Original paper

Favreauite, a new selenite mineral from the El Dragón mine, Bolivia

Mills, Stuart J.; Kampf, Anthony R.; Christy, Andrew G.; Housley, Robert M.; Thorne, Brent; Chen, Yu-Sheng; Steele, Ian M.

European Journal of Mineralogy Volume 26 Number 6 (2014), p. 771 - 781

published: Dec 1, 2014

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ArtNo. ESP147052606008, Price: 29.00 €

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Favreauite, ideally PbBiCu6O4(SeO3)4(OH)·H2O, 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 cm3. 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 Pb0.95Ca0.17Bi0.90Cu5.81Se4.10O16(OH)·1H2O, based on 18 O pfu. The Raman spectrum shows strong SeO3 bands at 847 cm1 (ν1), 764 and 795 cm1 (ν3), 493 and 542 cm1 (ν2), and 320 and 392 cm1 (ν4). Favreauite is tetragonal, space group P4/n, with the unit-cell parameters: a = 9.860(4) Å, c = 9.700(5) Å, V = 943.0(9) Å3 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 < 4σF o] and 0.0356 for all 1432 unique reflections. Favreauite is a close structural relative of nabokoite, KCu7Te4+O4(SO4)5Cl, and atlasovite, KCu6Fe3+BiO4(SO4)5Cl. In all cases, oxygen-centred tetrahedra share edges to form corrugated [Cu6 MO4] layers (M = Bi or Te) which can be derived from the framework structure of murdochite, Pb4+Cu2+ 6O8-x (Cl,Br)2x by selective deletion of atoms. In favreauite, additional OH and H 2O 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 3pyramids. The Bi3+ and interlayer Pb2+ form an approximately face-centred cubic array analogous to the Pb4+ sites in murdochite. Unlike Bi3+, Pb2+ is in a site with nonpolar 4 point symmetry, which suppresses the stereoactivity of its lone pair.


crystal structureselenitesynchrotronbolivianabokoitefavreauitemurdochiteanion-centred tetrahedrael dragón mine