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

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 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_0.95Ca_0.17Bi_0.90Cu_5.81Se_4.10O₁₆(OH) · 1H₂O, based on 18 O pfu. The Raman spectrum shows strong SeO₃ bands at 847 cm^-1 (v1), 764 and 795 cm^-1 (v3), 493 and 542 cm^-1 (v2), and 320 and 392 cm^-1 (v₄). 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) ų 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 R₁ = 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 [Cu₆MO₄] layers (M = Bi or Te) which can be derived from the framework structure of murdochite, Pb⁴⁺Cu^{2 +}₆O_8-x(Cl,Br)_2x by selective deletion of atoms. In favreauite, additional OH and H₂O between the layers are weakly bound to Cu, giving it Jahn-Teller distorted 4 + 2 coordination. The Cu-Bi-O layer is braced by SeO₃ pyramids. 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.