Phase Boundaries Among Three Hydrous Ferric Sulfates and Their Implications for Mars

Ferric sulfate minerals have been identified by orbital and landed missions at multiple locations on Mars and are the most common minerals in the Acid Mine Drainage (AMD) system on Earth. The occurrences and the speciation of ferric sulfates are very sensitive to variations in environmental conditions, such as temperature (T), relative humidity (RH%), redox potential (Eh), and potential of hydrogen ions (pH). In this study, two phase boundaries among kornelite, paracoquimbite, and ferricopiapite were experimentally derived in T–RH% space, using the well-established humidity buffer technique. The phase transformation and phase identification during experiments were determined by the gravimetric measurements and laser Raman spectroscopy, respectively. The two new phase boundaries clearly defined the edges of the stable fields of paracoquimbite that were ambiguously determined in a previous study. From the experimental data, we derived the entropy, enthalpy, and Gibbs free energy of the two reactions, and calculated the enthalpy changes and Gibbs free energy changes for each water of crystallization (either enter or escape from the structure) of these hydrous ferric sulfates. When compared with the same parameters of hydrous metal (Fe³⁺, Fe²⁺, Cu²⁺, Mg²⁺, Ni²⁺, Zn²⁺, Co²⁺, Mn²⁺, Cd²⁺, and Na⁺) sulfates derived by previous hydration/dehydration studies, we found a strong consistency, especially the Gibbs free energy changes. This finding implies the very consistent energetic barriers for the hydration/dehydration of those sulfates, post their first hydration/dehydration, regardless of their difference in cation and crystal structure.