Editorial for Special Issue “Research on Ikaite—Natural Occurrences and Synthetic Mineral Precipitation”

This Special Issue consists of five articles published in the period of 2022–2025, with a special focus on the metastable calcium carbonate mineral ikaite (CaCO₃·6H₂O) and the pseudomorph type known as glendonite. Glendonites are recognized throughout the geological record, all the way back to the Precambrian, and are suspected to have ikaite as their parent mineral, but they now consist of mainly calcite (CaCO₃). Ikaite crystals have been found in a wide variety of environments around the world. In general, the findings have been characterized by low soil/sediment/water temperatures, close to freezing point. For that reason, glendonites have been interpreted as being formed in cold paleoclimate conditions. In the five papers presented here, we receive a closer look into the conditions that form ikaite in the present day in marine and deep-sea environments, and what happens to a marine ecosystem with ongoing ikaite precipitation under changing climate conditions. In addition, we are provided with a fantastic overview of the huge museum collection of glendonites from different localities and geological periods. The authors have performed tremendously in describing the crystallographic details of more than a thousand glendonites from various museum collections, supplemented with impressive photo material and interpretations of the chemical conditions that lead first to ikaite, and then later to glendonite pseudomorph formation.

In summary, in the first paper by Stockmann et al. (2022) [1], ‘Mineral Changes to the Tufa Columns of Ikka Fjord, SW Greenland’, we visit the type locality for ikaite, namely Ikka Fjord in SW Greenland. In contrast to other findings on ikaite, the mineral at this locality precipitates in massive forms of up to 20 m tall tufa columns. What is even more unique is the rapid new growth of columns of 0.5 m ikaite precipitates per year, as a result of the mixing of alkaline groundwater of pH 10 with cold seawater. At the same time, the nearly a thousand columns are situated in the shallow part of a tiny fjord that is exposed to the effects of the Arctic amplification of both air and seawater temperatures. In the summer of 2019, all the seawater surrounding the submarine ikaite columns was above 6 °C. Based on in situ seawater temperature measurements of Ikka Fjord and the mineral analysis of ikaite columns over approximately 25 years, this is the upper stability threshold for the ikaite of Ikka Fjord. What was observed and described in this article was how the mineralogy of the columns had changed from primary ikaite in some parts into secondary monohydrocalcite (CaCO₃·H₂O) and aragonite (CaCO₃) [1]. This is interpreted as a direct consequence of the heating of seawater, leading to the dehydration of primary ikaite. Aragonite is formed instead of calcite as a result of the high concentrations of Mg²⁺ in seawater, blocking calcite growth. Hence, the mineral alterations in Ikka Fjord can give hints as to the possible pseudomorph formation of ikaite in a marine environment at a low temperature (<6 °C). There is no direct transformation of ikaite into calcite under these conditions.

In the second paper by Whiticar et al. (2022) [2], ‘Calcium Carbonate Hexahydrate (Ikaite): History of Mineral Formation as Recorded by Stable Isotopes’, we focus on another deposit of large ikaite crystals, this time the organic-carbon-rich sediments in the King George Basin of the Bransfield Strait in Antarctica. Here, the precipitation process is different from that of Ikka Fjord, and interpreted to be as a result of the early diagenetic decomposition of organic matter under cold water (−1.4 °C) and high pressure (200 bar) conditions. The authors use stable isotopes to determine the formation history of ikaite under these conditions. They conclude that the nucleation of the ikaite crystals in the Bransfield Basin sediments may be induced by high alkalinity, high phosphate concentrations, and dissolved organic compounds, in addition to low in situ water temperatures and high pressure [2]. What should not be overlooked in the precipitation history of ikaite for both the Bransfield Strait and the Ikka Fjord system, is the microbial effect. The authors argue for the case of the Bransfield Strait that intense microbial metabolism generates organic compounds such as aspartic acid and glutamic acid, which may play an important role in ikaite mineral precipitation, as they do in biological and extracellular carbonate mineral precipitation [2].

In the third and fourth articles by Schultz et al. (2023a) [3], ‘Links between Ikaite Morphology, Recrystallised Ikaite Petrography and Glendonite Pseudomorphs Determined from Polar and Deep-Sea Ikaite’, and Schultz et al. (2023b) [4], ‘Transgression Related Holocene Coastal Glendonites from Historic Sites’, the focus is on glendonite pseudomorphs. The glendonites studied by Schultz et al. (2023a) [3] represent samples of recrystallized ikaite from deep sea sediments extracted from the Nankai Trough in offshore Japan, the Congo deep-sea fan, the Laptev Sea, South Georgia, and the Okhotsk Sea, in addition to the coastal lagoon at Point Barrow in Alaska. At these sites, we find recrystallized ikaite occurring as mm large, zoned calcite crystals. The data presented highlight the details recorded on the origin, storage, and recrystallization process of these naturally formed ikaite crystals [3]. They describe the glendonite petrographic characteristics to be as a consequence of the structure and chemistry of the recrystallizing ikaite, rather than the physical or geochemical environment. One of the most significant results is the variation in calcite types within a single sample, leading to the observation that the variation is a consequence of impurities and geochemical variability within the ikaite crystal, rather than the external environment [3]. The authors conclude that the similarities in the morphology and internal textures between the recrystallized ikaite and glendonite confirm that glendonite may be used as an indicator of the presence of ikaite in the past [3]. In Schultz et al. (2023b) [4], they examine the findings of coastal glendonite from eleven historical sites. These have formerly been known under different names such as Pseudogaylussite, Fundylite, and Kool Hoot. The authors use ¹⁴C radiometric dating to better confirm the relative age between the localities. They range in age from 10 to 1 thousand years before the present (Ky BP), with the oldest glendonite locations being the Kool Hoot (Alaska) and the river Clyde (Scotland), and the youngest the Bay of Fundy in Canada [4]. The glendonites from the Wadden Sea are intermediate in age, while the age of the glendonites from the Olenitsa site on the Russian Kola Peninsula is still debated. However, the authors stress that although ¹⁴C radiometric dating provides an age, the results can be erroneous due to the inclusion of older carbon sources in the analysis [4]. Another finding of this paper is the method description of how the Mg/Ca ratio that provides data on the recrystallized ikaite preserved as calcite is influenced by diagenetic pore waters [4].

Finally, Schultz and Huggett (2025) [5] in ‘Advances in Glendonite Understanding and Its Potential for Carbon Capture’, make an impressive effort to describe the morphology and petrology of 1100 glendonite samples from various museum collections and present a definition on how samples can be classified as glendonite. Furthermore, the authors evaluate the knowledge gained from glendonite formation in the context of carbon capture via calcium carbonate mineralization [5]. Perhaps a future method for Carbon Capture and Storage (CCS) can be extracted from these enigmatic pseudomorphs of the past?