Images
br
Formula: Ca(CO3).6H2O
Hydrated carbonate
Crystal System: Monoclinic
Specific gravity: 1.77 measured, 1.833 calculated
Streak: White
Colour: Chalky white
Environments
Ikaite is a very rare carbonate that forms in sea water and lake water in anaerobic (oxygen deficient),
organic-rich periglacial (subject to repeated freezing and thawing) and glaciomarine (containing both glacial ice
and marine water) environments near 0oC, readily converting to
calcite at higher temperatures
of about 8oC
(HOM, Mindat, Webmin).
Localities
At Bransfield Strait, Antarctic Peninsula, Western Antarctica, Antarctica, ikaite has been discovered in 1,950 metres
of water and at a bottom temperature of minus 1.6°C as translucent, amber crystals up to 8 cm in length
(R&M 97.6.496-509).
At White Cliffs, New South Wales, Australia, opal
pseudomorphs after ikaite have been found
(KL p259).
At the type locality, Ikka Bund, Ikka Fjord, Sermersooq, Greenland, Denmark, chalklike underwater pillars occur in the
inner part of the fjord, 8 km south of Ivigtut. The pillars reach to within half a metre
of the water surface. The temperature at the base of the pillars was 3oC and 7o at the top.
Samples were collected by a frogman and shipped in a refrigerator at about 4oC
(AM 49.439).
The framework of newly formed ikaite tufa in Ikka Fjord is
uniformly composed of an interlocking mass of 0.1 to 2 mm transparent euhedral ikaite crystals, but discrete
macroscopic crystals have not been documented from here
(R&M 97.6.496-509).
At the Olitsna river, White Sea coast, Karelia Republic, Russia, calcite
pseudomorphs after ikaite, called glendonites, have been found
(KL p154).
At Bielo More, Kola Peninsula, Murmansk Oblast, Russia, pseudomorphs of
calcite after ikaite to 10 cm have been found
(Extra Lapis English-4 p23).
At Point Barrow, North Slope Borough, Alaska, USA, ikaite occurs predominatly as single bladelike crystals.
When it alters to calcite, even as water is released internally or into its
surroundings the
calcium carbonate molecules begin to grow together to form a second generation crystalline structure. Ikaite
crystals from Barrow that had been experimentally set out to dehydrate had recrystallised to the point where they were
firm enough to be handled after several months, although they were still extremely fragile
(R&M 97.6.496-509).
At Mono Lake, Mono county, California, USA, the tufa towers around
the Lake are now exposed due to lowering of lake levels. The tufa
towers lack any macroscopic crystals of ikaite, or their
pseudomorphs.
Some ikaite that had precipitated naturally under fridgid nighttime temperatures and returned to a US
laboratory subsequently converted to vaterite when allowed to dehydrate.
Microcrystalline ikaite identified in the lake waters was also found to convert directly to
vaterite and calcite
(R&M 97.6.496-509).
Formation
Ikaite may form under at least two sets of environmental circumstances. Both modes of formation require:
(1) temperatures near the freezing point of water
and
(2) some chemical additive or component in the water column or in the porewaters of the enclosing sediments that
inhibits the formation of calcite and allows instead for the formation of
ikaite.
There is some evidence that phosphate and/or Mg2+ ions favour ikaite precipitation through
inhibition of calcite formation.
The primary environment of ikaite formation appears to be authigenic formation in organic-rich marine or
marginal marine sediments that are undergoing microbial decomposition by sulphate-reducing bacteria. In some of the
subbottom seafloor occurrences of ikaite, the presence of carbon from the anaerobic microbial oxidation of
methane may also be a contributing factor in the formation of ikaite.
The second environment of ikaite formation occurs in aquatic environments, and appears to have formed from the
mixing of bicarbonate alkaline subsurface spring waters in either a marine or saline lake environment
(R&M 97.6.496-509).
Alteration
At temperatures only a few degrees above the freezing point of water, ikaite dehydrates to
calcite, releasing free water molecules. Without undue
disturbance, the external appearance of the original ikaite crystal will normally be maintained. However, any
internal structural similarity is lost, which fulfils the definition of becoming a
pseudomorph. Ikaite crystals undergo a considerable decrease in
interior volume as a result of water loss, and the new pseudomorph is
thus very porous and fragile. The process of the final transformation of ikaite to the
calcite found in glendonites is still unclear and may involve a stepwise
transformation via a hydrated carbonate such as monohydrocalcite or
one of the anhydrous paramorphs of CaCO3
(calcite, vaterite or
aragonite). Laboratory grown ikaite converts directly to
vaterite when allowed to rapidly dehydrate at room temperatures
(R&M 97.6.496-509).
Back to Minerals