Formation of akaganeite in Gale crater, Mars

Akaganeite (β-FeO(OH,Cl) a chloride-containing Fe(III) (hydr)oxide) has been detected in several locations in Gale Crater by the Chemistry and Mineralogy (CheMin) X-ray diffraction instrument onboard the Mars Science Laboratory Curiosity rover. Akaganeite was first identified in the drilled Cumberland and John Klein mudstone samples at Yellowknife Bay. Recenly akaganeite has been detected in higher abundance in the Rock Hall sample drilled on Vera Rubin Ridge. Formation mechanisms of akaganeite in Gale crater are still unclear. Comparison of martian observations with experimental data on akaganeite formation at different values of pH and dissolved Cl^- concentrations reveals that akaganeite in Gale crater could form from Fe(II)-bearing minerals under moderately saline, acidic to alkaline conditions. Experimental studies were designed to verify this formation mechanism. We have investigated formation of akaganeite from the natural Fe(II) sulfide mineral pyrrhotite. Pyrrhotite was incubated in HCl solution (pH ~1.3) under ambient conditions. At the end of the reaction, the liquid containing dissolved Fe(II) was separated from the solids and Fe(II) was oxidized to Fe(III) by H₂O₂. The pH was then increased to pH 2 and 4 and solution was incubated at 80 °C overnight. X-ray diffraction analysis of the precipitates revealed formation of akaganeite under acidic pH 2 conditions while ferrihydrite formed at moderately acidic pH 4. The results indicate that formation of akaganeite from Fe(II) sulfides is only plausible in acidic environments (pH < 4). Experiments will be also performed to investigate formation of akaganeite from Mars-analogue basalts.