Insights into the amorphous calcium carbonate (ACC) → ikaite → calcite transformations

Amorphous calcium carbonate (ACC) is a precursor material that plays a key role in polymorph selection and crystallization of carbonates. It is involved in the formation of the cryogenic carbonate ikaite (CaCO₃·6H₂O), but the role of ACC during the transformation of this room-temperature-metastable material is poorly understood. We report the occurrence of two ACC types that differ in their mode of formation, morphology, particle size, water content and stability. ACC precipitated in solution below 5 °C, referred to as ACC (I), is amorphous, forms as ∼100 nm sized spheres, and contains 1.12 mol adsorbed and 0.26 mol chemically bonded water. Aging of ACC (I) for 25 min under cold conditions forms euhedral ikaite, whereas calcite forms within 10 min at room temperature. Our findings suggest that ACC (I) is the same precursor phase for both crystalline forms and their selection can be controlled by temperature alone. Rapid dehydration of ikaite by organic solvents and vacuum pumping at room temperature and by increasing the temperature from 5 to 30 °C within 1 min forms amorphous carbonate, referred to as ACC (II). ACC (II) consists of micron-sized, porous grains that preserve the ikaite grain morphology and contains 0.42 mol physically adsorbed and 0.35 mol chemically bonded water. Temperature-dependent investigations performed between 25 and 350 °C indicate that ACC (I) is more stable than ACC (II) despite its higher water content. At 300-350 °C ACC (I) can be dehydrated to an amorphous material containing 0.03 mol water, which crystallizes to calcite without a transitional anhydrous phase. The different ACC types can be attractive for materials science applications. The finding of an amorphous phase during the ikaite → calcite transition suggests the alteration of the original geochemical signal and thus has implications for interpreting paleoclimatological data.