The Carbonatation of Anhydrite: Kinetics and Reaction Pathways

A textural and kinetic investigation of the carbonatation of anhydrite in the presence of carbonate- and bicarbonate-bearing solutions under static conditions was conducted. The replacement occurs via a coupled dissolution–precipitation mechanism. Textural and kinetic evidence indicates that the rate-limiting step in the replacement reaction was the dissolution of anhydrite and that the dissolution rate was likely controlled by the diffusion of ionic species in the aqueous phase. Calcium carbonate polymorphism was sensitive to temperature and solution composition. Bicarbonate-bearing solutions up to 80 °C only produced calcite, but aragonite formed alongside calcite in carbonate-bearing solutions, occurring in trace amounts at 25 °C and becoming the dominant polymorph at temperatures ≥60 °C. Furthermore, within the carbonate-bearing solutions (high pH) at elevated temperature, kinetic and textural evidence indicates that competition between calcite and aragonite nucleation and growth plays a greater role in defining the mineralogy and textures of the products than an aragonite to calcite ripening process such as the one previously reported for the carbonatation of gypsum. A lack of crystallographic relationship between the aragonite and calcite that formed at elevated temperatures, along with an apparent stabilization of the calcite/aragonite ratio at the early stages of the replacement, highlight the importance of the kinetics of precipitation (via nucleation and growth) and the role temperature and solution composition can play in stabilizing metastable product phases during mineral replacement reactions.