Brief communicationThe crystal structure of Ca(ReO4)2 · 2H2O

The crystal structure of Ca(ReO4)2 · 2H2O should be described in space group C2c, not in space group Cc (R. G. Matveeva et al., Sov. Phys. Dokl. 25, 321, 1980), or in C2 (J. P. Picard et al., J. Solid State Chem. 69, 380, 1987). A crystal structure description based on an averaging of the two previous determinations agrees better with known interatomic distances and allows the assignment of a probable hydrogen bonding scheme. Space groups Cc and C2 are both monoclinic subgroups of order two of C2c. Descriptions of crystal structures in polar space groups are often difficult to diagnose as being of unnecessarily low symmetry, because the overlooked symmetry elements are not in conspicuous places of the chosen unit cell.