![]() The formation of Martian salts and iron oxides from mafic silicates implies a release of SiO 2 into aqueous solution followed by precipitation of silica and/or silica-rich minerals. ![]() The presence of iron oxyhydroxides, specific Fe and Mg sulfates, and Mg phyllosilicates in Martian surface materials is consistent with aqueous alteration of primary mafic and ultramafic rocks. Temperatures above ∼0☌ are not required to form abundant silica through acid alteration of Martian rocks. ![]() More soluble minerals (e.g., ferric oxides, phyllosilicates) could precipitate downstream from partially neutralized, evaporated, or frozen solutions. Modeling shows that Ti oxides are also present in silica-rich deposits. Partial evaporation or freezing of released solutions would also cause precipitation of amorphous silica. Low pH favors dissolution of silicates and saturation of solution with respect to relatively insoluble silica, which then precipitates. High-water/rock conditions could represent acid flow through rocks, solution discharge from an acid spring, and/or surface flow of released solutions. Silica-dominated deposits, like those found at the Columbia Hills in Gusev crater on Mars, could form at solution pH below ∼2 and water/rock ratios of ∼10 2–10 4. The models show that some silica can form during low-temperature alteration of mafic to ultramafic rocks over a large range of pH and water/rock ratios. Theoretical geochemical modeling has been used to evaluate the formation conditions of amorphous silica during aqueous alteration of typical Martian igneous rocks at 0☌.
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