Description and crystal structure of nyholmite, a new mineral related to hureaulite, from Broken Hill, New South Wales, Australia

Abstract

Nyholmite, Cd3Zn2(AsO3OH)2(AsO4)2·4H2O, from the Block 14 Opencut, Broken Hill, New South Wales, Australia, is a new Cd-Zn arsenate species, isostructural with the minerals of the hureaulite group. The mineral occurs in a quartz-garnet-arsenopyrite matrix as white globules, tufted aggregates of fibrous crystals and radiating hemispheres of thin, colourless, bladed crystals. Associated minerals are goldquarryite, lavendulan-sampleite, scorodite-strengite and gypsum. Individual crystals are up to 0.2 mm in length and 0.05 mm across. The mineral is transparent to translucent with a vitreous lustre. It is brittle with an uneven fracture and a white streak. The Mohs hardness is 3–3.5 and the calculated density is 4.23 g cm–3 for the empirical formula. Electron microprobe analyses yielded CdO 34.58, ZnO 9.72, MnO 3.59, CuO 3.39, Al2O3 0.20, CaO 0.16, PbO 0.37, As2O5 34.55, P2O5 6.29 totalling 92.85 wt.%. The empirical formula, based on 20 oxygen atoms, is Ca0.03Pb0.02 Cd2.80Al0.04Zn1.24-Cu0.44Mn0.53[(AsO4)3.13(PO4)0.92]Σ4.05H1.91·3.79H2O. Nyholmite is monoclinic, C2/c, a = 18.062(4) Å, b = 9.341(2) Å, c = 9.844(2) Å, β = 96.17(3)°, V = 1651.2(6) Å3 (single-crystal data, at 123 K). The six strongest lines in the X-ray powder diffraction pattern are [d(Å),I,(hkl)]: 8.985,30,(200); 8.283,85,(110); 6.169,25,(111); 4.878,25,(002); 3.234,100,(2, 420); 3.079,65,(222, 511); 2.976’45’(113). The crystal structure was solved by Patterson methods and refined using 2045 observed reflections to R1(F) = 3.73%. The structure is characterized by a kinked, five-membered chain of edge-sharing Mφ6 (φ = unspecified anion) octahedra, or pentamer, that extends in the a direction. The pentamers link by sharing corners to form a sheet in the (001) plane. Pentamers are also linked, via corner-sharing, by (As,P)O4 groups forming thick slabs in the (001) plane. The slabs link in the c direction by cornersharing between octahedra and tetrahedra to form a dense heteropolyhedral framework. Moderate to weak hydrogen-bonding provides additional linkage between the slabs.

Ab initio density-functional studies of 13-atom Cu and Ag clusters

“…Transition-metal (TM) clusters have been studied extensively from computational and experimental points of view [1][2][3][4][5] due to their potential applications in catalysis [6], photonics [7] and carbon nanotubes [8]. Owing to the difficulties associated with the experimental determination of the structures of small clusters [1], Monte Carlo and molecular-dynamics simulations play important roles in the structural characterization of small clusters.…”

Structure of transition metal clusters: A force-biased Monte Carlo approach

“…In this section, we present the results from our simulations with an emphasis on the structural properties of the clusters. Following a comparison of the total-energy values of the FMC clusters with the same from the Cambridge Cluster Database [4,5], we analyze the two-and three-body correlation functions and show that the three-dimensional distribution of atoms in the FMC and CCD clusters are very close to each other as far as these correlation functions are concerned. We then address the distribution of the coordination numbers of the atoms in order to obtain further information about the atomic arrangement in the first-coordination shell of atoms.…”

“…The main purpose of the paper is to explore the usefulness of employing atomic forces in Monte Carlo simulations. In particular, we study here the application of a simple gradient-based Monte Carlo method, originally introduced by Rossky et al [2,3], to optimize the transition metal clusters of Fe, Ni, and Cu and compare the results with the putative global minimum of these clusters reported in recent literature [4,5].…”

“…Transition metal clusters (TMCs) have been studied extensively from computational and experimental points of view [4][5][6][7][8]. Theoretical efforts to study TMCs include empirical Monte Carlo and moleculardynamics simulations to semi-empirical tight-binding and full ab initio calculations.…”

“…The putative global minima of Ni and Cu clusters of varying sizes have been studied systematically by Doye and Wales [5] using the Sutton-Chen potential. Likewise, Elliott et al [4] have recently addressed the computation of the global minima of several Fe clusters using the Finnis-Sinclair potential. In both the cases, the authors have employed an improved version of the basin-hopping algorithm of Li and Scheraga [12] using Monte Carlo simulations coupled with the Conjugate-Gradient optimization [13].…”