A Comparison of Dehydration Processes of Al-, Fe2+-, & Mg-Sulfates under Mars Relevant Conditions Yuhang Zhou and Alian Wang Abstract #1797 Dept. Earth and Planetary Sciences and McDonnell Center for Space Sciences, Washington University, St. Louis, MO, 63130, USA ([email protected])([email protected])

Introduction structural changes during the dehydration • Hydrous sulfates (Ca-, Mg-, Fe-sulfates and recently Al-sulfates) , as markers of aqueous processes on Mars, have been observed on a variety of locations on Mars; A. alunogen  meta-alunogen • This set of six experiments studies the dehydration processes and dehydration rates of a Al-sulfate (Alunogen, Al2(SO4)3•17H2O) and a Fe2+-sulfate (Melanterite, FeSO •7H O); Change of Raman spectra of 4 2 H O modes SO4 modes 2 Al (SO ) .nH O (n = 17 to 12.5) • The dehydration rate of hydrous sulfates is a function of environment pressure (P), temperature (T), and partial water pressure (P ). 2 4 3 2 H2O during dehydration at 298 K. Our experiments were conducted at Mars relevant P, PH2O, and at three Ts; XRD analysis confirms the • We compared our results with those from the previous experiments on a Mg-sulfate (epsomite, MgSO •7H O); 4 2 structural change from alunogen • Our goal is to understand the potential hydration degrees of these sulfates within Mars subsurface and the current water budget of [Al (SO ) ·17H O] to meta- Mars. 2 4 3 2 alunogen Al2(SO4)3·12H2O].

Samples & Experiments Experiment Results Crystal structure study of alunogen reveals that among the total Samples are prepared to make sure: Among the 6 dehydration structural 17 H2O per molecule, 5 1. Within the same grain size range; experiments , alunogen H2O that are hydrogen bonded. 2. At the highest hydration degrees dehydrated from 17 H2O to Their weak bonding strength (RH buffer tech.) 12.53 H2O at 21°C. makes them easy to loss, while 3. With confirmed ID and Melanterite dehydrated other 12 H2O are coordinated with homogeneity (laser Raman 100- from 7 H2O to 2.72 H2O at Al, thus more structurally stable. point check). 21°C. Experimental design:   2+ Mass reductions of alunogen and melantorite during the B. Melanterite rozenite mixture of rozenite & amorphous Fe - sulfates dehydration H O modes Change of Raman spectra 1.1 1.1 2 SO4 modes alunogen dehydration at 3 temperature melanterite dehydration at 3 temperature of FeSO4.nH2O (n = 7, 4, 7 H2O 1 1) during dehydration at 17 H2O 1 0.9 6 H2O 298 K (Choi et al., 2006).

261 K 0.8 261 K

0.9 0.7 n(t)/n(0) 0.6

n(t)/n(0) 273 K 4 H2O 0.8 Crystal structure study of melanterite reveals that 0.5 273 K The P and PH2O in our experiments are relevant to general Mars P and among the total 7 structural H2O, only one is 0.4 PH2O. 298 K hydrogen bonded, which will be removed first 12 H2O time(s) 298 K 0.7 0.3 during the dehydration. Then, structural change 60 600 6000 60000 600000 60 600 6000 60000 600000 happens from ferrohexahydrite [FeSO ·6H O, C2/c] time(s) 4 2 to rozenite [FeSO4·4H2O, P21/c]. The appearance of n(t) = the number of structural H2O per molecule at a time (t) broad Raman peak at 1014 cm-1 suggests the 2+ n(0) = the number of structural H2O per molecule at a time (0) formation of amorphous Fe -sulfates with hydration Time (s) = time duration of experiment (second) degree between 1-4.

Alunogen dehydrates fast until reaching 12w, then almost stable Two sets of experiments (for alunogen and melanterite) are conducted at our exp. conditions; at 3 temperatures: 25°C, 0°C, and -12°C，which are within the T range Melanterite dehydration appears as 3 stages: from 7w to 6w, to 4 Conclusion at Mars surface. w, and then further. A comparison of the dehydration processes of alunogen, melanterite, and epsomite reveals: Comparison -- three dehydration paths • The dehydration rates of all three processes are strong 1.1 1.1 temperature dependent. At low T, all three dehydrations 298 K Al2(SO4)3.nH2O 273 K 1 1.0 invariably go very slow; FeSO4.nH2O 0.9 0.9 • Under the similar P, T, P conditions, the pathways of 0.8 MgSO4.nH2O 0.8 H2O dehydration of three hydrous sulfates are very different: 0.7 0.7 alunogen lost only the hydrogen bonded H2O; melanterite went Room temperature 0°C experimental setup 0.6 0.6 through three steps in which crystalline structures were (immersed in H2O liquid + ice bath)

experimental setup n(t)/n(0) 0.5 0.5 maintained during almost entire duration; epsomite lost first

0.4 0.4 n(t)/n(0) Experiments at -12°C were made by placing the vacuum desiccators in a hydrogen bonded H2O and then become amorphous during freezer (at -12°C ± 1°C). 0.3 0.3 almost entire duration. The differences in the bonding strength time(s) time(S) 0.2 0.2 are the causes for their different dehydration pathways. •For each of six experiments, 15 sample 0 5000 10000 15000 20000 0 100000 200000 300000 400000 500000

bottles were used: (60±5 mg per bottle • Under the similar P, T, PH2O conditions, the dehydration of Under very similar P, T, PH2O for alunogen, 120±5 mg per bottle for 1.1 epsomite goes faster than that of melanterite, the dehydration of 261 K conditions: melanterite): melanterite goes faster than that of alunogen. 1 •at each of 9-16 steps during the • All three dehydrations are strongly T dependent; 0.9 dehydration, all 10 sample bottles were • With very different dehydration Acknowledgement taken out for mass measurements, from 0.8 pathways: This work was partially supported by NASA Mars Fundamental Research Project which we obtained the standard deviation o Alunogen – fast at beginning, then NNX10AM89G (AW) and by the Chinese Scholarship Council (YHZ). We want to thank the

of gravimetric measurements; 0.7 stable at ~ 12w; help given by Ms. Y. L. Lu and Mr. Paul Carpenter in Raman, IR, and XRD laboratories. n(t)/n(0) o Melanterite – experience three 0.6 • at five steps during the dehydration, one of the remaining 5 sample stages; References o [1] Bribing et al., 2006, Science Vol 312. P400-404; [2] Murchie et al., 2009, JGR; [3] Clark et al., 2007, J. bottles was taken out for laser Raman measurements, from which we 0.5 time(s) Epsomite – a smooth continuous Geophys. Res., 112, E06S01; [4] Samuel, et al., 2010,GRL, VOL. 37, L09201; [5] Wang, A., and Z. C. Ling,2011, determined the crystal structural changes. 0 200000 400000 600000 dehydration (amorphozation, Wang et al., 2006, 2009). J. Geophys. Res., 116 , E 00 F17; [6]Wang, 2012,#2172; LPSC; [7] Chio et al., 2006, LPS XXXVII.