American Mineralogist, Volume 92, pages 1630–1639, 2007 0003-004X/07/0010–1630$05.00/DOI: 10.2138/am.2007.2508 1630 INTRODUCTION The oxidation of polymetallic deposits results in the formation of a great diversity of secondary minerals; deposits like Tsumeb (Namibia) and Broken Hill (Australia) truly are among the min- eralogical rainforests of the planet (Pring 1995). The study of the mineralogy of these deposits contributes to our understanding of heavy metal mobility in the near-surface environment, with direct applications to using soil sampling from geochemical prospecting, and as analog for heavy metal mobility around waste deposits (e.g., Williams 1990). Occasionally, new minerals from this setting present their own intrinsic interest. For example, bernalite Fe(OH) 3 from Broken Hill, Australia, represents a surprising addition to the well-known Fe-O-H system, and an interesting perovskite-like structure with A-site vacancy (Birch et al. 1993; Welch et al. 2005). In this paper, we present a description of the mineralogy and crystal structure of bouazzerite, a new hydrated Mg-Fe-Bi-arse- nate from the Bou Azzer mine, Anti Atlas, Morocco. The mineral is remarkable because it contains Bi 3+ -As 5+ -Fe 3+ nanoclusters in which Fe 3+ exists in trigonal prismatic coordination. Bouazzerite is the first example of trigonal prismatic coordination for a first- * E-mail: joel.brugger@adelaide.edu.au Mineralogy and crystal structure of bouazzerite from Bou Azzer, Anti-Atlas, Morocco: Bi-As-Fe nanoclusters containing Fe 3+ 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, Bi 6 (Mg,Co) 11 Fe 14 [AsO 4 ] 18 O 12 (OH) 4 (H 2 O) 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 Va- riscan 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/cm 3 (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 6.14 Fe 12.6 Mg 8.45 Co 0.48 Ni 0.12 Ca 0.23 (As 17.0 Cr 0.64 Si 0.32 ) Σ=18.0 O 174.6 H 184 . Bouazzerite is monoclinic, P2 1 /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) Å 3 . The eight strongest lines in the X-ray powder diffraction pattern are [d in Å (I)(hkl)]: 11.79(100)(021), 10.98(80)(101/101), 10.16(80)(120), 7.900(80)(022), 12.45(70)(110), 15.78(60)(011), 3.414(40)(333/400), 3.153(40)(353/225). The crystal structure of bouazzerite is based upon [Bi 3 Fe 7 O 6 (OH) 2 (AsO 4 ) 9 ] 11– anionic nanoclusters that are built around [ trigonal prismatic Fe 3+ ( octahedral Fe 3 3+ (OH)O 12 ) 2 ] 29– groups, containing one Fe 3+ ion in trigonal prismatic coordination and six Fe 3+ ions in octahedral coordination. The nanoclusters have a diameter of about 1.3 nm and are linked together by chains of Mg(O,H 2 O) 6 octahedra. The resulting arrange- ment 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. Keywords: Bouazzerite, new mineral, crystal structure determination, trigonal prismatic coordina- tion, Bou Azzer province, Anti-Atlas, Morocco