidellite (from anorthite), and szomolnokite, FeSO4 or FeSO4·H2O (from iron sulfides). Albite, microcline, and apatite should be stable with respect to gas-solid decomposition, suggesting that gas-solid weathering products on Mars may be depleted in Na, K, and P (and, possibly, Cl and F). Certain montmorillonite-type clay minerals are thermodynamically favorable intermediate gas-solid decomposition products of Al-bearing pyroxenes and may be metastable intermediate products of special mineral surface reaction mechanisms. However, the predicted high thermodynamic susceptibility of these clay minerals to subsequent gas-solid decomposition implies that they should ultimately decompose in the present Martian surface environment. Kaolinite is apparently the only clay mineral which should be thermodynamically stable over all ranges of temperature and water vapor abundance in the present environment at the Martian surface. Considering thermodynamic criteria, including possible gas-solid decomposition reactions, it is doubtful that significant amounts of goethite and clay minerals can be currently forming on Mars by mechanisms known to operate to Earth. If major amounts of goethite and clay minerals occur on Mars, they probably owe their existence to formation in an environment characterized by the presence of liquid water or by mechanism possibly unique to Mars. In any case, any goethite or montmorillonite-type clay mineral on Mars must ultimately decompose. |
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