Experimental stability of magnesium sulfate hydrates that may be present on Mars

Since the Viking missions in 1976, magnesium Sulfates have been predicted to exist on the surface of Mars. Recent orbital measurements suggest that Mg-sulfates are rather ubiquitous on the martian surface. Chemical analyses by landers support the infermce that Mg-sulfate hydrates may be one source of the significant quantities of equatorial near-surface hydrogen observed by the neutron and gamma-ray spectrometers on the Mars Odyssey spacecraft. The present study was undertaken to examine stability relations among the various Mg-sulfate hydrates. Using saturated salt solutions to control water-vapor pressure at temperatures of 3, 23, 50, 63, and 75 degrees C, Mg-sulfate phases were allowed to equilibrate from 2 to 3 months to see which hydration states were formed or were stable. Starting materials consisted of hexahydrite (6H(2)O), starkeyite (4H(2)O), kieserite (1H(2)O), a second monohydrate-polymorph available as a chemical reagent, and an anhydrous MgSO4 reagent. Products created in this study included these minerals, along with epsomite (7H(2)O), sanderite (2H(2)O), amorphous MgSO4 (1-2H(2)O), several previously undescribed phases, one of which was quite persistent (2.4H(2)O), and trace amounts of pentahydrite (5H(2)O). As expected, Mg-sulfate stability is strongly dependent on water vapor pressure and temperature. Lower temperatures favor the more hydrated Mg-sulfates. However, the MgSO4 system was found to be surprisingly complicated and is strongly dominated by metastability, sluggish kinetics, and reaction pathways. Unexpected results were frequently encountered, in addition to the formation of previously undescribed phases. Several of the hydrates also show significant metastable extensions, such that phase boundaries can only be approximated. For example, kieserite, which has been reported on Mars from OMEGA data, in addition to having a distinct stability region, is resistant to transformation and persists throughout temperature-RH space until very high relative humidities are achieved. Results of this study show that MgSO4 hydrates in addition to epsomite, hexahydrite, and kieserite can persist and should not be overlooked when assessing possible Mg-sulfate minerals that call occur on Mars. Published by Elsevier Inc.