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

Abstract

Favreauite, ideally PbBiCu_6O_4(SeO_3)_4 • H_2O, is a new secondary selenite mineral from the El Dragon mine, Antonio Quijarro Province, Potosi 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 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.