Mineralogy and crystal structure of bouazzerite from Bou Azzer, Anti-Atlas, Morocco: Bi-As-Fe nanoclusters containing Fe3+ in trigonal prismatic coordination

Joël Brugger 1 , Nicolas Meisser 2 , Sergey Krivovichev 3 , Thomas Armbruster 4  and Georges Favreau 5
  • 1 Department of Geology and Geophysics, Adelaide University, North Terrace, SA-5001 Adelaide, Australia and South Australian Museum, North Terrace, SA-5000 Adelaide, Australia
  • 2 Musée Géologique Cantonal and Laboratoire des Rayons-X, Institut de Minéralogie et Géochimie, UNIL-Anthropole, CH-1015 Lausanne- Dorigny, Switzerland
  • 3 Department of Crystallography, St. Petersburg State University, University Emb. 7/9, 199034 St. Petersburg, Russia
  • 4 Laboratorium für chemische und mineralogische Kristallographie, Universität Bern, Freiestrasse 3, CH-3012 Bern, Switzerland
  • 5 421 Avenue Jean Monnet, 13090 Aix-en-Provence, France

Abstract

Bouazzerite, Bi6(Mg,Co)11Fe14[AsO4]18O12(OH)4(H2O)86, is a new mineral occurring in “Filon 7” at the Bou Azzer mine, Anti-Atlas, Morocco. Bouazzerite is associated with quartz, chalcopyrite, native gold, erythrite, talmessite/roselite-beta, Cr-bearing yukonite, alumopharmacosiderite, powellite, and a blue-green earthy copper arsenate related to geminite. The mineral results from the weathering of a Variscan hydrothermal As-Co-Ni-Ag-Au vein. The Bou Azzer mine and the similarly named district have produced many outstanding mineral specimens, including the world’s best erythrite and roselite.

Bouazzerite forms monoclinic prismatic {021} crystals up to 0.5 mm in length. It has a pale apple green color, a colorless streak, and is translucent with adamantine luster. dcalc is 2.81(2) g/cm3 (from X-ray structure refi nement). The new mineral is biaxial with very weak pleochroism from yellow to pale yellow; the refractive indices measured on the (021) cleavage face range from nmin = 1.657 to nmax = 1.660; the Gladstone-Dale relationship provides a value of 1.65. The empirical chemical formula is Bi6.14Fe12.6Mg8.45Co0.48Ni0.12Ca0.23(As17.0Cr0.64Si0.32)Σ=18.0O174.6H184. Bouazzerite is monoclinic, P21/n, Z = 2, with a = 13.6322(13) Å, b = 30.469(3) Å, c = 18.4671(18) Å, β = 91.134(2)°, and V = 7669.0(13) Å3. The eight strongest lines in the X-ray powder diffraction pattern are [d in Å (I)(hkl)]: 11.79(100)(021̅), 10.98(80)(101/1̅01), 10.16(80)(1̅20), 7.900(80)(022̅), 12.45(70)(11̅0), 15.78(60)(01̅1), 3.414(40)(333/400), 3.153(40)(353/22̅5).

The crystal structure of bouazzerite is based upon [Bi3Fe7O6(OH)2(AsO4)9]11- anionic nanoclusters that are built around [trigonal prismaticFe3+(octahedralFe3+3(OH)O12)2]29- groups, containing one Fe3+ ion in trigonal prismatic coordination and six Fe3+ ions in octahedral coordination. The nanoclusters have a diameter of about 1.3 nm and are linked together by chains of Mg(O,H2O)6 octahedra. The resulting arrangement displays channels down [100] that contain structural water. Bouazzerite is the fi rst mineral based upon Bi- and As-containing ferric nanoclusters. Its discovery provides a unique insight into transport mechanisms of toxic elements in the oxidation zones of sulfi de mineral deposits in the form of complex Fe-As nanoparticles.

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