Reaction of Akaganeite with Mars-Relevant Anions

Akaganeite is an Fe­(III) (hydr)­oxide with a tunnel structure typically occupied by chloride. The mineral can undergo anion-exchange reactions in aqueous solution, resulting in incorporation of other anions together with Cl– into the tunnels. Identification of anions present in akaganeite tunnels may permit characterization of solution compositions in which akaganeite precipitated and/or existed. Akaganeite has been reported in several locations on Mars, including Yellowknife Bay in Gale crater. However, the nature of the tunnel anions has not been investigated. In order to constrain the nature of the tunnel anions in martian akaganeite, synthetic akaganeite (72 mg/g total Cl– content) was reacted with Mars-relevant anions (F–, OH–, and SO42–). Release of Cl– into solution was monitored with ion chromatography. Anion-reacted akaganeite was characterized with instruments analogous to instruments onboard robotic space crafts including X-ray diffraction (XRD), Mössbauer spectroscopy, thermal and evolved gas analysis (TA/EGA), and visible and near-infrared reflectance spectroscopy (vis–NIR). The results demonstrated that 17–71% of Cl– was released from akaganeite during 96 h incubation in water and anion-bearing solutions. A combination of XRD and TA/EGA, similar to the instruments onboard the Mars Science Laboratory rover Curiosity, can distinguish between unreacted akaganeite and akaganeite reacted with SO42–, F–, and OH– based on diffraction peak positions and evolved HCl, HF, and SO2. The vis–NIR analogous to the instrument on the Mars Reconnaissance Orbiter is only sensitive to Cl– replacement by OH– but not by SO42– and F– as evident from the changes in the shape and position of OH combination band. Mössbauer measurements at room temperature with instruments similar to those onboard the Mars Exploration Rovers did not distinguish among different tunnel anion compositions. The laboratory data are collectively consistent that akaganeite detected in Yellowknife Bay contains only Cl– in tunnels and likely formed and/or was in contact with Cl-bearing solutions during late stage diagenesis or aqueous alteration