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

• Main Authors: Tanya S. Peretyazhko (1732987), Douglas W. Ming (6298832), Richard V. Morris (6298838), David G. Agresti (6298841), Wayne P. Buckley (11166630)
• Format: Other Non-Article Part of Journal/Newspaper
• Language: unknown
• Published: 2021
• Subjects: Biophysics; Biochemistry; Cell Biology; Molecular Biology; Physiology; Ecology; Marine Biology; Cancer; Environmental Sciences not elsewhere classified; Chemical Sciences not elsewhere classified; Physical Sciences not elsewhere classified; oxidative alteration; oxidative alteration experiments; pyrrhotite; 4. X-ray diffraction; Akaganeite; sulfide; pH 0 2; goethite; oxidation; akageneite formation; Gale Crater; Fe; acidic Cl-bearing groundwater; Vera Rubin ridge; pH 0 1.5; Rock Hall site; pH 0 4. Well-crystallized akageneite; Yellowknife; Yellowknife Bay; Rubin
• Online Access: https://doi.org/10.1021/acsearthspacechem.1c00075.s001

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 pH 0 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 pH 0 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 pH 0 3, and formation was suppressed at pH 0 4. Well-crystallized akageneite precipitated at pH 0 1.5, while akaganeite of poorer crystallinity formed at pH 0 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.