12. Calcium Sulfate Precipitation Throughout Its Phase Diagram

Calcium sulfate phases are among the most dominant evaporitic minerals and occur in large amounts both on Earth and Mars. In addition, they find broad application across various fields of industrial relevance. Despite its obvious significance, the CaSO₄–H₂O system has received surprisingly little attention in the recent flurry of studies addressing alternative mechanisms of solution-mediated nucleation and growth. Nevertheless, there is increasing evidence that distinct precursors and temporary intermediates may also occur on the way to the final stable phase, suggesting a rather complex mineralization process along with time- and size-dependent changes in solid composition and structure. In this chapter, we first review the current state of knowledge on the CaSO₄–H₂O phase diagram, including a detailed account of the respective transition temperatures and the influence of salinity on relative stability fields. Subsequently, we summarize both long-standing and more recent observations on the possible pathways that lead to the precipitation of the different CaSO₄ phases from solution under various conditions. In particular, the effects of temperature, ionic strength, solvent polarity and additives on precipitation dynamics and phase stability are addressed. Based on all this evidence, we propose a tentative unified model for calcium sulfate crystallization across the CaSO₄–H₂O phase diagram and identify water activity and corresponding changes in the hydration of CaSO₄ precursors as key aspects during phase selection. Finally, we highlight the central questions that, according to our opinion, still need to be resolved before a complete picture of the nucleation, growth, and transformation mechanisms of solid phases in the CaSO₄–H₂O system is attained.