On the formation and persistence of ikaite

Ikaite (CaCO₃·6H₂O) is a metastable mineral, occurs in cold regions of Earth, and readily transforms into more stable minerals such as calcite or vaterite. The conditions for formation and transformation of ikaite, however, are insufficiently explored. Important implications of the occurrence of ikaite, such as its potentially large effect on the polar carbon cycle or its applicability as low temperature proxy, therefore, cannot be assessed adequately.

To contribute to a better understanding, three key aspects of ikaite formation and transformation were investigated: i) the effect of the presence of mineral surfaces on ikaite nucleation, ii) the effect of increasing temperatures (T = 0-20 °C) on ikaite stability in aqueous solutions, iii) the effect of phosphate on ikaite growth.

Our experiments revealed that mineral surfaces can significantly promote nucleation of ikaite vs. calcite and vaterite. Furthermore, the persistence of ikaite is reduced by the temperature dependent increase of vaterite and calcite precipitation. Inhibition of these minerals, therefore, is inevitable for an increased ikaite persistence. No significant incorporation of aqueous phosphate (an effective calcite inhibitor) into growing ikaite was detected.

The findings agree with an ikaite formation mechanism via an assembling of aqueous CaCO₃ complexes. Ikaite nucleation per se is possible at increased temperatures, if solution supersaturation and inhibitor concentration are sufficiently high. In natural environments, however, occurrence of such conditions becomes less likely with increasing temperature.