Cu3MgCl2(OH)6 and the bond-valence parameters of the OH-Cl bond

The title compound is a new mineral consisting of sheets made from edge-sharing Cu(OH)(4)Cl-2 and Mg(OH)(6) octahedra. The sheets are only weakly linked by O-H-Cl bridges, giving rise to perfect cleavage along (001) and to strong texture effects in powder diffraction measurements. Owing to low bond-valence sums at the Cl sites Of Cu3MgCl2(OH)(6) and of several related compounds, bond-valence parameters for the H/Cl pair have been refined to R-0 = 1.336 and b = 0.53 angstrom based on the crystal structures of selected hydroxychlorides.