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Petewilliamsite, (Ni,Co)₃₀(As₂O₇)₁₅, a new mineral from Johanngeorgenstadt, Saxony, Germany: description and crystal structure

¹ Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario K1A 0E8, Canada
² Department of Civil Engineering and Geological Sciences, 156 Fitzpatrick, University of Notre Dame, Notre Dame, Indiana 46556–0767, USA
³ Canadian Museum of Nature, P.O. Box 3443, Station D, Ottawa, Ontario K1P 6P4, Canada
⁴ Department of Mineralogy, The Natural History Museum, Cromwell Road, London SW7 5BD, UK
⁵ Box 3921, Duke Medical Center, Durham, North Carolina 27710, USA

^* E-mail: aroberts{at}NRCan.gc.ca

Petewilliamsite, ideally (Ni,Co)₃₀(As₂O₇)₁₅, monoclinic, space group C2, a = 33.256(5), b = 8.482(1), c = 14.191(2) Å, ß = 104.145(3)°, V = 3881.6(11) ų, a:b:c = 3.9209:1:1.6731, Z = 2, is a new mineral found on a single nickeline-veined quartz specimen from Johanngeorgenstadt, Saxony, Germany. The mineral possesses a pronounced subcell-supercell: a (subcell) = 1/5 a (supercell); b (subcell) = b (supercell); c (subcell) = 1/3 c (supercell), and the strongest six lines of the X-ray powder-diffraction pattern are [d in Å (I) (hkl)]: 4.235(30)(020); 3.118(100)(513, 023); 3.005(60)( 03); 2.567(50)(1020); 1.637(50)(536); 1.507(30b)(553, 1533, 20 06). It occurs predominantly as scattered patches of mm-sized aggregates which are intimately associated with varicoloured xanthiosite; additional associations include bunsenite, aerugite, rooseveltite, native bismuth, paganoite and two undefined arsenates. Subhedral equant crystals with rounded faces are intimately intergrown in 1 mm-sized aggregates and individual grains do not exceed 0.5 mm in maximum diameter. The average crystal size is variable from 20 µm to 0.3 mm. The colour varies from dark violet-red to dark brownish-red and the streak is pale reddish-brown to pale purplish-brown. Crystals are translucent, brittle, vitreous, and do not fluoresce under ultraviolet light. The mineral shows neither twinning nor cleavage, has an uneven fracture, and the calculated density (for the empirical formula) is 4.904 g/cm³. Electron-microprobe analyses gave NiO 19.45, CoO 18.39, CuO 3.40, CaO 0.17, FeO 0.04, As₂O₅ 60.32, total 101.77 wt.%. The empirical formula, derived from crystal-structure analysis and electron-microprobe analyses, is (Ni_14.66²⁺ Co_13.82²⁺ Cu_2.41²⁺ Ca_0.17 Fe_0.03²⁺) based on O = 105 atoms per formula unit _31.09(As⁵⁺_1.97O₇)₁₅, (a.p.f.u.). In reflected plane-polarized light in air, petewilliamsite is dark grey with orange to spectral (multicoloured) internal reflections and no obvious bireflectance, anisotropy or pleochroism. Measured reflectance values in air are tabulated; the index of refraction calculated at 589 nm is 1.88. The mineral name honours Professor Peter (‘Pete’) Allan Williams of the University of Western Sydney, New South Wales, Australia, for his contributions to the study of secondary minerals.

The crystal structure of petewilliamsite has been solved by direct methods and refined on the basis of F² using 9212 unique reflections measured with Mo-K X-radiation on a diffractometer equipped with a CCD-based detector. The final R1 was 7.68%, calculated for 1273 observed reflections. The structure contains 15 symmetrically distinct As⁵⁺ cations, each of which is tetrahedrally coordinated by four O atoms, and pairs of these AsO₄ tetrahedra share a vertex which results in As₂O₇ pyroarsenate groups that are in layers parallel to (010). The structure also has 16 distinct transition-metal M (M: Ni,Co) sites of which there are one tetrahedral, four square bipyramidal, and 11 octahedral arrangements. Adjacent pyroarsenate groups are linked through bonds to M cations. The structure of petewilliamsite is not closely related to other naturally occurring arsenates and it is the first pyroarsenate mineral.

KEYWORDS: petewilliamsite, new mineral, (Ni, Co)₃₀(As₂O₇)₁₅, crystal structure, electron-microprobe data, reflectance data, Johanngeorgenstadt (Saxony, Germany)

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