Transformation of Amorphous Calcium Carbonate in Air

In air, the mechanisms of amorphous calcium carbonate (ACC) transformation into crystalline polymorphs of CaCO₃ and whether the atomic ordering is attributable to a solid-state transformation or a dissolution and reprecipitation process are still under debate. While some studies observed a significant influence of relative humidity on ACC transformation, other studies suggested a dehydration process of ACC prior to crystallization. In the present study, we focus on the metastability of additive-free ACC in air and in particular on its interaction with relative humidity. Our findings indicate that the transformation of ACC into crystalline CaCO₃ is triggered only after the physisorption of a critical H₂O level. Consequently, ACC metastability was prolonged by retarding H₂O uptake and by keeping the physisorbed H₂O below the critical level, ACC remained in its metastable state. Therefore, the conceptual formation of a “thin film” of about four monolayers of physisorbed H₂O is considered to govern the transformation of ∼90 nm sized ACC particles via partial dissolution and reprecipitation. Furthermore, we observed simultaneous formation of calcite, vaterite, and aragonite from ACC, where distinct proportions correspond to different H₂O exposure conditions. Thus, polymorph formation from ACC depends also on physicochemical boundary conditions during transformation rather than on prestructural formation within ACC alone