ALBERT

Description: Favreauite, ideally PbBiCu 6 O 4 (SeO 3 ) 4 · H 2 O, is a new secondary selenite mineral from the El Dragón mine, Antonio Quijarro Province, Potosí Department, Bolivia. The mineral occurs in vughs in a matrix of (Co, Cu)-rich penroseite, dolomite and goethite. Associated minerals are: ahlfeldite, allophane, calcite, chalcomenite, malachite, molybdomenite and an unnamed Al selenite. Favreauite forms tiny green square tabular crystals, flattened on {001}, up to 0.1 mm on edge and 0.01 mm thick, occurring in subparallel and divergent groups. The Mohs hardness of favreauite is estimated as 3; it has perfect cleavage on {001}, an irregular fracture and a vitreous lustre. The calculated density based on the empirical formula is 4.851 g cm –3 . Favreauite is uniaxial (–), with mean refractive index estimated as 1.854 from the Gladstone–Dale relationship. It is pleochroic in shades of green, O 〈 E . Electron microprobe analyses gave the empirical formula Pb 0.95 Ca 0.17 Bi 0.90 Cu 5.81 Se 4.10 O 16 (OH) · 1H 2 O, based on 18 O pfu . The Raman spectrum shows strong SeO 3 bands at 847 cm –1 ( 1 ), 764 and 795 cm –1 ( 3 ), 493 and 542 cm –1 ( 2 ), and 320 and 392 cm –1 ( 4 ). Favreauite is tetragonal, space group P 4/ n , with the unit-cell parameters: a = 9.860(4) Å, c = 9.700(5) Å, V = 943.0(9) Å 3 and Z = 2. The eight strongest lines in the X-ray powder diffraction pattern are [ d obs /Å ( I ) ( hkl )]: 5.67(100)(111), 3.470(76)(220,202), 3.190(35)(003), 2.961(40)(311,113), 2.709(33)(302,203), 2.632(34)(231,312), 2.247(36)(331,133), and 1.6652(33)(305,513,531). The crystal structure was refined to R 1 = 0.0329 for 1354 observed reflections [ F o 〉 4 F o ] and 0.0356 for all 1432 unique reflections. Favreauite is a close structural relative of nabokoite, KCu 7 Te 4+ O 4 (SO 4 ) 5 Cl, and atlasovite, KCu 6 Fe 3+ BiO 4 (SO 4 ) 5 Cl. In all cases, oxygen-centred tetrahedra share edges to form corrugated [Cu 6 M O 4 ] layers ( M = Bi or Te) which can be derived from the framework structure of murdochite, Pb 4+ Cu 2+ 6 O 8 - x (Cl,Br) 2 x by selective deletion of atoms. In favreauite, additional OH and H 2 O between the layers are weakly bound to Cu, giving it Jahn-Teller distorted 4 + 2 coordination. The Cu–Bi–O layer is braced by SeO 3 pyramids. The Bi 3+ and interlayer Pb 2+ form an approximately face-centred cubic array analogous to the Pb 4+ sites in murdochite. Unlike Bi 3+ , Pb 2+ is in a site with nonpolar $$\overline{4}$$ point symmetry, which suppresses the stereoactivity of its lone pair.