On the nucleation of ikaite (CaCO3x6H2O) – A comparative study in the presence and absence of mineral surfaces

Abstract

The formation of ikaite (CaCO3x6H2O) was studied in the presence and absence of quartz and mica surfaces using desupersaturation curves from cryo-mixed-batch-reactor experiments. Upon nucleation and growth within the reactor, the solution approached solubility of the precipitating carbonate phase. For ikaite, a solubility constant of log Ksp ikaite = −7.3 ± 0.1 was found (T = 0 °C). At supersaturations Ωikaite < 15, the nucleation of ikaite was significantly promoted by the presence of quartz or mica. This promotion prevented a competing nucleation of anhydrous calcium carbonates. In the presence of quartz or mica, therefore, ikaite forms over a much broader supersaturation range than in the absence. Similarly strong promotors of ikaite nucleation rather than anhydrous carbonate nucleation were previously attributed to calcite-inhibiting substances only.At supersaturations Ωikaite ≥ 8, application of classical nucleation theory on induction periods of ikaite formation yielded an effective interfacial energy of 15 ± 3 mJ/m2. Compared to data of anhydrous CaCO3 phases, this interfacial energy is low and expresses the highly hydrated character of ikaite. At supersaturations Ωikaite ≥ 18, a transient amorphous phase appeared besides ikaite.Our results show that a comprehensive understanding of ikaite formation in natural settings requires consideration not only of supersaturation and presence of calcite-inhibitors but also of the presence or absence of mineral surfaces capable of promoting heterogeneous nucleation of ikaite.