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)4Cl2 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 R0=1.336 and b=0.53 A based on the crystal structures of selected hydroxychlorides.