The crystal structure of keyite, Cu-3(2+)(Zn,Cu2+)(4)Cd-2(AsO4)(6)(H2O)(2), an oxysalt mineral with essential cadmium

The crystal structure of keyite, Cu-3(2+)(Zn,Cu2+)(4)Cd-2(AsO4)(6)(H2O)(2), a 11.654(3), b 12.780(5), c 6.840(3) Angstrom, beta 99.11(3)degrees, V 1005.8(6) Angstrom(3), I2/a, Z = 2, was solved by direct methods and refined to an R index of 9.3% based on 1123 observed reflections measured with MoK alpha X-radiation. There are two Cu2+ sites; Cu(1) is octahedrally coordinated by four oxygen atoms and two H2O groups, and is exactly half-occupied by Cu2+; Cu(2) is coordinated by four oxygen atoms in a square-planar arrangement. There is one Zn site, occupied by Zn and minor Cu2+, and octahedrally coordinated by six oxygen atoms. There is one Cd site occupied dominantly by Cd2+ and coordinated by six oxygen atoms in a trigonal prismatic arrangement. There are two As sites, both occupied by As5+ and tetrahedrally coordinated by oxygen atoms. The Cu(1)phi(6) (phi: unspecified ligand) octahedra share comers to form a [Cu phi(5)] chain in which only alternate octahedra are occupied. This chain is flanked by AsO4 tetrahedra and CdO6 polyhedra to form a heteropolyhedral ribbon that extends along the c axis. These ribbons are cross-linked by CuO4 groups into a sheet parallel to (100). These sheets alternate with parallel sheets of [Zn2O10] octahedral dimers and AsO4 tetrahedra to form a very complex heteropolyhedral framework. The formula of keyite is extensively revised from the original description, but the revised formula agrees closely with the results of original chemical analysis and of electron-microprobe analyses obtained as part of this work. The new formula shows that keyite contains essential Cd.