The order–disorder potential of the crystal structure of monohydrocalcite, CaCO₃·H₂O

Monohydrocalcite, CaCO3·H2O, forms a P31 structure composed of composite rods in which a spiral arrangement of Ca ions is accompanied by spiral arrangements of CO3 groups and of H2O molecules. The atom arrangement on the surface of the rods has rod symmetry P3121 whereas in the rod interior the atoms and atom groups have symmetry P31, which allows for two opposite orientations of a composite rod in respect to the [00.1] direction, connected with minimal change in the inter-rod bonding scheme.

The ordered crystal structure contains both rod orientations in a complicated periodic arrangement but various degrees of disorder should occur, generating only small energy penalty.