A NEW CRYSTAL-CHEMICAL VARIATION OF THE ALUNITE-TYPE STRUCTURE IN MONOCLINIC PbZn0.5Fe3(AsO4)2(OH)6

The crystal structure of a segnitite- related mineral with composition Pb[Zn(0.5)square(0.5)]Fe(3)(AsO(4))(2)(OH)(6), square = vacancy, from Broken Hill, Australia, has been determined using single-crystal X-ray data. The mineral has monoclinic symmetry, space group C2/c, with a 25.8898(6), b 14.8753( 2), c 12.1700(2) angstrom, b 110.681(1), Z = 16. All crystals examined were found to be multiply twinned by 120 degrees rotations about c*, which corresponds with ch of the hexagonal alunite-type subcell. The structure was determined on a twinned crystal with one dominant individual, and refined to R = 0.055 for 4089 reflections with F > 4 sigma F. The general features of the monoclinic structure match those of the prototype rhombohedral alunite-type structure. This involves a rhombohedral stacking of hexagonal tungsten-bronze-type sheets of corner-connected octahedra that share their apical anions with AsO4 tetrahedra on either side of the layer of octahedra, with Pb atoms located between the layers. However, the structure differs from those of other alunite-type minerals in having zinc atoms ordered in trigonal bipyramidal sites in half of the available six-membered rings of the layers of octahedra. These sites are unoccupied in previously reported alunite-type structures. Ordered displacements of the Pb atoms occur in response to the Zn-square ordering.