Calcium sulfates in planetary surface environments

Abstract

Calcium sulfates play a key role in both industrial processes and the global sulfur cycle. In arid to hyper-arid planetary surface environments, they can be used to assess the availability of water to the environment. Yet, the processes through which calcium sulfate minerals (i.e. gypsum, bassanite, anhydrite) and the anhydrous γ-CaSO4 form and transform remain insufficiently understood. Especially the dissolution-reprecipitation reaction from gypsum to anhydrite has to this date been achieved in the laboratory only under very specific conditions.Recent evidence suggests the importance of solution flow as opposed to batch reactions in aiding the precipitation of anhydrite at room temperature. Our own results, however, clearly show, that solution flow alone cannot be the sole catalyst. In light of the recent abundance of research towards the nucleation and crystallization of calcium sulfates, we present this review in an attempt to summarize and contextualize these new results and to identify necessary directions for further research with focus on conditions of arid and hyper-arid planetary environments.