Geochemistry of Carbonate Minerals

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

To contribute to a better understanding of the occurrence of ikaite, key aspects of the formation and transformation kinetics of ikaite were investigated. One focus was on the investigation of the effect of the presence of mineral surfaces on ikaite nucleation. Knowledge of this effect is important because mineral surfaces are omnipresent in natural ikaite formation environments. The experiments revealed for the first time that the presence of mineral surfaces can significantly promote nucleation of ikaite vs. calcite and vaterite and, thus, can substantially contribute to the occurrence of ikaite. The complete study can be downloaded from

https://doi.org/https://doi.org/10.1016/j.chemgeo.2022.121089

A second project focus was lying on investigations of the effect of aqueous phosphate on the growth of ikaite crystals. Aqueous phosphate is a well-known inhibitor of calcite growth. Its effect on ikaite growth kinetics, therefore, is of high interest. The experiments revealed no significant incorporation of phosphate into growing ikaite crystals at the applied conditions. The observed lack of incorporation of phosphate agrees with the previously suggested low-energy formation mechanism of ikaite via an assembling of aqueous CaCO3 complexes which likely does not facilitate substantial substitution of carbonate by phosphate ions. The complete study can be downloaded from

https://doi.org/10.1007/s10498-023-09418-z

A further project focus was lying on the investigation of the effects of increasing temperatures (T = 0-20 °C) on the stability of ikaite in aqueous solutions. The experiments revealed for the first time that the persistence of ikaite is significantly reduced by the temperature dependent increase of the nucleation and growth rate of the competing phases vaterite and calcite. An extensive inhibition of the less soluble CaCO3 minerals, therefore, is inevitable for an increase of ikaite persistence. Thus, a fundamental limitation of ikaite persistence follows at increasing temperatures, even though ikaite nucleation per se is possible as long as the solution is sufficiently supersaturated. Consequently, pseudomorphs of ikaite (glendonites) very likely may serve as useful proxies of low temperatures in most cases. The complete study can be downloaded from

https://pubs.acs.org/doi/10.1021/acsearthspacechem.4c00097