STUDY OF TWO STRUCTURAL POLYMORPHS OF MgSO4·H2O BY RAMAN, IR, XRD, AND HUMIDITY BUFFER EXPERIMENTS – IMPLICATION FOR MARTIAN KIESERITE

Mystery of martian kieserite: Kieserite (MgSO4· H2O) was identified on Mars based on NIR reflectance spectra obtained by OMEGA instrument on the Mars Express and CRISM instrument on the MRO [1, 2, 3]. Nevertheless, the pathway of forming kieserite on Mars is quite perplexing. Laboratory experiments [4, 5, 6, 7] have demonstrated that if epsomite (MgSO4. 7H2O) or hexahydrite (MgSO4.6H2O) precipitated from SO4-rich aqueous solutions were the origin of martian kieserite, then under current (& past) Mars surface temperatures, martian kieserite was not formed by direct dehydration of these two phases. Our experiments show that at T ≤ 50°C [4,5], the dehydration products of epsomite or hexahydrite is starkeyite (MgSO4·4H2O), which is stable within -10°C to 50°C range and 5% -51% RH (RH range varies with T). In addition, amorphous Mg-sulfates (holding up to 3 structural waters) can be formed by fast dehydrations of epsomite or hexahydrite [8]. Pathways for forming kieserite at T≤ 50°C: Our experiments [5] indicated that kieserite can be formed from epsomite or hexahydrite through two pathways: (1) through an intermediate stage -amorphous Mgsulfates; (2) by dehydrating a mixture of epsomite with anhydrite (CaSO4). Furthermore, on-going experiments in our laboratory suggest that when epsomite was mixed with ferrous and ferric sulfates (FeSO4·7H2O, Fe2(SO4)3·7H2O, and Fe2(SO4)3·5H2O), kieserite can also be produced by low T dehydration of epsomite. It appears that the anhydrous calciumand hydrous iron-sulfates provided a mini-scale local environment surrounding epsomite grains. This local environment has overridden the effect of large scale environmental conditions (provided by humidity buffers in laboratory experiments, or by atmospheric and surface conditions .on Mars), and has facilitated the dehydration process at low T (e.g. dehydration of starkeyite) that is otherwise barred by the activation energy. Structural polymorphs of MgSO4·H2O seen by Raman, XRD and IR: The mystery of martian kieserite is further complicated by two structural polymorphs of MgSO4H2O (as LH-monohydrate and HH-monohydrate in figures, named after their formation pathways, where LH stands for Low Humidity and HH stands for High Humidity), which have distinct XRD, Raman, and IR spectra,; and have different formation pathways and stability fields. These two polymorphs are the topic of this study. Both polymorphs were produced from the same pure reagent monohydrate magnesium sulfate. A set of baking experiments (and possible TGA) is going-on, to obtain the exact H2O/MgSO4 ratios. Figure 1 shows the Raman spectra of two polymorphs. The major spectral differences are (1) the positions of strongest Raman peak, ν1 of SO4 tetrahedron (symmetric stretching vibration), at 1046 or 1042 cm; (2) the peak shapes of structural water in 3400-3000 cm spectral region (singlet or doublet); (3) the positions of other peaks with mid to low intensities. Figure 2 shows the XRD patterns of two polymorphs. They are also compared with a natural kieserite from Lehrte, Germany [9], whose XRD pattern is a perfect match to the standard kieserite in XRD database and also to the calculated XRD pattern based on structural refinement [10]. The XRD pattern of HHmonohydrate basically matches with terrestrial natural kieserite. In addition, the XRD pattern of LHmonohydrate does not match with the standard MgSO4·1.25H2O. Figure 3 shows the mid-IR spectra (ATR) of two polymorphs. The differences in spectral patterns and peak positions are obvious, especially the strongest ν3 peak of SO4 tetrahedron (asymmetric stretching vibration). The weaker but sharp ν1 peak positions are consistent with Raman observations. Formation pathways and stability fields of two polymorphs: Chipera et al (2007, [7]) have noticed that a reagent monohydrate Mg-sulfate has a different XRD pattern from the standard kieserite in XRD PDF database. In their humidity buffer experiments (using XRD for phase identification), this monohydrate partially converted to standard kieserite at mid RH levels (37 -53 %, vary with T), and was totally converted at higher RH levels (43 – 74%, vary with T). In our humidity buffer experiments (using Raman