The Crystal Structure of Calcium Carbonate Hexahydrate at similar to-120 degrees

The crystal structure of CaCO3.6H(2)O has been determined from 1420 X-ray diffraction data collected photographically by the oscillation technique from a single crystal held at similar to-120 degrees. The unit cell parameters are a = 8.87 (2) angstrom, b = 8.23 (1) angstrom, c = 11.02 (2) angstrom, and beta = 110.2 (2)degrees, and the space group is C2/c with Z = 4. The calculated density at,similar to,-120 degrees is 1.80 g cm(-1), and the observed density at similar to 0 is 1.82 g cm(-3). The final R factor is 0.10. The structure contains discrete CaCO2 ion pairs, each surrounded by an envelope of 18 water molecules. Thus, Ca2+ is coordinated to only one CO32-. Six of the surrounding H2O molecules are bonded to Ca2+, eight are hydrogen bonded to oxygens of the CO3 - group, and four are bonded to adjacent ion pairs and to other water molecules in the envelope. CaCO2 center dot 6H(2)O was found to be more stable than CaCO2 H2O in water near 0 degrees. The formation of CaCO2.6H(2)O from an equivalent amount of calcite (CaCO2) and water is accompanied by a 20% decrease in volume. This may be important in explaining the scarcity of calcareous material in life in the ocean depths.