Characterizing Ferrihydrite Transformation Products in Near-Saturated Brine Environments: Implications for Fe-Oxide Formation on Mars

Ferrihydrite has been observed within the Martian regolith; therefore, ferrihydrite transformation pathways are likely critical to iron cycling and mineral transformation on Mars and other extraterrestrial systems. Data from Mars rovers and orbiters indicate that ferrihydrite is associated with significant salt deposits. Previous studies show these salts likely formed as the planet desiccated and may rehydrate to form modern brines today that strongly influence(d) mineral alteration. We hypothesize that the salts observed on Mars' surface may help preserve ferrihydrite for longer periods than typically observed on Earth. This study investigates the effects of brine chemistry on ferrihydrite alteration through laboratory experiments. Lab-synthesized ferrihydrite was reacted with near-saturated brines and ultra-pure water at 20°C for 30 days in a series of batch reactor experiments. X-ray diffraction and Raman spectroscopy showed that ferrihydrite was preserved without evidence of dissolution/transformation in near-saturated solutions of MgSO₄, Na₂SO₄, and NaClO_4, while additional iron-oxyhydroxide phases formed in other brines. We also compared mineral reaction products formed from freeze-dried ferrihydrite and undried ferrihydrite slurry. The freeze-dried ferrihydrite was more likely to be preserved, whereas ferrihydrite in a slurry resulted in the precipitation of goethite and lepidocrocite, indicating that particle aggregation and/or drying history affect ferrihydrite stability and alteration. Overall, ferrihydrite remained largely unaltered in the presence of concentrated sulfate and perchlorate brines. In the context of soils/regolith observed on Mars, our research demonstrates that ferrihydrite is more likely to be preserved when found in areas where these salts are dominant, and desiccated in a cold/arid environment prior to brine exposure.