Iranite, CuPb10(CrO4)6(SiO4)2(OH)2, isomorphous with hemihedrite

Abstract

This study presents the first structural report of iranite, ideally CuPb10(CrO4)6(SiO4)2(OH)2 [copper decalead hexachromate bis(orthosilicate) dihydroxide], based on single-crystal X-ray diffraction data. Iranite is isomorphous with hemihedrite, with substitution of Cu for Zn and OH for F. The Cu atom is situated at the special position with site symmetry 1. The CrO4 and SiO4 tetrahedra and CuO4(OH)2 octahedra form layers that are parallel to (120) and are linked together by five symmetrically independent Pb2+ cations displaying a rather wide range of bond distances. The CuO4(OH)2 octahedra are corner-linked to two CrO4 and two SiO4 groups, while two additional CrO4 groups are isolated. The mean Cr-O distances for the three nonequivalent CrO4 tetrahedra are all slightly shorter than the corresponding distances in hemihedrite, whereas the CuO4(OH)2 octahedron is more distorted than the ZnO4F2 octahedron in hemihedrite in terms of octahedral quadratic elongation.