The crystal structure of parageorgbokiite, β-Cu5O2(SeO3)2Cl2

The crystal structure of parageorgbokiite, beta-Cu5O2(SeO3)(2)Cl-2, a new mineral species from the fumaroles of the Great Fissure Tolbachik eruption, in the Kamchatka Peninsula, Russia, has been solved from single-crystal X-ray-diffraction data by direct methods and refined to R-1 = 0.032 for 1388 unique reflections with vertical bar F-o vertical bar >= 4 sigma(F). The mineral is monoclinic, space group P2(1)/c, a 5.3982(5), b 8.0543(8), c 11.1277(10) angstrom, beta 99.258(2)degrees, V 477.52(8) angstrom(3), Z = 2. The structure contains three symmetrically unique Cu positions, each octahedrally coordinated by O2- and Cl- anions. There is one symmetrically independent Se4+ cation coordinated by three 0 atoms, to form SeO3 trigonal pyramids. The structure of parageorgbokiite is best described in terms of arrangements of the strong M-O bonds (M = Se4+ and Cu2+). The O1 atom is not bonded to Se4+ cations and is tetrahedrally coordinated by four Cu2+ cations, which results in the formation of an oxocentered OlCu(4) tetrahedron. Two OlCu(4) tetrahedra share a Cu3... Cu3 edge to form O2Cu6 dimers that are further linked by sharing the Cu2 corners to form O2Cu5 infinite chains parallel to the a axis. The SeO3 pyramids are attached to the faces of the oxocentered tetrahedra in a "face-to-face" position to form complex ([O2Cu5][SeO3]2)(2+) chains and a three-dimensional framework with channels occupied by the Cl- anions. A comparison of parageorgbokiite and georbokiite shows that the two minerals have the same space group P2(1)/c (monoclinic), but differ in their unit-cell parameters. The structures of both minerals are based upon the same complex ([O2Cu5][SeO3](2))(2+) chains. However, the arrangements of the chains in the structures are different. The channels in parageorgbokiite are larger than those in georgbokiite. From the lower density of parageorgbokiite in comparison to georbokiite, we suggest that parageorgbokiite is a high-temperature modification Of Cu5O2(SeO3)(2)Cl-2 that is either chemically or kinetically stabilized at ambient conditions.