Abstract

The crystal structure of chloroxiphite, Pb3CuO2(OH)2Cl2, from Merehead Quarry (monoclinic, P21/m, a = 6.6972(8), b = 5.7538(5), c = 10.4686(14) Å, β = 97.747(10)°, V = 399.72(8) Å3) has been refined to R1 = 0.041. The structure contains three symmetrically unique Pb sites and one Cu site. The strong distortion of the Pb2+ coordination polyhedra is due to the stereoactivity of the s2 lone electron pairs on the Pb2+ cations. The Cu-site is coordinated by four OH groups to form an almost planar Cu(OH)4 square that is complemented by two apical Cl anions, forming an elongated [Cu(OH)4Cl2] octahedron. Because of the large size and variability of coordination polyhedra around Pb2+ cations and the strength of the Me–O bonds in comparison to the Me–Cl bonds (Me = metal), it is convenient to describe the structure of chloroxiphite in terms of oxocentred OPb4 tetrahedra. The O1 atom is tetrahedrally coordinated by four Pb2+ cations forming relatively short and strong O–Pb bonds. The OPb4 tetrahedra link together via common edges to form [O2Pb3]2+ double chains. The difference between chloroxiphite and other natural oxyhalides is the presence of Cu2+ cations which form an independent structural unit that links to units formed by OPb4 tetrahedra. In this sense, chloroxiphite can be considered as a modular structure consisting of both strong cation- and anion-centred units.

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