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Ikaite: the mystery mineral

Ikaite is a rare mineral that is chemically identical to calcite, but has a different crystal form which contains water in its structure. It is found naturally growing in places that are characterized by low temperatures and unusual chemistry. First discovered forming tufa towers in the Ikka Fjord in Greenland (from which it gets its name), it has since been discovered growing in seafloor sediments around continents. Yet, when it is collected, it rapidly breaks down to a wet crystal mush. Little is understood about what it breaks down to and how it breaks down. So a team of researchers designed a series of experiments to observe ikaite breaking down in the laboratory, in order to find answers.

Few studies examine natural ikaite (Ca(CO3)•6(H2O)) because it is so difficult to work with without it decomposing before you undertake the experiments on it. We were able to access a number of marine ikaites sampled from all over the world, including from Antarctica, Siberia, and Africa. The ikaites were frozen immediately upon collection, to stop them decomposing. Then, to transport the ikaites between collection sites and laboratories, they were carried in a thermos flask that had been pre-frozen and packed with ice. Despite warm weather and long delays during travelling, our samples made it intact to the lab (Fig. 1a&b).

Figure 1: (A) and (B) Ikaite samples taken straight from the freezer. (C) Powdering the ikaite in a polystyrene box flooded with liquid nitrogen. The mortar and pestle were soaked in liquid nitrogen to bring them down to -40 °C before the ikaite sample was added, and then powdered using the mortar and pestle. The second mortar was used to place the glass capillaries in and pour on liquid nitrogen in order to cool them. (D) The powdered sample being mounted in the diffractometer under the cryojet, which was set at -6 °C. (Credit: Vickers et al., 2022)

The most interesting question we wanted to answer, what ikaite breaks down to, we tried to find out using stepped heating in a powder X-ray diffractometer (PXRD), which can tell us the crystal structure of a sample. Yet, in order to use this instrument, the ikaites had to first be powdered without it getting too hot and transforming. So, we set up a “cold box” – a polystyrene box with liquid nitrogen (-40 °C) in, in which we put the mortar and pestle to grind the ikaites (Fig. 1c). Then the powder was put into capillaries (still in the cold box and using giant gloves to prevent cold burns), and mounted under a cryojet (a pipe blowing freezing cold air onto the sample, Fig. 1d). We also put one small sample into the scanning electron microscope to watch it transform at 21 C (Fig. 2).

Our experiments, as published in Vickers et al. (2022) show that, unlike previous studies on synthetic (i.e. laboratory-made) ikaite, natural ikaite transforms directly to calcite, without any intermediate phases, via a quasi- solid-state transformation. Our study therefore shows that laboratory-made ikaite may not be representative of the diverse ikaites found in nature, and our experiments mostly show that further studies are needed to really understand the behaviour of this strange mineral.

Image may contain: Slope, Rectangle, Font, Parallel, Triangle.
Figure 2: (A) Typical diffraction pattern for ikaite part way through transformation, showing only calcite and ikaite as the mineral phases present. (B) Ikaite undergoing transformation in the Scanning Electron Microscope, after 1 h 45 minutes. A crystal of calcite is forming as water is kicked out from the ikaite crystal structure. The space around the calcite crystal is because calcite, having no water, takes up much less space than ikaite. (Credit: Vickers et al., 2022)

Publication details: Vickers, M.L., Vickers, M., Rickaby, R.E., Wu, H., Bernasconi, S.M., Ullmann, C.V., Bohrmann, G., Spielhagen, R.F., Kassens, H., Schultz, B.P. and Alwmark, C., 2022. The ikaite to calcite transformation: implications for palaeoclimate studies. Geochimica et Cosmochimica Acta, 334, 201–216. https://doi.org/10.1016/j.gca.2022.08.001

 

By Madeleine Vickers
Published Aug. 28, 2022 8:23 PM - Last modified Aug. 28, 2022 8:23 PM
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The CEED blog covers some behind-the-scenes about our latest research and activities. The contributors are a mix of students and staff from The Centre for Earth Evolution and Dynamics, Dept. of Geosciences, University of Oslo, Norway.