NEW CU2+ COORDINATION POLYHEDRA IN THE CRYSTAL STRUCTURE OF BURNSITE, KCDCU7O2(SEO3 )2CL9

Abstract

The crystal structure of burnsite, KCdCu7O2(SeO3)(2)Cl-9, a new mineral species from fumaroles of the Tolbachik volcano, Kamchatka, Russia, has been solved by direct methods and refined on the basis of F-2 for all unique data collected with monochromatic MoKalpha X-radiation and a CCD (charge-coupled device) detector to an agreement factor (R I) of 1.9%, calculated using the 460 unique observed reflections (\F-O\ = 4sigma(F)). The mineral is hexagonal, space group P6(3)\mmc, a 8.7805(8), c 15.521(2) Angstrom, V 1036.3(2) Angstrom(3), Z = 2. There are two non-equivalent Cu2+ cations in the structure of burnsite. The Cu(l) cation is coordinated by three atoms of 0 and three of Cl; this is the first example of this type of mixed-ligand Cu(2+)phi(6) (phi: O2-, Cl-) octahedron in a mineral. The octahedron is (2 + 4)-distorted owing to the Jahn-Teller effect, in contrast to the vast majority of Cu2+ oxysalt minerals that contain (4 + 2)- distorted Cu2+ octahedra. The Cu(2) cation is coordinated by two 0 and three Cl anions arranged at the vertices of a trigonal bipyramid; this is the first occurrence of a mixed-ligand Cu2+O2Cl3 trigonal bipyramid in a mineral. The single symmetrically independent Cd2+ cation is coordinated by six Cl anions located at the vertices of a regular octahedron. The structure contains one symmetrically independent K cation that is coordinated by six Cl anions in a trigonal prismatic arrangement. There is one unique Se4+ cation that is strongly bonded to three 0 anions on one side of the cation, owing to the presence of a s(2) lone-electron pair. The structure is described both in terms of cation-centered polyhedra and oxocentered OCu4 tetrahedra. The structure of burnsite is closely related to the structures of other natural copper oxide chloride selenites discovered in the Tolbachik fumaroles (chloromenite, georgbokiite, ilinskite).