The Behavior of Hydrated Na-Mg Sulfate Phases Under Mars-Relevant Conditions

An increasing inventory of hydrous evaporite and silicate minerals has been identified from orbital and lander data on Mars. Several hydrous sulfate minerals are thought to be present on Mars, based on spectral, chemical, and geomorphic observations (e.g., CRISM, OMEGA, and Mars Exploration Rover results). We are examining the behavior of hydrous minerals on Mars and their potential participation in the H₂O cycle to augment and complement these data. With limited liquid water stability on the martian surface, hydration and dehydration of hydrous minerals with changes in temperature (T) and relative humidity (RH) during a Mars day can have a significant influence on the bioavailability of water and potentially on atmospheric H₂O. This research focused on the Na₂MgSO₄nH₂O system, predicted by King et al. (2004) to occur on Mars. Phases in this system were also predicted by Clark et al. (1981), based on the concentration of Mg on Mars' surface. They proposed that Mg would be more stable in the form of a double salt, as a result of solubilities in the MgSO₄nH₂O system. Our experiments included blödite, konyaite, a decahydrate, and potential new phases. Blödite (Na₂MgSO₄4H₂O) was analyzed by X-ray powder diffraction (XRD) under controlled RH-T conditions to investigate its behavior and to understand mineral reactions. Crystal structures and phase abundances were determined using Rietveld methods. When blödite was allowed to deliquesce (RH>80%) and then exposed to low temperatures (T<0°C), two different results were observed. One set of experiments produced a new, likely higher hydrate Na₂MgSO₄ phase. This new phase was first observed at -10°C (47-78% RH), it formed within minutes, and it persisted on decreasing T to at least -30°C. In an attempt to produce only this phase, a second set of experiments was performed by storing blödite (either dry or in mush form) at -10°C for several days. These experiments produced a mixture of mirabilite (Na₂SO₄10H₂O; 56 wt% H₂O) and a hydrous MgSO₄ phase, either meridianiite (11H₂O; 62 wt% H₂O) or epsomite (7H₂O; 51 wt% H₂O), depending on RH conditions. We also observed circumstantial evidence for amorphous sodium sulfate under lower-RH conditions in association with epsomite. This separation occurred for dry and wet blodite, implying that time is the key factor in determining the low-T assemblage. The result of the first experiment was apparently a metastable phase, based on its rapid formation and its ultimate transformation to a two-phase mixture. Meridianiite and mirabilite, and likely the new hydrate, contain more H₂O by weight than the highest known hydrate of Na₂MgSO₄ (the decahydrate, 41 wt% H₂O). Depending on the abundance of water, Na-Mg sulfate solutions have the potential to precipitate several different highly hydrated phases, all of which undergo dehydration/hydration reactions on heating/cooling. Thus, these phases add to the list of hydrated phases on the martian surface that can participate in the H₂O cycle and contribute to the mineral H₂O storage inventory.