Ikaite

Ikaite
Calcite after Ikaite var. Glendonite concretion
General
Category	Carbonate mineral, hydrous carbonates subgroup
Formula (repeating unit)	CaCO3·6H2O
Strunz classification	5.CB.25
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) (same H-M symbol)
Space group	C2/c
Identification
Color	White when pure
Crystal habit	Nearly square prism; pyramidal; sigmoidal: square prism capped with oppositely canted pyramids; massive, tubular (tinolite vr.)
Mohs scale hardness	3
Luster	Dull
Streak	White
Specific gravity	1.83
Optical properties	Biaxial (-)
Refractive index	nα = 1.455 nβ = 1.538 nγ = 1.545
Birefringence	d = 0.090
Other characteristics	Decomposes into water and calcite above 8 °C
References

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.

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 SW Greenland, close to Ivigtut, the locality of the famous cryolite deposit. Here ikaite occurs in truly spectacular towers 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.