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
Z=4
Atomic Positional Parameters
Reference
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.