Effects of H₂O, pH, and oxidation state on the stability of Fe minerals on Mars

On Mars, there is evidence that solutions may have been present on the surface episodically in the past. These solutions were derived by weathering minerals such as olivine and sulfides found in mafic-ultramafic rocks, and possibly sulfates and meteorite fragments. Upon removal of water and/or supersaturation of solutions, secondary minerals formed. We present a theoretical model for the formation of the solutions and the subsequent precipitation of Fe-bearing phases. The first Fe-bearing phases to weather via oxidation are sulfides (troilite or pyrrhotite) which produce secondary Fe (hydr)oxides or FeS2 (pyrite or marcasite) and Fe²⁺ sulfates such as melanterite. Melanterite may be replaced by Fe sulfates that have decreasing H₂O/(3Fe³⁺ + 2Fe²⁺), increasing oxidation, and increasing bulk OH/(OH + 2SO₄). At Meridiani Planum the presence of jarosite indicates that the solutions were oxidized with pH < 4.5. The solutions were likely Fe-Mg-(Ca)-SO₄-(Cl)-rich and precipitated Fe (hydr)oxides, Fe phosphates, Fe sulfates with low OH/(OH + SO₄), Ca-Mg sulfates, and possible halides, along with Si-rich phases. The minerals indicating an acid environment were preserved by either removing components such as salts or water from the system (e.g., in dust storms or episodic aqueous events); isolating the mineral surfaces from fluids; using disequilibrium processes; or a combination. At other localities, jarosite is below detection limits, and schwertmannite may be present. If schwertmannite is present, the solution had nearneutral pH (∼4 < pH < 10) and Fe (hydr)oxides would have crystallized rapidly, leaving Mg-Na-(Ca)-SO₄-Cl-rich solutions that likely precipitated Ca phosphates, Ca-Mg-Na sulfates, Fe sulfates with moderate-high OH/(OH + SO₄), halides, Si-rich phases, and possibly Fe-Mg-Ca carbonates.