Investigations of the heterogeneous formation of ikaite and of the ikaite transformation as important processes for the genesis of stable calcium carbonate minerals
Final Report Abstract
The conditions for the formation and transformation of metastable ikaite (CaCO3·6H2O) are insufficiently explored although various occurrences are known in the cold regions of Earth. 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 fully assessed. To contribute to a better understanding of the occurrence of ikaite, aim of the project was the investigation of key aspects of the formation and transformation kinetics of ikaite. In this context, one project focus was on investigations of ikaite nucleation in the presence of mineral surfaces. For the experiments conducted at T = 0 °C, quartz and mica have been selected as two important members of commonly occurring minerals. The results 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. Moreover, first quantitative interfacial energies for ikaite were obtained by using experimental induction periods for nucleation. These experimental data allowed for important conclusions on basic formation kinetics of ikaite. In this way, the results confirmed the lowenergy formation mechanism via the assembling of aqueous ion-pair complexes which previously had been postulated based on structural implications and DFT-calculations. Furthermore, complementary data on ikaite growth kinetics measured in the project support this formation mechanism, which significantly differs from the formation mechanism of anhydrous CaCO3 minerals due to the omission of a complete dehydration of the ionic constituents. 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. At increasing temperatures, such an inhibition requires increasing inhibitor concentrations. Therefore, 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. All investigations, thus, show that extensive ikaite formation at rising temperatures is increasingly difficult. Consequently, pseudomorphs after ikaite (glendonites) very likely may serve as useful proxies of low temperatures in most cases.
Publications
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On the nucleation of ikaite (CaCO3x6H2O) – A comparative study in the presence and absence of mineral surfaces. Chemical Geology, 611, 121089.
Strohm, Samuel B.; Inckemann, Sebastian E.; Gao, Kun; Schweikert, Michael; Lemloh, Marie-Louise; Schmahl, Wolfgang W. & Jordan, Guntram
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A Study on Ikaite Growth in the Presence of Phosphate. Aquatic Geochemistry, 29(4), 219-233.
Strohm, Samuel B.; Saldi, Giuseppe D.; Mavromatis, Vasileios; Schmahl, Wolfgang W. & Jordan, Guntram
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On the Ephemeral Occurrence of Ikaite in Aqueous Solutions between 0 and 20 °C. ACS Earth and Space Chemistry, 8(9), 1725-1736.
Strohm, Samuel B.; Berghofer, Sebastian A.; Kaiser, Christian & Jordan, Guntram
