Ikaite is the
mineral name for the hexahydrate of
calcium carbonate, CaCO3·6H2O. Ikaite tends to form very steep or spiky pyramidal crystals, often radially arranged, of varied sizes from
thumbnail size
aggregates to gigantic salient spurs. It is only found in a
metastable state and decomposes rapidly by losing most of its water content once removed from near-freezing water. This "melting mineral" is more commonly known through its
pseudomorphs.
Distribution
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[5] in the
Ikka Fjord in southwest
Greenland, close to
Ivittuut, the locality of the famous
cryolite deposit.[6][7] 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.[8][9] At the Ikka Fjord, it is supposed that the ikaite towers are created as the result of a groundwater seep, rich in carbonate and
bicarbonateions, entering the fjord bottom in the form of springs, where it hits the marine fjord waters rich in calcium.[9] Ikaite has also been reported as occurring in high-latitude marine
sediments at Bransfield Strait,
Antarctica;[10]Sea of Okhotsk, Eastern
Siberia, off
Sakhalin;[11] 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).[12] 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.[13] Ikaite has also been reported as a cryogenic deposit in caves where it precipitates from freezing carbonate-rich water.[14]
Structure
Ikaite crystallizes in the
monocliniccrystal system in
space group C2/c with lattice parameters a~8.87A, b~8.23A, c~11.02A, β~110.2°.[15][16] The structure of ikaite consists of an ion pair of (Ca2+CO32−)0 surrounded by a cage of hydrogen-bonded water molecules which serve to isolate one ion pair from another.[17]
Stability
Synthetic ikaite was discovered in the nineteenth century in a study by Pelouze.[18] Ikaite is only
thermodynamically stable at moderate pressures, so when found near the Earth's surface is always
metastable.[19][20] Nevertheless, as it appears to be at least moderately common in nature, it is clear that the conditions for metastable
nucleation and growth cannot be too restrictive. Cold water is certainly required for formation, and nucleation inhibitors like phosphate ions for the growth of anhydrous
calcium carbonate phases, such as
calcite,
aragonite, and
vaterite probably aid its formation and preservation. It is thought that perhaps the structure of calcium carbonate in a concentrated aqueous solution also consists of an ion pair, and that this is why ikaite readily nucleates at low temperatures, outside of its thermodynamic stability range. When removed from its natural cold water environment, ikaite rapidly disintegrates into
monohydrocalcite or anhydrous calcium carbonate phases and water, earning the nickname of the melting mineral.
Pseudomorphs
The presence of ikaite may be recorded through geological time through the presence of
pseudomorphs of other
calcium carbonate phases after it.[21] Although it can be hard to uniquely define the original mineral for every specimen, there appears to be good evidence for ikaite as the precursor for the majority of the following locality names of pseudomorphs:
Glendonite, after type locality, Glendon, New South Wales, Australia.
Thinolite, (Gr. Thinos = shore) found in the
tufa of
Mono Lake, California, US[22][23]
The common ingredient appears to be cold temperatures, although the presence of traces of other chemicals such as nucleation inhibitors for anhydrous calcium carbonate may also be required. It has also been reported as forming in winter on
Hokkaido at a saline spring.
Since cold water can be found at depth in the oceans even in the tropics, ikaite can form at all latitudes. However, the presence of ikaite pseudomorphs can be used as a
paleoclimate proxy or
paleothermometry representing water near freezing conditions.[29][30]
Thinolite deposits
Thinolite is an unusual form of calcium carbonate found on the shore (Greek: thinos = shore) of
Mono Lake,
California. This and other lakes now largely in the desert or semi-desert environments of the southwestern US were part of a larger post-glacial lake that covered much of the region near the end of the last glaciation. It is thought that at this time, conditions similar to that of the Ikka fjord allowed for the growth of massive ikaite.
Isotope geochemistry
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.[31] Studies of the ratio of 13C to 12C 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.[32] 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 18O to 16O, 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.
^Buchardt, B., Seaman, P., Stockmann, G., Wilken, M.V.U., Duwel, L., Kristiansen, A., Jenner, C., Whiticar, M. J., Kristensen R.M., Petersen, G.H., and Thorbjorn, L. (1997). "Submarine columns of ikaite tufa". Nature. 390 (6656): 129–130.
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abBuchardt, B., Israelson, C., Seaman, P., and Stockmann, G. (2001). "The Ikaite tufa towers in Ikka Fjord, SW Greenland: Formation by mixing of seawater and alkaline spring water". Journal of Sedimentary Research. 71 (1): 176–189.
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^Bischoff, J. L., Stine, S., Rosenbauer, R. J., Fitzpatrick, J. A., Stafford Jr, T. W. (1993). "Ikaite precipitation by mixing of shoreline springs and lake water, Mono Lake, California, USA". Geochimica et Cosmochimica Acta. 57 (16): 3855–3856.
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^Ekaterina Bazarova, Alexander Kononov and Oksana Gutareva (2016). Cryogenic Mineral Formations in the Okhotnichya Cave in the primorsky Mountain Ridge (Western Baikal Region, Russia), Eurospeleo Magazine 3: 47–59.
^
abDickens, B.; Brown, W.E. (1970). "The crystal structure of calcium carbonate hexahydrate at ~120 °C". Inorganic Chemistry. 9 (3): 480–486.
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^Kaplan, M.E. (1979). "Calcite pseudomorphs (pseudogaylusite, jarrowite, thinolite, glendonite, gennoishi) in sedimentary rocks. The origin of pseudomorphs (in Russian)". Lithology and Mineral Resources. 5: 125–141.
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abE.S. Dana (1884). "A crystallographic study of the thinolite of Lake Lahontan". U.S. Geological Survey Bulletin (12): 429–450.
^Browell, E. J. J. (1860). "Description and analysis of an undescribed mineral from Jarrow Slake". Tyneside Naturalists Field Club. V: 103–104.
^Shearman,D.J.; Smith, A.J. (1985). "Ikaite, the parent mineral of jarrowite-type pseudomorphs". Proceedings of the Geological Association, London. 96 (4): 305–314.
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^Kennedy,G.L.; Hopkins, D.M.; Pickthorn, W.J. (1987). "Ikaite, the glendonite precursor, in estuarine sediments at Barrow, Arctic Alaska". Annual Meeting Abstract Program. 9. Geological Society of America: 725. {{
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^Swainson, I.P.; Hammond, R.P. (2001). "Ikaite, CaCO3·6H2O: Cold comfort for glendonites as palaeothermometers". American Mineralogist. 86 (11–12): 1530–1533.
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^Shearman, D.J.; McGugan, A.; Stein, C.; Smith, A.J. (1989). "Ikaite, CaCO3·6H2O, precursor of the thinolites in the Quaternary tufas and tufa mounds of the Lahontan and Mono Lake Basins, western United States". Geological Society of America Bulletin. 101 (7): 913–917.
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^Whiticar, M.J.; Suess, E. (1998). "The cold carbonate connection between Mono Lake, California and the Bransfield Strait, Antarctica". Aquatic Geochemistry. 4 (3/4): 429–454.
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^Schubert, C.J.; Nunberg, D.; Scheele, N.; Pauer, F.; Kriews, M., C. J.; Nürnberg, D.; Scheele, N.; Pauer, F.; Kriews, M. (1997). "13C isotope depletion in ikaite crystal: evidence for methane release from the Siberian shelves?". Geo-Marine Letters. 17 (2): 169–174.
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