Characterizing Ferrihydrite Transformation Products in Near-Saturated Brine Environments

Abstract

Ferrihydrite transformation pathways are critical in understanding iron cycling and mineral transformation processes in extraterrestrial systems such as those observed on Mars. While common throughout terrestrial near-surface environments, ferrihydrite is highly susceptible to transformation to more stable iron oxides like goethite and hematite via aqueous alteration. Data from Mars rover missions show ferrihydrite associated with significant salt deposits on Mars’ surface. Previous studies have shown that salt compounds are ubiquitous on Mars’ surface and likely formed from ancient and modern brines that strongly influence(d) mineral alteration. This study investigates the effects of brine chemistry on synthesized ferrihydrite alteration. Batch reaction experiments were conducted for 30 days with near saturated brines (NaCl, CaCl2, MgCl2, NaNO3, Na2CO3, Na2SO4, NaClO3, NaClO4, and MgSO4) as well as ultra-pure water (UPW) as a control. We also investigated the effects of varying brine concentrations by using 1:2 and 1:10 diluted mixtures of the near-saturated brines. X-ray diffraction (XRD) and Raman spectroscopy showed that ferrihydrite was preserved in near-saturated solutions of MgSO4, Na2SO4, and NaClO4 and did not show evidence of dissolution/transformation, while ferrihydrite partially transformed into other iron oxide phase products in the remaining brines. We also compared the mineral reaction products between freeze-dried ferrihydrite and undried ferrihydrite slurry through a series of replicate experiments with NaCl, NaNO3, NaClO4, MgSO4, and MgCl2. The freeze-dried ferrihydrite produced significant differences in reaction products in XRD samples. Samples from the slurry batch showed greater abundance of goethite and hematite compared to the dried initial and replicate batches, indicating that particle size and/or drying history also has a significant effect on ferrihydrite stability and dissolution/transformation. Solubility of iron oxide products was low in all the samples, despite varying pH conditions. Determining the iron oxide phases present in the samples required combining XRD and Raman analyses. Overall, ferrihydrite remained unaltered in higher sulfate and perchlorate concentrations. In the context of soils and regolith observed on Mars, ferrihydrite may be more likely to resist transformation to hematite and goethite when found in areas where these salts are dominant. Ferrihydrite preservation may also be more likely when the ferrihydrite has been desiccated in a cold, arid environment prior to exposure to brines.