The crystallization of ikaite (CaCO3 x 6H2O) in presence of mineral surfaces: Effects on formation and implications for nucleation pathways

Ikaite is an essential calcium carbonate mineral especially in cold regions of Earth. However, the conditions leading to the formation of ikaite are poorly constrained. In a previous study (Chaka 2018), it was reported that the nucleation mechanism of ikaite likely does not compare to widely recognized mechanisms of more stable anhydrous CaCO3-minerals. Nucleation was rather attributed to a low energy pathway, which leads to the highly hydrated ikaite structure by assembling aqueous ion pair complexes. Whether such a nucleation mechanism is affected by foreign surfaces, as it was already shown for the formation of anhydrous calcium carbonate minerals (Li et al. 2014), remained unclear so far. This lack of knowledge is particularly serious as a better understanding of the nucleation of ikaite in presence of mineral surfaces could lead to profound insights of ikaite formation under natural conditions. In our study, new data on ikaite formation in presence of mineral surfaces were gained from cryo-mixed-batch-reactor and cryo-AFM experiments using quartz and mica surfaces as proxies for natural settings. Our experiments revealed similar effects of the presence of quartz and mica on ikaite nucleation. Both minerals caused a significant expansion of the supersaturation range from which ikaite forms instead of calcite or vaterite. For ikaite formation, therefore, the presence of mineral surfaces is at least as important as calcite-inhibiting substances. The latter, so far, were the only ikaite promoters recognized in natural settings. Furthermore, our results revealed low interfacial energies which imply a nucleation mechanism including a minimal energy barrier.