The stability and crystallography of Mars relevant hygroscopic salts: Implications for environmental conditions of formation and their subsequent role in the water cycle.

摘要

This study examines the crystal structure and stability of phases in the CaSO4*nH2O, Mg(ClO4) 2*nH2O, and MgSO4*nH 2O systems under a wide range of temperatures and %RH, using a combination of X-ray diffraction and thermogravimetric experimental techniques. The dehydration/hydration behavior of these minerals is explored in the context of possible contributions to the H2O cycle on Mars over diurnal, seasonal, and obliquity cycles.;Detailed structural analysis of the CaSO4*nH 2O system show the existence of a semihydrate phase (0.5 n 0.67). Diffraction data eliminates trigonal structures from consideration, and site-occupancy refinements show that all possible H 2O sites in the unit cell are occupied to varying degrees. The crystal structures of the lower hydrates in the Mg(ClO4)2* nH2O system were determined using the X-ray powder diffraction charge-flipping structure solution technique.;Dehydration results show that the hydrated phases studied here are resistant to diurnal changes in hydration states driven solely by temperature-induced changes in relative humidity, and the amount of H2O cycling between the atmosphere and regolith is limited. Extrapolation of gypsum dehydration data to Mars-relevant surface conditions at low latitudes indicates that even under the most favorable conditions at the equator, dehydration would take ∼400 days to begin. Gypsum will therefore act as a sink as opposed to contributing to the flux of H2O between the regolith and atmosphere. Dehydration could have occurred in the past due to impact/volcanic events, and in such an instance, low temperature (260K) rehydration studies show that bassanite (n = 0.5) can readily cycle between 0.5 to 0.67 H2O in response to diurnal relative humidity fluctuations at low temperature. The Mg(ClO4)2*6H2O phase will be dominant on the surface of Mars. Deliquescence of Mg-perchlorate hexahydrate over a diurnal cycle is possible resulting in the removal of H2O from the surrounding regolith/atmosphere. Hydration/dehydration data for the meridianiite phase (MgSO4*11H2O) show a tendency to remain hydrated at higher latitudes due to sluggish dehydration rates, however a diurnal cycle between meridianiite and epsomite may be possible at mid-latitudes and would represent a large flux between the atmosphere and regolith.