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.