The Hydration and Dehydration of Hydrous Mixed-cation Sulfates

Introduction: Viking, Pathfinder, and the Mars Exploration Rovers and martian orbiters (e.g., Odys-sey) all support the historical and present-day existence of water on Mars. Mars Odyssey detected up to ~10 wt% equivalent H 2 O in equatorial regions of Mars where water ice is not stable [1], and a variety of hy-drated sulfate and chloride minerals have been suggested as possibly contributing to the Mars surface water reservoir [2, 3, 4]. Several studies have suggested the presence of open-system evaporite deposits on Mars, all of which assumed the existence of mixed-cation sulfate and perhaps chlorides. King et al. [5] proposed a model for primordial oceans and lakes on Mars that were enriched in Mg, sulfate, and chloride. They predicted a concentration sequence involving a variety of salts, including many different mixed-salt hydrates. Several Mg and Ca sulfates have been identified in the martian regolith [6, 7, 8] and previous work [3, 9] demonstrated that sulfate minerals can contain considerable H 2 O under martian surface conditions. Some evaporite minerals, including blödite, have also been suggested to form by atmospheric deposition [10]. Knowledge of martian regolith mineralogy is essential to understanding Mars' hydrogeologic history, and hydrous minerals can serve as valuable records of past aqueous alteration events. Indeed, important inferences about past conditions can be made from the presence of specific minerals [5]. Laboratory data measured under simulated martian surface conditions are crucial to provide constraints on hydrous mineral stability [11]; guided by current martian soil chemistry data and the simulations of [5], we selected several hydrated mixed-cation sulfates and sulfate-chlorides for study. County, NM, were selected for study. None of these contained independent H 2 O molecules and were not expected to dehydrate reversibly. They were chosen to determine whether the H 2 O molecules in their structures can survive low-pressure, slightly elevated-temperature conditions. Samples were ground dry and were mounted on an Anton-Paar TTK 450 heating stage on a Bruker D8 diffractometer with a VANTEC-1 position-sensitive detector (Cu radiation). X-ray diffraction (XRD) data collection ranges were tailored to each mineral to encompass the strongest peaks, and collection times were reduced to ≤30 min to allow rapid, repetitive meas