On the Ephemeral Occurrence of Ikaite in Aqueous Solutions between 0 and 20 °C

Nucleation and persistence of metastable ikaite (CaCO3·6H2O) in place of less soluble calcium carbonate polymorphs have important implications for the crystallization pathways of carbonate minerals and for the carbon flux in Earth’s cold regions. In this study, inhibitor-free aqueous solutions (initial supersaturation Ωikaite = 18) were used to investigate the occurrence of ikaite between 0 and 20 °C. Within this entire temperature range, a transient occurrence was detected which decreased from 28 h at 0 °C to less than 4 min at 20 °C. The driver for the transiency was the concomitant nucleation of the less soluble anhydrous CaCO3 minerals vaterite and calcite which hints at a limited applicability of the classical Ostwald rule of stages. The multiphase nucleation of ikaite, calcite, and vaterite at T = 0 °C was quantitatively modeled by applying classical nucleation theory. An adequate fit can be obtained with the kinetic prefactor of ikaite being at least 3 orders of magnitude larger than the ones for calcite and vaterite. This kinetic advantage might be a consequence of the low-energy pathway of ikaite precipitation via an assemblage of aqueous CaCO3·6H2O complexes without extensive time- and energy-consuming dehydration. In summary, the results indicate that near-freezing temperature is not a prerequisite for the formation but rather for the persistence of ikaite. Besides temperature, factors such as the presence of an adequate quantity of substances inhibiting the more stable phases might be essential for prolonged persistence.