Chemical weathering on Mars: Thermodynamic stabilities of primary minerals (and their alteration products) from mafic rocks

Chemical weathering on Mars is examined theoretically from the standpoint ofheterogeneous equilibrium between solid mineral phases and gaseous 02, H20, and CO2 in the Martian atmo-sphere. 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 forma-tion are estimated for certain minerals and details of these estimation procedures are given. Partial pressure stability diagrams are presented to show pertinent mineral reaction bound-aries at 298 and at 240°K. In the present Martian environment, he thermodynamically stable products of gas-solid weathering of individual minerals at 240°K should be Fe2Os, as henmtite 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 szomolnokitc, FeSO ~ or FeSO4-H20 (from iron sulfides). Albite, microcline, and apatite should be stable with respect to gas-solid ecomposition, suggesting that gas~solid weathering products on Mars may be depleted in Na, K, and P (and, possibly, C1 and F)