Effect of Solution pH and Chloride Concentration on Akaganeite Precipitation: Implications for Akaganeite Formation on Mars

Akaganeite, a chloride-bearing Fe(III) (hydr)oxide, has been reported on Mars in Yellowknife Bay in Gale crater by X-ray diffraction (XRD), in Robert Sharp crater by Compact Reconnaissance Imaging Spectrometer for Mars, and possibly at the Gusev crater and Meridiani Planum by combined Mossbauer and bulk chemistry analyses. Because formation conditions remain unknown, we investigated akaganeite precipitation as a function of solution pH and chloride concentration. Akaganeite was synthesized through hydrolysis of Fe(III) perchlorate in the presence of 0.02, 0.05, and 0.1 M chloride and at initial solution pH of 1.6, 4, 6, and 8 at 90 degrees C. Mineralogy of the precipitated Fe(III) (hydr)oxides was characterized by XRD, infrared spectroscopy, and Mossbauer spectroscopy. Total chloride and perchlorate contents were determined by ion chromatography. XRD revealed that akaganeite formed alone or in mixtures with ferrihydrite, hematite, and/or goethite at initial pH 1.6 with 0.02, 0.05, and 0.1 M Cl- and at initial pH from 4 to 8 with 0.05 and 0.1 M CI . Infrared analysis showed that akaganeite bands at similar to 2 and similar to 2.45 pm were sensitive to amount of akaganeite, total chloride content, and presence of other Fe(III) (hydr)oxides. The results indicate that akaganeite in Yellowknife Bay in Gale crater likely formed under acidic to alkaline (1.6 < pH < 8) oxidizing conditions in solutions containing >0.05 M Cl- by the dissolution of basaltic minerals. Akaganeite in Robert Sharp crater may have formed in acidic (pH < 4) solutions with similar to 0.1 M Cl- through oxidative dissolution of Fe(II) sulfides and/or acidic oxidizing dissolution of basalts. Plain Language Summary Akaganeite, a chloride-bearing Fe(III) (hydr)oxide, has been reported in several locations (e.g., Gale and Robert Sharp craters) on Mars, but formation conditions are unknown. The objective of this work was to investigate formation of akaganeite at variable pH and dissolved chloride concentrations and to characterize the past aqueous conditions in the locations where akaganeite was detected on Mars. We found that akaganeite on Mars could form under substantially different environmental conditions. Akaganeite in Yellowknife Bay likely formed under acidic to alkaline moderately saline conditions by the dissolution of basaltic minerals. Akaganeite in Robert Sharp crater may have formed in acidic saline solutions through oxidative dissolution of Fe(II) sulfides and/or acidic oxidizing dissolution of basalts.