Stability Fields and Phase Transition Pathways of Ferric Sulfates in 50°c to 5°c Temperature Range

Introduction: One of the mysteries of Fe-sulfate observations on Mars is the discrepancy between the findings of jarosite and a variety of ferric sulfates at both Mars exploration rover sites (Gusev Crater and Meridiani Planum) [1,2] and the rarity of findings of Fesulfates by orbital remote sensing . By orbital remote sensing, Mgand Ca-sulfates have been identified with wide vertical and horizontal spread in Valles Marineris, Meridiani Planum, and in north polar regions . In contrast, a few occurrences of Fe-sulfates were reported by the CRISM team. The overall quantity of Fe-sulfates at the potential occurrences is not comparable with those of Mgand Ca-sulfates. Another mystery in Fe-sulfate observations is the temporal spectral changes of Fesulfate-enriched salty soils after they were excavated from the subsurface and exposed to current atmospheric conditions at the surface by Spirit. Laboratory experiments suggested that dehydration is the best candidate to explain the spectral changes, i.e., a phase transition of ferric sulfates has occurred indicating that it was originally NOT in equilibrium with atmospheric condition at the martian surface. There have been numerous studies of the phase relationships of Mg-sulfates of various hydration states and on the stability fields and phase transitions of these minerals. However, to our knowledge, phase boundaries for ferrous sulfates have only been studied for one pair and there have been no such studies for ferric sulfates. The lack of information on fundamental thermodynamic properties of ferric sulfates has made it difficult to interpret mission observations and thus to link the occurrence of these secondary minerals with paleoclimatic conditions on Mars. Study of the fundamental thermodynamic properties of ferric sulfates: We have been conducting a systematic laboratory experimental study on the fundamental thermodynamic and kinetic properties of hydrated ferric sulfates. This study has gone through three steps. We firstly synthesized eight ferric sulfates with different degrees of hydration, acidity, and crystallinity (ferricopiapite, paracoquimbite, kornelite, lausenite, pentahydrate, rhomboclase, mikasaite, and amorphous Fe-sulfates). Their identities were confirmed using XRD, and then spectroscopic characterization was done using Raman, MIR, NIR, and UV-VIS-NIR spectroscopy . Because of the importance of copiapite among common ferric sulfates, we then synthesized its four chemical endmembers (ferricopiapite, copiapite, magnesiocopiapite, and aluminocopiapite) and characterized them spectroscopically . We then used five of the synthesized ferric sulfates to set up 150 experiments at three temperatures and ten relative humidity levels to study their stability fields and phase transition pathways. We report here the results of this study after over two years’ reaction time. These results provide the basic information for the third step of our investigation in which an experimental study of the phase boundary between a pair of ferric sulfates (Fe2(SO4)3·5H2O and Fe2(SO4)3·7H2O) was designed and conducted. The results are reported in a separate abstract. 150 Experiments: Because the state of Fe-sulfates is a function of fH2O, pH, and fO2, the sulfates with Fe and mixed Fe/Fe were avoided in our experiments for the purpose of removing at least one variable. We selected five of the synthetic ferric sulfates on the basis of these considerations: (1) they are the most common species precipitated from aqueous Fe-S-O solutions in a wide temperature and pH range; (2) they are common species observed in terrestrial settings; (3) they can occur in an extremely arid environment like amorphous Mg-sulfates; and (4) they are easy to make in the laboratory. Ten relative humidity (RH) buffers (saturated aqueous solution with excess salts: LiBr, LiCl, MgCl2, Mg(NO3)2, NaBr, KI, NaCl, KCl, KNO3, and H2O) provide a RH range of 6% to 100%. Three temperature zones at 50±1°C, 21±2°C, and 5±1°C were maintained in an oven, at ambient conditions on a lab bench, and under refrigeration. Dry powders of the starting ferric sulfate phases were placed in a reaction vial, which was uncapped and was placed inside of a glass bottle filled with RH buffers. The tightly sealed 150 sample-buffer vial pairs were put into designated temperature zones for long period to equilibrate.