Pathways to Form Kieserite from Epsomite at mid to Low Temperatures, with Relevance to Mars,

Kieserite on Mars: The hydrous Mg-sulfate that has been identified definitively on Mars by OMEGA (Mars Express orbiter) is kieserite (MgSO4·H2O) [1]. An additional class of OMEGA spectra has been attributed to “polyhydrated sulfate.” This class has spectral features that match Mg-sulfates of higher hydration or multi-cation sulfates [2,3]. Ca-sulfates, especially gypsum (CaSO4·2H2O), are also identified in polar regions on Mars [4], but anhydrite (CaSO4) does not possess an absorption in the OMEGA spectral region. Of interest is an apparent systematic trend in the geomorphic siting of different types of sulfates at regional and global scales on Mars [5,6,7]. Kieserite has been found mostly on steep slopes or on plateaus, whereas “polyhydrated sulfates” occur on shallow slopes or on valley floors. This trend indicates a potential commonality in the geological processes that is responsible for transitions (hydration state, etc.) between these two phases, and also between these and other co-existing phases that would be invisible to OMEGA, e.g., anhydrite. The new findings from missions demonstrate that the study of potential martian sulfates, their composition, hydration states, crystallinity, and phase transition pathways are very important to understanding the early history, hydrologic evolution, and present-day surface of Mars. Laboratory study of Mg-sulfate: Laboratory experiments [8-16] on hydrous sulfates under well-controlled conditions provide fundamental knowledge of phase boundaries, reaction pathways, and conditions of phase transitions, which will help to interpret surface and orbital observations. The limitations of laboratory experiments include difficulties in simulating real, complex geologic processes and the extremely long durations needed for low-temperature experiments. We have studied the stability field and phasetransition pathways of hydrous Mg-sulfates using humidity-buffer techniques and vacuum desiccation. Laser Raman spectroscopy is used as the major analytical tool, accompanied by mass-loss measurements, XRD, and IR spectroscopy. We have done 126 experiments between 800-2500 hrs duration at three temperatures (50 ̊C, 21 ̊C, and 5 ̊C) and using ten relative humidity (RH) buffers covering a RH range from 5.5% to 100%. The powdered pure samples epsomite, starkeyite, kieserite and amorphous Mgsulfate (with two structural waters) were used as starting phases. Six samples of powdered epsomite mixed with powdered anhydrite, bassanite, and gypsum with molar ratios of Mg:Ca at ~8:2 and ~4:6 were studied at 30% RH and 50 ̊C. Can kieserite form by dehydration of higher hydrates? Although kieserite has been identified at many locations on Mars by orbital remote sensing, it cannot be formed in our experiments from the direct dehydration of epsomite, hexahydrite, or starkeyite at 5 ̊C ≤ T≤ 50 ̊C (kieserite can be produced from dehydration of epsomite at T ≥75 ̊C). Figure 1a & b shows the hydrous Mg-sulfate phases identified in the latest reaction products of our experiments based on their Raman spectra. Except for a few cases that will be discussed in later section, starkeyite is the apparent stable phase of Mg-sulfate at 5 ̊C ≤ T≤ 50 ̊C and midlow relative humidity 5.5% ≤ RH ≤ 50% (or 35% depending on T). Starkeyite has a very stable fourmember-ring substructure [17], thus additional activation energy is needed to break the ring and to form a tighter framework in the kieserite structure where all SO4 tetrahedra and MgO5Ow octahedra share their coordinating oxygen. It appears unlikely that temperatures high enough to provide these activation energies exist at the surface of Mars today. Therefore if the primary hydrous Mg-sulfates precipitated from aqueous solution on Mars were epsomite (or MgSO4·11H2O at low T [18,19]), are there dehydration pathways that could produce abundant kieserite detected by OMEGA today? 1 Pathway for kieserite formation at mid to low T: Amorphous Mg-sulfate is the first pathway to form kieserite from epsomite in our experiments (5 ̊C ≤ T≤ 50 ̊C). During the two experiments shown in 0 20 40 60 80