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Davidlloydite, ideally Zn3(AsO4)2(H2O)4, a new arsenate mineral from the Tsumeb mine, Otjikoto (Oshikoto) region, Namibia: description and crystal structure

Published online by Cambridge University Press:  05 July 2018

F.C. Hawthorne ,
M. A. Cooper ,
Y. A. Abdu ,
N. A. Ball ,
M. E. Back  and
K. T. Tait
F.C. Hawthorne*
Affiliation:
Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada
M. A. Cooper
Affiliation:
Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada
Y. A. Abdu
Affiliation:
Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada
N. A. Ball
Affiliation:
Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada
M. E. Back
Affiliation:
Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada
K. T. Tait
Affiliation:
Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada
*
*E-mail: [email protected]
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Abstract

Davidlloydite, ideally Zn3(AsO4)2(H2O)4, is a new supergene mineral from the Tsumeb mine, Otjikoto (Oshikoto) region, Namibia. It occurs as elongated prisms (∼10:1 length-to-width ratio) that are flattened on {010}, and up to 100 × 20 × 10 μm in size. The crystals occur as aggregates (up to 500 μm across) of subparallel to slightly diverging prisms lying partly on and partly embedded in fine-grained calcioandyrobertsite. Crystals are prismatic along [001] and flattened on {010}, and show the forms {010} dominant and {100} subsidiary. Davidlloydite is colourless with a white streak and a vitreous lustre; it does not fluoresce under ultraviolet light. The cleavage is distinct on {010}, and no parting or twinning was observed. The Mohs hardness is 3 – 4. Davidlloydite is brittle with an irregular to hackly fracture. The calculated density is 3.661 g cm–3. Optical properties were measured with a Bloss spindle stage for the wavelength 590 nm using a gel filter. The indices of refraction are α = 1.671, β = 1.687, γ = 1.695, all ±0.002; the calculated birefringence is 0.024; 2Vobs = 65.4(6)°, 2Vcalc = 70°; the dispersion is r < v, weak; pleochroism was not observed. Davidlloydite is triclinic, space group P1, with a = 5.9756(4), b = 7.6002(5), c = 5.4471(4) Å, α = 84.2892(9), β = 90.4920(9), γ = 87.9958(9)°, V = 245.99(5) Å3, Z = 1 and a:b:c = 0.7861:1:0.7167. The seven strongest lines in the X-ray powder diffraction pattern [listed as d (Å), I, (hkl)] are as follows: 4.620, 100, (011, 10); 7.526, 71, (010); 2.974, 49, (200, 01); 3.253, 40, (021, 120); 2.701, 39, (10, 002, 1); 5.409, 37, (001); 2.810, 37, (210). Chemical analysis by electron microprobe gave As2O5 43.03, ZnO 37.95, CuO 5.65, H2O(calc) 13.27, sum 99.90 wt.%. The H2O content and the valence state of As were determined by crystal structure analysis. On the basis of 12 anions with H2O = 4 a.p.f.u., the empirical formula is (Zn2.53Cu0.39)Σ2.92As2.03O8(H2O)4.

The crystal structure of davidlloydite was solved by direct methods and refined to an R1 index of 1.51% based on 1422 unique observed reflections collected on a three-circle rotating-anode (MoKα radiation) diffractometer equipped with multilayer optics and an APEX-II detector. In the structure of davidlloydite, sheets of corner-sharing (As5+O4) and (ZnO4) tetrahedra are linked by ZnO2(H2O)4 octahedra. The structure is related to that of parahopeite.

Keywords

Davidlloyditenew mineral speciesarsenateTsumeb mineOtjikoto (Oshikoto) regionNamibiaCrystal Structureelectron microprobe analysisoptical properties
Type
Research Article
Information
Mineralogical Magazine , Volume 76 , Issue 1 , February 2012 , pp. 45 - 57
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2012

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