Transformation of Ferrihydrite to Crystalline Fe Oxides in Salt Solutions: Implications for the Scarcity of Goethite on Mars

Abstract

Data from Mars exploration rovers and satellite observations suggest that iron oxides were formed during aqueous activity on early Mars. Although hematite (Hm) and ferrihydrite are common on the Mars surface, goethite (Gt, a common iron oxide on Earth’s surface) has been recognized there at very few locations. To gain insight into these differences, we undertook a quantitative investigation of the transformation products of ferrihydrite under controlled values of pH (4, 6, and 8) and ionic strength (I < 0.005 and I = 0.01, 0.1, and 1 mol/kg) in NaCl and CaCl₂ solutions at 70 °C. Hematite formation was dominant at neutral pH (6 and 8) in both NaCl and CaCl₂ solutions. The hematite ratio [Hm/(Hm + Gt)] was independent of I in NaCl solutions but increased with increasing I in CaCl₂ solutions. Under acidic pH (3 and 4) conditions, goethite was predominant at lower ionic strengths, which is consistent with previous studies. On the other hand, the hematite ratio increased systematically with increasing I and became predominant at higher ionic strengths (I > 0.1 mol/kg) in both NaCl and CaCl₂ solutions. These results suggest that highly saline aqueous solutions inhibit goethite formation in Cl-rich conditions regardless of pH, which can explain the absence of goethite in the Karasburg member, Murray formation in Gale crater, where the water chemistry of the ancient pore water during temporal wet events at about 3–2 Ga was characterized to acidic (pH 3–5) and saline Na–Cl type at low temperatures (<70 °C).