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

Abstract

Favreauite, ideally PbBiCu_6O_4(SeO_3)_4 • H_2O, 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_2O, based on 18 O pfu. The Raman spectrum shows strong SeO_3 bands at 847 cm^(−1) (ν_1), 764 and 795 cm^(−1) (ν_3), 493 and 542 cm^(−1) (ν_2), and 320 and 392 cm^(−1) (ν_4). Favreauite is tetragonal, space group P4/n, with the unit-cell parameters: ɑ = 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 > 4σF_o] and 0.0356 for all 1432 unique reflections. Favreauite is a close structural relative of nabokoite, KCu_7Te^(4+)O_4(SO_4)_5Cl, and atlasovite, KCu_6Fe^(3+)BiO_4(SO_4)_5Cl. In all cases, oxygen-centred tetrahedra share edges to form corrugated [Cu_6MO_4] layers (M = Bi or Te) which can be derived from the framework structure of murdochite, Pb^(4+)Cu^(2+)_6O_(8-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_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 Formula point symmetry, which suppresses the stereoactivity of its lone pair.

Additional details