MgSO4•11H2O – POWDER XRD, RAMAN AND VIS-NIR SPECTROSCOPIC CHARACTERIZATION

Why MgSO4·11H2O? Hydrous Mg-sulfate with 11 structural waters (11w hydrate), a new low temperature hydrated Mg-sulfate phase, was first reported in 2006 by Peterson & Wang [1]. They did a structural refinement using single-crystal XRD and suggested that the crystal molds in outcrops at Meridiani [2] could indicate melting and loss of the 11w hydrate. Chou et al. [3] proposed a phase boundary between epsomite (MgSO4·7H2O) and a highly hydrated Mgsulfate (with 12 or 11 structural waters), and suggested this highly hydrated phase might be one of the forms of Mg-sulfates that could be stable at the current Mars surface and in its subsurface, especially in martian polar regions. Vaniman and Chipera [4] also observed the formation of this phase in their low temperature experiments. We are interested in the spectral features of MgSO4·11H2O, including Raman and VIS-NIR spectroscopy [5,6,7,8]. The VIS-NIR spectral features would specifically help to interpret spectra obtained from OMEGA (Mars Express orbiter) and CRISM (MRO orbiter). Confirming the existence of MgSO4·11H2O on Mars would further our understanding of the hydrologic evolution of Mars. Preparation of Crystals: MgSO4·11H2O samples were prepared from a solution of epsomite (MgSO4·7H2O, Sigma-Aldrich, SigmaUltra) in water using a 7:3 mass ratio. Epsomite was first ground to a particle size < 150 μm to ensure its total dissolution in water. The 7:3 mass ratio of epsomite to water yields a H2O to MgSO4 molar ratio of 12.9 to 1 -higher than the needed 11 to setup a margin allowing a small amount of evaporative water loss observed during the preparation process. The solutions in petri dishes were put into two different freezers kept at -5 ̊C and -15 ̊C respectively and allowed to equilibrate. After a few days, a polycrystalline mat formed in petri dishes of all prepared samples (Fig.1). In addition, the mat climbs up along the wall of petri dishes, indicating a considerable volume increase compared to epsomite. Structural data suggests a 43% cell volume increase on conversion of epsomite to 11w hydrate. On the basis of the total consumption of water from the initial H2O to MgSO4 molar ratio (12.9:1) and the obvious volume increase, we conclude that a highly hydrated Mgsulfate was formed at temperatures below 0 ̊C. When the H2O to MgSO4 molar ratio was much higher than 12.9 in the initial solution, a thin layer of water ice was formed beneath the polycrystalline mat. XRD confirmation of MgSO4·11H2O: We built a special XRD sample mount to measure the diffraction pattern of powders at temperatures below 0 ̊C. The mount attaches to a small box holding powdered dry ice, and can keep the sample temperature below 0 ̊C either for ~ 24 minutes when thermally isolated, or for ~ 17 min when inserted into the sample chamber of XRD diffractometer. When analyzing the polycrystalline sample formed at low T with this mount, 3-4 repeated XRD scans from 10 ̊ to 40 ̊ 2θ using 0.1 ̊steps be made before the sample totally converts to epsomite.