Stability and Phase Transition Pathways of Oh-bearing Ferric Sulfates under the Conditions Relevant to Diurnal, Seasonal, and Obliquity Cycles on Mars

2634 for 44 Lunar and Planetary Science Conference (Houston, TX, March 18-22, 2013) STABILITY AND PHASE TRANSITION PATHWAYS OF OH-BEARING FERRIC SULFATES UNDER THE CONDITIONS RELEVANT TO DIURNAL, SEASONAL, AND OBLIQUITY CYCLES ON MARS. Yanli Lu and Alian Wang, Dept. of Earth and Planetary Sciences and the McDonnell Center for the Space Sciences, Washington University in St. Louis, One Brookings Drive, St. Louis, MO, 63130, USA ([email protected], [email protected]). Different forms of ferric sulfates at surface & subsurface on Mars: Among the Fe-sulfates identified on Mars, those being found at the surface are mostly OH-bearing, e.g., jarosite [(K, Na, H3O)Fe3(SO4)2(OH)6] in Meridiani outcrop [1] and ferric hydroxysulfate [FeOHSO4] in exposed stratigraphic layers at Aram Crater [2]. In contrast, ferric sulfates of other forms were suggested existing in excavated subsurface regolith (Gusev crater), e.g., ferricopiapite [Fe4.67(SO4)6(OH)2·20H2O], paraconquimbite and coquimbite [Fe2(SO4)3·9H2O], fibroferrite [FeOHSO4·5H2O], and rhomboclase [FeH(SO4)2·4H2O] [3, 4]. Furthermore, the dehydration of highly hydrated phase (ferricopiapite) was suggested by long-duration repeating observations of the Spirit rover [5]. These mission observations emphasized the importance of understanding the fundamental properties of ferric sulfates, especially their preservations and phase transitions under the conditions relevant to Mars’ surface and subsurface environments. To build these understanding through laboratory experiments will link the mission observations to the interaction processes between Mars atmosphere and surface materials, and would further shed light into the current water budget of Mars. Previous laboratory investigation of the fundamental properties of ferric sulfates: A few early studies [6,7] investigated the solubility relationships of minerals in the Fe2O3-SO3-H2O system, but only in 200-50 °C temperature range. Xu et al. [8] studied the rehydration of two anhydrous ferric sulfates at 21 °C; Wang et al [9] studies the stability fields and phase transition pathways of five hydrous ferric sulfates at 50 °C, 21 °C, and 5 °C. Kong et al. [10] defined the first phase boundary below 50 °C between two normal ferric sulfates. The ferric sulfates studied in above recent works [8-10] belong to normal [Fe2(SO4)3·xH2O], acidic [FeH(SO4)2·4H2O], and slightly basic Fe4.67(SO4)6(OH)2·20H2O] types. New experimental study of OH-bearing ferric sulfates: Since 2011, we started a new set of experiments on two major types of OH-bearing ferric sulfates: jarosite group and butlerite-fibroferrite group. We have successfully synthesized three jarosites KFe3(SO4)2(OH)6, NaFe3(SO4)2(OH)6, and H3OFe3(SO4)2(OH)6, and a ferric hydroxysulfate FeOHSO4 [11]. Using these newly synthesized basic ferric sulfates, we started three sets of experiments in 2012, which were designed to simulate the post-depositional saltsatmosphere interaction processes on Mars, i.e., the deposited ferric sulfates experienced only the changes in atmospheric conditions (temperature T and relative humidity RH), as those would happened during the diurnal, seasonal, obliquity cycles on Mars. The solidliquid interactions, that would bring-in additional ions, were not simulated by our experiments. The three sets of new experiments are: (1) 90 experiments on the stability field and phase transition pathways of K-, Na, H3O-jarosites, at three temperatures (50 °C, 21 °C, 5 °C) and ten relative humidity levels (from 5% to 100%); (2) 54 experiments on the stability field and phase transition pathways of K-, Na, & H3O-jarosites, ferricopiapite, paracoquimbite, kornelite [Fe2(SO4)3·7H2O], crystalline and amorphous pentahydrated ferric sulfates [Fe2(SO4)3·5H2O], and rhomboclase at -10 °C and six RH levels (from 11 % to 98%); (3) 12 experiments on the rehydration of FeOHSO4 at three Ts (95 °C, 50 °C, 21 °C) and four RH levels from 50% to 100%. In these experiments, the humidity buffer technology was used to “drive” the processes towards dehydration or rehydration; the gravimetric measurements at regular time interval were conducted to monitoring the change of H2O/OH content in each sample; the laser Raman spectroscopic measurements were made on the intermediate and final reaction products for phase identifications. We report here the preliminary results. Result #1, Stable K-, Na, H3O-jarosites: Figure 1 shows the data from ten experiments on Na-jarosite within ten different RH buffers at 21 ± 1 °C. Similar plots are obtained from the experiments on Na-jarosite at 50 °C, 5 °C, and -10 °C, as well as from the similar two sets of exper0 5 10 15 1 10 10