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Ianbruceite, ideally [Zn2(OH)(H2O)(AsO4)](H2O)2, 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

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
F. C. Hawthorne*
Affiliation:
Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada
M. E. Back
Affiliation:
Department of Natural History (Mineralogy), Royal Ontario Museum, 100 Queen’s Park, Toronto, Ontario M5S 2C6, Canada
K. T. Tait
Affiliation:
Department of Natural History (Mineralogy), Royal Ontario Museum, 100 Queen’s Park, Toronto, Ontario M5S 2C6, Canada
J. Schlüter
Affiliation:
Mineralogical-Petrological Institute, University of Hamburg, Grindelallee 48, D-20146 Hamburg, Germany
T. Malcherek
Affiliation:
Mineralogical-Petrological Institute, University of Hamburg, Grindelallee 48, D-20146 Hamburg, Germany
D. Pohl
Affiliation:
Mineralogical-Petrological Institute, University of Hamburg, Grindelallee 48, D-20146 Hamburg, Germany
G. Gebhard
Affiliation:
Groβenseifen, 51545 Waldbröl, Germany

Abstract

Ianbruceite, ideally [Zn2(OH)(H2O)(AsO4)](H2O)2, is a new supergene mineral from the Tsumeb mine, Otjikoto (Oshikoto) region, Namibia. It occurs as thin platy crystals up to 80 μm long and a few μm thick, which form flattened aggregates up to 0.10 mm across, and ellipsoidal aggregates up to 0.5 mm across. It is associated with coarse white leiteite, dark blue köttigite, minor legrandite and adamite. Ianbruceite is sky blue to very pale blue with a white streak and a vitreous lustre; it does not fluoresce under ultraviolet light. It has perfect cleavage parallel to (100), is flexible, and deforms plastically. The Mohs hardness is 1 and the calculated density is 3.197 g cm-3. The refractive indices are α = 1.601, β = 1.660, γ = 1.662, all ±0.002; 2Vobs = 18(2)°, 2Vcalc = 20°, and the dispersion is r < v, weak. Ianbruceite is monoclinic, space group P21/c, a = 11.793(2), b = 9.1138(14), c = 6.8265(10) Å, β = 103.859(9)°, V = 712.3(3) Å3, Z = 4, a:b:c = 1.2940:1:0.7490. The seven strongest lines in the X-ray powder diffraction pattern [d(Å), I, (hkl)] are as follows: 11.29, 100, (100); 2.922, 17, (130); 3.143, 15, (202); 3.744, 11, (300); 2.655, 9, (230); 1.598, 8, (152); 2.252, 7, (222). Chemical analysis by electron microprobe gave As2O5 36.27, As2O3 1.26, Al2O3 0.37, ZnO 49.72, MnO 0.32, FeO 0.71, K2O 0.25, H2Ocalc 19.89, sum 108.79 wt.%; the very high oxide sum is due to the fact that the calculated H2O content is determined from crystal-structure analysis, but H2O is lost under vacuum in the electron microprobe.

The crystal structure of ianbruceite was solved by direct methods and refined to an R1 index of 8.6%. The As is tetrahedrally coordinated by four O anions with a mean As O distance of 1.687 Å. Zigzag [[5]Zn[6]Znϕ7] chains extend in the c direction and are linked in the b direction by sharing corners with (AsO4) tetrahedra to form slabs with a composition [Zn2(OH)(H2O)(AsO4)]. The space between these slabs is filled with disordered (H2O) groups and minor lone-pair stereoactive As3+. The ideal formula derived from chemical analysis and crystal-structure solution and refinement is [Zn2(OH)(H2O)(AsO4)](H2O)2.

Type
Research Article
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2016

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