Heterogeneous nucleation and transformation of ikaite (CaCO3 x 6H2O) on mineral surfaces

One of the most puzzling calcium carbonate minerals is ikaite (CaCO3 x 6H2O). Its formation is of particular importance, because anhydrous calcium carbonate minerals occur as pseudomorphs after ikaite. Consequently, ikaite may be an important and frequently forming precursor for more stable carbonate minerals especially in cold environments of Earth (Sánchez-Pastor et al. 2016). Despite the importance of ikaite, its formation conditions are not well constrained and knowledge about its decomposition and transformation is limited. Previous studies showed that cold temperatures and increased alkalinity promote ikaite formation and that dissolved Mg2+ and/or phosphate suppress a competing precipitation of calcite and/or vaterite (Purgstaller et al. 2017). However, these studies typically concern homogeneous ikaite formation. Therefore, it is still unknown whether mineral-water interfaces, which are ubiquitous in nature, affect both nucleation and transformation of ikaite. Using cryo-mixed batch-reactor experiments (CMBR), we investigate which effects mineral surfaces exert on nucleation of ikaite, its subsequent transformation into more stable carbonates and the product phase selectivity. Besides ex-situ analyses of solution compositions and phase inventories of the reactor, in-situ monitoring of solution pH and Ca2+ concentrations gives insights into differences and similarities of ikaite behaviour in experiments with and without added minerals such as quartz and mica. Complementary to CMBR, in-situ flow-through cryo-atomic-force-microscopy (CAFM) can reveal the temporal evolution of the reactions on defined substrates in high spatial resolution. Initial results obtained by a newly developed CAFM will be presented.