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

Author： Elliott P. Turner P. Jensen P. Kolitsch U. Pring A.

Publisher： Mineralogical Society

ISSN： 0026-461X

Source： Mineralogical Magazine, Vol.73, Iss.5, 2009-10, pp. : 723-735

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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 Ca_0.03Pb_0.02 Cd_2.80Al_0.04Zn_1.24-Cu_0.44Mn_0.53[(AsO₄)_3.13(PO₄)_0.92]_Σ4.05H_1.91·3.79H₂O. 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,(222, 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.