Investigations of the heterogeneous formation of ikaite and of the ikaite transformation as important processes for the genesis of stable calcium carbonate minerals

钙钛矿（CaCO3x6H2O）是地球寒区重要的碳酸钙矿物。大量的似海岩及其假晶的出现证明了这种矿物作为极地碳循环的重要组成部分所起的重要作用。尽管其广泛存在，但其形成条件和稳定性却鲜为人知。特别是，对爱海石的非均相成核和转变的认识是缺失的。这妨碍了对自然界中这种矿物的形成条件和稳定性的可靠评估。先前的研究表明，与均匀条件相比，石英和云母表面在T = 0°C时显著扩大了海岩形成的过饱和范围。因此，有无矿物表面是岩样形成的一个基本的重要标准。然而，在0°C以上的温度下，矿物表面对衣海岩成核的影响在多大程度上仍然是未知的。此外，在有矿物表面存在的情况下，温度升高对钙钛石转变为更稳定的无水碳酸钙的影响尚不清楚。然而，在高于水冰点的温度下，了解岩的非均相成核和转变，对于正确评价这种矿物作为低温指标的适用性是非常重要的。此外，了解在T ~ 0 ~ 0°C的冰矾的非均质形成和转变，有助于更好地理解地球寒冷地区无水CaCO3多晶的各种形成路径。因此，该项目的目的是研究在T ~ 0 ~ 0°C有矿物表面存在的钛矿的形成和转化过程。在成功进行的低温混合批堆（CMBR）实验的基础上，我们将研究石英和云母在T = 0°C时的形成条件。此外，在这些矿物表面存在的情况下，将获得艾凯特石的稳定性和转变过程的知识。为此，除了cmbr实验外，还将使用相分析（XRD）和显微镜（cryo-SEM和cryo-AFM）来研究底物对成核和转变过程的影响。

Ikaite (CaCO3x6H2O) is an important calcium carbonate mineral in the cold regions of Earth. Numerous occurrences of ikaite and its pseudomorphs are documenting the significant role of the mineral as a substantial part of the polar carbon cycle. Despite its widespread occurrence, conditions of formation and stability of ikaite are poorly known. Especially, knowledge of the heterogeneous nucleation and transformation of ikaite is missing. This impedes a reliable assessment of the conditions of formation and stability of this mineral in nature. Previous work showed that quartz and mica surfaces enlarge the supersaturation range of ikaite formation at T = 0 °C significantly if compared to homogeneous conditions. Presence or absence of mineral surfaces, therefore, is a fundamentally important criterion for ikaite formation. It is still unknown, however, to which extent the effects of mineral surfaces are significant for ikaite nucleation at temperatures above 0 °C. Additionally, the effects of increased temperature on transformation of ikaite into more stable anhydrous calcium carbonates in presence of mineral surfaces are unknown. Knowledge of heterogeneous nucleation and transformation of ikaite at temperatures above the freezing point of water, however, are of immense importance for a correct evaluation of the applicability of this mineral as an indicator of cold temperatures. Moreover, knowledge of heterogeneous formation and transformation of ikaite at T > 0 °C can contribute to a significantly better understanding of the various formation paths of anhydrous CaCO3 polymorphs in the cold regions of Earth. Aim of the project, therefore, is investigating the formation and transformation process of ikaite in presence of mineral surfaces at T > 0 °C. Based on the successfully conducted cryo-mixed-batch-reactor (CMBR) experiments on heterogeneous nucleation of ikaite at T = 0 °C, we will investigate the formation conditions at T > 0 °C in the presence of quartz and mica. Furthermore, knowledge of the stability and the transformation process of ikaite in the presence of these mineral surfaces will be obtained. For these purposes, the impact of the substrates on the nucleation and transformation processes will be investigated using phase analysis (XRD) and microscopy (cryo-SEM and cryo-AFM) additionally to the CMBR-experiments.