Description and crystal structure of liversidgeite, Zn6(PO4)4{middle dot}7H2O, a new mineral from Broken Hill, New South Wales, Australia

Abstract

Liversidgeite, ideally Zn6(PO4)4·7H2O, is a new mineral from Block 14 Opencut, Broken Hill, New South Wales, Australia. The mineral occurs as white, thin, bladed crystals and as hemispherical aggregates of radiating crystals in cavities in sphalerite-galena ore. Associated minerals are anglesite, pyromorphite, greenockite, sulfur, and an unknown Zn phosphate sulfate. Individual crystals are up to 0.1 mm in length and 0.05 mm across. Liversidgeite is transparent to translucent, with a vitreous luster and a white streak. It is brittle with an irregular fracture, the Mohs hardness is ~3-3.5, and the observed and calculated densities are 3.21(2) and 3.28 g/cm3, respectively. Chemical analysis by electron microprobe gave ZnO 54.62, MnO 0.49, PbO 0.18, P2O5 32.62, As2O5 0.65, SO3 0.35, H2O 14.04, total 102.95 wt%, with H2O content derived from the refined crystal structure. The empirical formula calculated on the basis of 23 O atoms is Pb0.01(Zn5.86Mn0.06)Σ5.92(P4.01As0.05S0.04)Σ4.10O16.20·6.8H2O. Liversidgeite is triclinic, space group P1̄, with a = 8.299(1), b = 9.616(1), c = 12.175(1) Å, α = 71.68(1), β = 82.02(1), γ = 80.18(1)°, V = 905.1(2) Å3 (single-crystal data), and Z = 2. The six strongest lines in the X-ray powder diffraction pattern are [d(Å), (I), (hkl)]: 8.438 (80) (011), 3.206 (60) (013̄), 2.967 (75) (21̄2, 114), 2.956 (75) (212̄), 2.550 (85) (233, 21̄3), 2.537 (100) (221̄, 014̄, 311̄). The crystal structure of liversidgeite was refined to an R1 index of 5.95% based on 3054 observed (Fo > 4σFo) reflections measured with MoKα X-radiation. The structure is based on two distinct, infinite zigzag chains of edge-sharing Znφ6 (φ = unspecified anion) octahedra that extend in the a direction. The chains link to each other via common corners and also via corner-sharing PO4 tetrahedra, forming sheets parallel to the (011) plane. The sheets link via [Zn2φ8] dimeric building units, comprising edge-sharing Znφ5 trigonal bipyramids and Znφ4 tetrahedra, resulting in an open framework. Large ellipsoidal channels extend along the a direction and are occupied by interstitial H2O groups and the H atoms of the H2O groups that coordinate to the Zn cations. An extensive network of hydrogen bonds provides additional linkage between the sheets in the structure, via the interstitial H2O groups. The topology of the liversidgeite structure is identical to that of synthetic, monoclinic Zn2Co4(PO4)4(H2O)5·2H2O.