Formation of Fe(III) (Hydr)oxides from Fe(II) Sulfides: Implications for Akaganeite Detection on Mars

Abstract

Akaganeite on Mars could form from Fe(II) sulfides, but formation conditions remain unconstrained. We investigated akageneite formation by oxidative alteration of natural pyrrhotites exposed to HCl and oxidation–hydrolysis of Fe(II) HCl-leached from pyrrhotites at initial pH0 1.5, 2, 3, and 4. X-ray diffraction and Mössbauer analyses revealed the formation of poorly crystallized akageneite in oxidative alteration experiments. Air exposure of the HCl-reacted dry pyrrhotites led to an increase in akageneite formation and precipitation of Fe(II) hydrated sulfates, goethite, and hydronium jarosite. Iron(II) oxidation–hydrolysis was sensitive to Si dissolved from phyllosilicates in one pyrrhotite sample. Akaganeite and goethite formed at pH0 1.5 and 2 with akageneite more abundant at a dissolved Si/Fe ratio of 0.08 and goethite more abundant at a Si/Fe of 0.01. Akaganeite formed together with hematite, ferrihydrite, and goethite at pH0 3, and formation was suppressed at pH0 4. Well-crystallized akageneite precipitated at pH0 1.5, while akaganeite of poorer crystallinity formed at pH0 2 and 3. Akageneite on Mars could form from sulfides by both mechanisms during late diagenetic events triggered by interactions of acidic Cl-bearing groundwater with Fe(II) sulfides. Akaganeite in Yellowknife Bay, Gale Crater, could have formed by Fe(II) oxidation–hydrolysis either as a sole Fe(III) (hydr)oxide at pH < 2 or along with ferrihydrite and hematite at 2 < pH < 4 under Si-enriched conditions. Akaganeite formation at the Vera Rubin ridge, Gale Crater, could have occurred through oxidative alteration of sulfides in Cl-bearing pH 1.2–1.5 solutions. The presence of well-crystallized akageneite in the Rock Hall site at the Vera Rubin ridge indicates that Fe(II) oxidation–hydrolysis contributed to akageneite formation.