On ikaite crystallization

Strohm, Samuel (2025): On ikaite crystallization. Dissertation, LMU München: Fakultät für Geowissenschaften

Abstract

The crystallization of ikaite (CaCO3·6H2O) instead of less soluble calcium carbonate polymorphs has important implications for formation pathways of calcium carbonate minerals, carbon dynamics in polar regions of Earth or paleoclimatic reconstructions. However, the crystallization of ikaite is far from fully understood. Although several conditions that promote the formation of ikaite have been identified, the occurrence of ikaite remains unclear, as fundamental kinetic aspects of the crystallization of ikaite remain largely unexplored to date. In order to contribute to a better understanding of the occurrence of ikaite, this doctoral thesis investigated the crystallization of ikaite by unraveling key aspects of nucleation, growth and persistence using experimental approaches. As mineral surfaces are potentially effective for heterogeneous nucleation, the formation of ikaite was investigated in the presence of quartz and mica substrates at 0 °C. The study showed that mineral surfaces are an important formation parameter, which can promote the nucleation of ikaite: At supersaturations with respect to ikaite Ωikaite ≥ 15, ikaite was formed irrespective of the presence of these mineral substrates. At Ωikaite < 15, in contrast, ikaite precipitated only in the presence of the mineral substrates, while precipitation experiments in absence of these surfaces revealed the formation of the anhydrous CaCO3 minerals vaterite and calcite. Thus, the promotion of ikaite nucleation by quartz and mica prevented competing precipitation of anhydrous CaCO3 minerals and led to ikaite formation within a supersaturation range, which was much wider than under pseudohomogeneous conditions. Furthermore, induction periods measured in both supersaturation regimes enabled the determination of the interfacial energy of ikaite nucleation. Using classical nucleation theory, the interpretation of induction periods from ikaite-forming precipitation experiments (Ωikaite ≥ 8) yielded an effective interfacial energy of nuclei of 15 ± 3 mJ/m2. This interfacial energy of ikaite nuclei is significantly lower than values reported for anhydrous CaCO3 phases and, therefore, may support the hypothesis of a low energy formation pathway of ikaite via an ordering of aqueous ion pair complexes without extensive dehydration. Moreover, such a formation mechanism is corroborated by the results of the study of ikaite growth kinetics in phosphate containing solutions at 1 °C. Applying the empirical equation R=k(Ω-1)^n, measured growth rates yielded a rate constant k = 0.10 ± 0.03 µmol/m2/s and a reaction order n = 0.8 ± 0.3. This reaction order implies a transport- or adsorption-controlled growth mechanism which supports the formation of ikaite involving the assemblage of aqueous ion pair complexes. Apart from this finding, the experimental growth rates of ikaite provided no sign that ikaite growth is retarded by the presence of phosphate. A potential depletion of aqueous phosphate due to incorporation into ikaite was not detected. Thus, it must be assumed that carbonate anions in ikaite are not substantially substituted by phosphate, which supports the key role of phosphate as an inhibitor of anhydrous CaCO3 mineralization and as a powerful promotor of ikaite formation. Without such an inactivation of anhydrous CaCO3 mineralization, the occurrence of ikaite was generally transient in precipitation experiments at different solution temperatures (0–20 °C). The persistence of ikaite decreased from 28 h at 0 °C to less than 4 min at 20 °C due to concomitant nucleation of less soluble vaterite and calcite. This concomitant multiphase nucleation indicated a limited applicability of Ostwald’s rule of stages. Applying classical nucleation theory, an adequate fit of nucleation rates was obtained for T = 0 °C with a kinetic pre-factor A of ikaite being at least 3 orders of magnitude larger than A of vaterite and calcite. Such a kinetic advantage of ikaite potentially originates from the specific nucleation mechanism without dehydration. Furthermore, the ephemeral occurrence of ikaite between 0 and 20 °C supports that nucleation of ikaite is not limited to near-freezing environments per se. However, low temperatures are important for a prolonged persistence of ikaite. In order to ensure the persistence of ikaite, an inactivation of competing precipitation, for example by inhibitors, might be indispensable. An inhibition ensuring persistence of ikaite, though, most likely becomes more complex to achieve with temperature. In summary, the studies showed that the fundamental crystallization kinetics of ikaite differs significantly from anhydrous CaCO3 minerals. The specific formation mechanism of ikaite involving the assemblage of aqueous ion pair complexes might be essential for the crystallization. The investigations also revealed that mineral surfaces have a substantial effect on ikaite formation and play a more important role for the nucleation of ikaite in manifold environments of Earth than previously assumed. Furthermore, the relation between temperature and ephemerality of ikaite was shown for the first time in detail. The observed ephemerality indicated that the extent of ikaite persistence decreases significantly with temperature. Therefore, pseudomorphs after ikaite (glendonites) very likely may serve as reliable proxies of low temperatures in most cases.

• Dokumententyp: Dissertationen (Dissertation, LMU München)
• Themengebiete: 500 Naturwissenschaften und Mathematik ; 500 Naturwissenschaften und Mathematik > 550 Geowissenschaften
• Fakultäten: Fakultät für Geowissenschaften
• Sprache der Hochschulschrift: Englisch
• Datum der mündlichen Prüfung: 24. Juni 2025
• 1. Bericht­erstatter:in: Jordan, Guntram