Ikaite

Ikaite crystallizes in the monoclinic crystal system in space group C2/c with lattice parameters a~8.87A, b~8.23A, c~11.02A, β~110.2°. The structure of ikaite consists of an ion pair of (CaCO3) surrounded by a cage of hydrogen-bonded water molecules which serve to isolate one ion pair from another.

It is usually considered a rare mineral, but this is likely due to difficulty in preserving samples. It was first discovered in nature by the Danish mineralogist Pauly in the Ikka (then spelt Ika) fjord in southwest Greenland, close to Ivittuut, the locality of the famous cryolite deposit. Here ikaite occurs in truly spectacular towers or columns (up to 18 m or 59 ft tall) growing out of the fjord floor towards the surface water, where they are naturally truncated by waves, or unnaturally by the occasional boat. At the Ikka Fjord, it is believed that the ikaite towers are created as the result of a groundwater seep, rich in carbonate and bicarbonate ions, entering the fjord bottom in the form of springs, where it hits the marine fjord waters rich in calcium. Ikaite has also been reported as occurring in high-latitude marine sediments at Bransfield Strait, Antarctica; Sea of Okhotsk, Eastern Siberia, off Sakhalin; and Saanich Inlet, British Columbia, Canada. In addition it has been reported in a deep sea fan off the Congo, and therefore probably has worldwide occurrence. The most recent occurrence has been reported by Dieckmann et al. (2008). They found the mineral ikaite directly precipitated in grain sizes of hundreds of micrometers in sea ice in the Weddell Sea and throughout fast ice off Adélie Land, Antarctica. In addition, ikaite can also form large crystals within sediment that grow to macroscopic size, occasionally with good crystal form. There is strong evidence that some of these marine deposits are associated with cold seeps. Ikaite has also been reported as a cryogenic deposit in caves where it precipitates from freezing carbonate-rich water.

Isotope geochemistry can reveal information about the origin of the elements that make up minerals. The isotopic composition of ikaite and the pseudomorphs is actively studied. Studies of the ratio of C to C in ikaite relative to a natural, standard ratio can help to determine the origin of the carbon pool (organic/inorganic) which was consumed to form ikaite. Some studies have shown that oxidizing methane is the source of both modern day ikaite and glendonites in high latitude, marine sediments. Similarly the ratio of O to O, which varies in nature with temperature and latitude, can be used to show that glendonites were formed in waters very close to the freezing point, in agreement with the observed formation of ikaite.