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Ikaite [CaCO3.6H2O]

Structure Monoclinic
Space Group : C12/c (No. 15)
a=8.8700Å, b=8.2300Å, c=11.0200Å
a=90.0, b=110.2, g=90.0

Atomic Positional Parameters
Ca(1) 4e 0.5000 0.6472 0.2500
C(1)  4e 0.5000 0.3067 0.2500
O(1)  4e 0.5000 0.1508 0.2500
O(2)  8f 0.5263 0.3849 0.1582
O(3)  8f 0.6163 0.7229 0.0916 [H2O]

O(4)  8f 0.7883 0.5576 0.3825 [H2O]
O(5)  8f 0.6703 0.8842 0.3593 [H2O]
H(1)  8f 0.5680 0.6960 0.0020
H(2)  8f 0.6440 0.8350 0.0920
H(3)  8f 0.8720 0.5980 0.3540

H(4)  8f 0.8220 0.5880 0.4720
H(5)  8f 0.7740 0.8910 0.3500
H(6)  8f 0.6160 0.9810 0.3190

Dickens B and Brown WE (1970) Inorganic Chemistry, 9, 480

Mineral Chemistry
Ikaite is a hydrous calcium carbonate mineral (CaCO3.6H2O) which forms under unusual conditions in nature. Ikaite has been found in the Ikka (formerly Ika) fjord in south-west Greenland, and in Antarctic marine sediments. Unusual calcite mounds associated with lakes present during periods of glaciation are now thought to have formed as ikaite structures with subsequent transformation to calcite replacing ikaite.

Experiments replicating ikaite formation in nature have shown that ikaite may be synthesised by reaction of alkaline carbonate-rich aqueous fluids with calcium ions at 0oC. Ikaite has a field of stability at high pressures: CaCO3.6H2O occupies a smaller volume than CaCO3 + 6 H2O.

The crystal structure of ikaite consists of an array of calcium ions each coordinated by one carbonate group and six water molecules. Hydrogen bonding ties the discrete CaCO3.6H2O units together.

The CHIME figure shows calciums as large grey spheres, oxygens as small red spheres, and carbons as small light grey spheres. Hydrogens are white spheres. Note the eight-fold coordination of calcium by oxygen. The unit cell is outlined.