Research on Ikaite—Natural Occurrences and Synthetic Mineral Precipitation

Dear Colleagues,

This Special Issue of Minerals focuses on research on the mineral ikaite (CaCO₃·6H₂O), which despite being metastable is found at several localities around the world. Its formation is linked to aqueous environments, and it precipitates in the whole range of natural aqueous environments from marine to fresh water. Ikaite can precipitate close to the water surface, in sea ice, on the beach and deep within sediments. Oddities such as Greenlandic shrimps and industrial pipelines are other known sites for the growth of ikaite. In general, these localities are characterized by low temperature ranging from below zero to approximately +10 °C. At some localities, ikaite forms solely during the wintertime. Nevertheless, laboratory experiments have found ikaite to precipitate at 25 °C or even higher if certain inhibitors of calcite are present in solution. A great deal of research has focused on the phosphate-inhibiting effects on calcite in connection with ikaite formation, but recent research has shown that other inhibitors of calcite ± aragonite can lead to the precipitation of ikaite, such as the presence of Mg in marine systems. Researchers working on calcium carbonate phases in laboratory experiments often come across ikaite, especially when working at low temperatures as stated above. Because ikaite is a metastable mineral phase at all P-T conditions found on Earth, kinetics is at play when ikaite is forming (i.e., rather than thermodynamics). With this Special Issue, we would like to narrow down what geochemical and perhaps even biogeochemical factors control ikaite formation at localities where ikaite is found in Nature. We must be aware that ikaite might not be the first mineral phase to precipitate, but it could be amorphous calcium carbonate (ACC,) as seen from laboratory experiments. Therefore, it is crucial to add to this Special Issue what these experiments have taught us about the different calcium carbonate phases, and what controls ikaite to form either directly from solution or from ACC, and why ikaite when encountered is favored as a calcium carbonate phase over calcite, aragonite, vaterite, and/or monohydrocalcite(s). For what time intervals can we expect ikaite to remain stable, and what mineral alteration pathways can be expected? As already observed both in nature and in experiments, ikaite does not always transform directly into calcite, but takes other mineral pathways depending on, for example, the chemistry of the aqueous environment or solution, possible temperature or pH changes inflicted on the system, and the rate of these changes. The compiled knowledge of this Special Issue of Minerals will be of importance to anyone working on ikaite and research attempting to use the presence of ikaite or what are believed to be pseudomorphs of ikaite as paleoclimate indicators. A Special Issue on Ikaite is highly called for in order to provide an overview of the multiple factors that can lead to its formation, and what could preserve them to eventually allow them to form pseudomorphs composed of calcite or aragonite.

Dr. Gabrielle J. Stockmann
Dr. Juan Diego Rodríguez-Blanco
Guest Editors

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