MORPHOLOGICAL FEATURES DUE TO EPSOMITE HYDRATION/DEHYDRATION REACTIONS IN SIMULATED EVAPORITES AT THE HAND-LENS SCALE - IMPLICATIONS FOR MARTIAN SEDIMENTS

Recent investigations have shown that Martian sedimentary rocks and surface materials can contain abundant evaporite minerals, including various hydrated salts. Hydration/dehydration of these minerals can produce significant volume changes and may produce characteristic morphological features. In order to better understand the potential morphological expression of these reactions, we conducted a variety of experiments using artificial sediments containing mixtures of hydrated salts and silicate minerals that serve as martian soil analogs. These mixtures were allowed to hydrate and dehydrate under controlled conditions, and the resulting morphological features were documented. Epsomite (MgSO4 • 7H2O) was chosen as the salt mineral because it converts to hexahydrite (MgSO4 • 6H2O) at ~50% relative humidity, causing ~10% volume loss. This volume decrease creates sufficient contractive stress to create cracks. We observed surficial compression ridges, shrinkage cracks, blistering, flake formation, and remineralization of tensional cracks in our experiments. Radial cracks and non-closed polygons formed in both pure salt and salt-sand mixtures. These features are at cm to mm scale and are well resolved at resolutions of approximately 30 ìm per pixel, the resolution of the microscopic imager on the MER rovers.

We also observed significant textural differences between sediments that were saturated with brine and those that were only slightly moistened, with the latter forming thin surface crusts that easily blistered and flaked; these could potentially produce aeolian evaporite particles. In the case of brine-saturated sediments, fibrous epsomite grew within shrinkage cracks during dehydration, suggesting that this feature may potentially be used as an indicator of fluid saturation in ancient sediments.

This research should prove useful in identifying morphological features at the hand-lens scale on the Martian surface that may have formed as a result of dehydration/hydration cycling of evaporites or other hydrous minerals. From a process perspective it may help us to better understand polygon formation in Martian sediments and sedimentary rocks.