PARAKHINITE, CU32+PBTE6+O6(OH)(2) - CRYSTAL-STRUCTURE AND REVISION OF CHEMICAL FORMULA

The crystal structure of parakhinite, Cu32+PbTe6+O6(OH)(2), trigonal, a 5.765(2), c 18.001(9) Angstrom, V 518.0(4) Angstrom(3), Z = 3, space group P3(2), has been solved by direct methods and refined by least-squares methods to an R index of 8.1% and a wR index of 7.7% using 744 unique observed [I greater than or equal to 2.5 sigma(I)] reflections collected on a twinned crystal. The chemical formula of parakhinite has been revised as a result of this determination of the crystal structure. Parakhinite contains one Pb position; the Pb is coordinated by six oxygen anions and two hydroxyl groups. The single Te6+ position is octahedrally coordinated by six oxygen anions. There are three unique Cu2+ positions, each of which is coordinated by a (4 + 2)-distorted octahedral arrangement of anions, the distortion being due to the Jahn-Teller effect. Cu(1) and Cu(2) are coordinated by four oxygen anions and two hydroxyl groups, and Cu(3) is coordinated by six oxygen anions. Parakhinite contains corrugated sheets of octahedra parallel to (001). These sheets are linked via interstitial Pb atoms bonded to four anions in each adjacent sheet, and via hydrogen bonds. The parakhinite structure is approximately compatible with P6(2) symmetry. The group-subgroup relationships and optical characteristics suggest that parakhinite goes through a series of phase transitions on cooling.