On the\nEphemeral Occurrence of Ikaite in Aqueous Solutions\nbetween 0 and 20 °C

Nucleation and persistence of metastable ikaite (CaCO<sub>3</sub>·6H<sub>2</sub>O) in place of less soluble calcium carbonate\npolymorphs have important implications for the crystallization pathways\nof carbonate minerals and for the carbon flux in Earth’s cold\nregions. In this study, inhibitor-free aqueous solutions (initial\nsupersaturation Ω<sub>ikaite</sub> = 18) were used to investigate\nthe occurrence of ikaite between 0 and 20 °C. Within this entire\ntemperature range, a transient occurrence was detected which decreased\nfrom 28 h at 0 °C to less than 4 min at 20 °C. The driver\nfor the transiency was the concomitant nucleation of the less soluble\nanhydrous CaCO<sub>3</sub> minerals vaterite and calcite which hints\nat a limited applicability of the classical Ostwald rule of stages.\nThe multiphase nucleation of ikaite, calcite, and vaterite at <i>T</i> = 0 °C was quantitatively modeled by applying classical\nnucleation theory. An adequate fit can be obtained with the kinetic\nprefactor of ikaite being at least 3 orders of magnitude larger than\nthe ones for calcite and vaterite. This kinetic advantage might be\na consequence of the low-energy pathway of ikaite precipitation via\nan assemblage of aqueous CaCO<sub>3</sub>·6H<sub>2</sub>O complexes\nwithout extensive time- and energy-consuming dehydration. In summary,\nthe results indicate that near-freezing temperature is not a prerequisite\nfor the formation but rather for the persistence of ikaite. Besides\ntemperature, factors such as the presence of an adequate quantity\nof substances inhibiting the more stable phases might be essential\nfor prolonged persistence.