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

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

Correspondence: ^* E-mail: joel.brugger{at}adelaide.edu.au

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. d_calc is 2.81(2) g/cm³ (from X-ray structure refinement). 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 n_min = 1.657 to n_max = 1.660; the Gladstone-Dale relationship provides a value of 1.65. The empirical chemical formula is Bi_6.14Fe_12.6Mg_8.45Co_0.48Ni_0.12Ca_0.23(As_17.0Cr_0.64Si_0.32) _=18.0O_174.6H₁₈₄. Bouazzerite is monoclinic, P2₁/n, Z = 2, with a = 13.6322(13) Å, b = 30.469(3) Å, c = 18.4671(18) Å, ß = 91.134(2)°, and V = 7669.0(13) ų. The eight strongest lines in the X-ray powder diffraction pattern are [d in Å (I)(hkl)]: 11.79(100)(02), 10.98(80)(101/01), 10.16(80)( 20), 7.900(80)(02), 12.45(70)(10), 15.78(60)(01), 3.414(40)(33/400), 3.153(40)(35/25).

The crystal structure of bouazzerite is based upon [Bi₃Fe₇O₆(OH)₂(AsO₄)₉]^11– anionic nanoclusters that are built around [^{trigonal prismatic}Fe³⁺(^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 first 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 sulfide mineral deposits in the form of complex Fe-As nanoparticles.

Key Words: Bouazzerite • new mineral • crystal structure determination • trigonal prismatic coordination • Bou Azzer province • Anti-Atlas • Morocco