Transformation of Ferrihydrite in Sulfate Solutions Containing Dissolved Fe(II): Implications for the Rare Presence of Goethite on Mars

Abstract

Goethite, a common iron oxide formed by the aqueous reaction, is rarely found on the Martian surface. The ubiquity of minerals containing Fe²⁺ and sulfates on Mars suggests that dissolved Fe(II) and SO₄^2- may have been present in ancient Martian water. This type of water is commonly found in acid mine drainages on Earth, where goethite is the dominant iron oxide. Crystalline iron oxides are formed by the transformation of ferrihydrite, which is controlled by water chemistry conditions such as pH and salinity. Additionally, recent reports indicate that the transformation of ferrihydrite is also influenced by dissolved Fe(II). In this study, we investigated the alteration behavior of ferrihydrite in the Fe(II)-SO₄ system under various water chemistry conditions. We also attempted to explain the rarity of goethite on Mars.