Dehydration of Na-jarosite, ferricopiapite, and rhomboclase at temperatures of 50 and 95 °c: Implications for Martian ferric sulfates

Ferric sulfates with various hydration degrees were found on Mars by surface exploration and orbital remote sensing. The processes that they experienced during their formation and alteration relate to Martian hydrologic history and thus have been important topics for laboratory investigations. This study reports three sets of dehydration experiments conducted on Na-jarosite, ferricopiapite, and rhomboclase under two relative humidity conditions (5-6% RH and 10-11% RH) at two temperatures (50 and 95 °C), for which laser Raman spectroscopy and gravimetric measurements were the only two techniques used, in situ and non-invasively, to monitor the progress of these experiments. Our dehydration experiments show that OH-bearing Na-jarosite is relatively stable at 95 °C and RH < 11%, while H₂O/OH-bearing ferricopiapite and rhomboclase have converted almost entirely into anhydrous phases. These results have important applications on uncovering the mystery of rarely detected Martian ferric sulfates and on interpreting Mars remote sensing data.