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SELENOPOLYBASITE, [(Ag,Cu)₆(Sb,As)₂(S,Se)₇][Ag₉Cu(S,Se)₂Se₂], A NEW MEMBER OF THE PEARCEITE–POLYBASITE GROUP FROM THE DE LAMAR MINE, OWYHEE COUNTY, IDAHO, USA

Luca Bindi^1,, Michel Evain² and Silvio Menchetti³

¹ Museo di Storia Naturale - sezione di Mineralogia e Litologia, Università degli Studi di Firenze, via La Pira 4, I–50121 Firenze, Italy
² Laboratoire de Chimie des Solides, I.M.N., UMR 6502 CNRS – Université de Nantes, 2, rue de la Houssinière, B.P. 32229, F–44322 Nantes Cedex 3, France
³ Dipartimento di Scienze della Terra, Università degli Studi di Firenze, via La Pira 4, I–50121 Firenze, Italy

E-mail address: lbindi{at}geo.unifi.it

Selenopolybasite, ideally [(Ag,Cu)₆(Sb,As)₂(S,Se)₇][Ag₉Cu(S,Se)₂Se₂], is a new mineral species from the De Lamar mine, Owyhee County, Idaho, USA. It occurs as black anhedral to subhedral grains up to 400 µ m across, closely associated with naumannite, covellite, pyrite and calcite. Selenopolybasite is opaque with a metallic luster and possesses a black streak. It is brittle, and neither fracture nor cleavage was observed; the Vickers microhardness (VHN₁₀₀) is 131 kg/mm² (range 125–141) (corresponding Mohs hardness: 3–3). The density could not be measured owing to the small grain-size. The calculated density is 6.548 g/cm³ (on the basis of the empirical formula). In plane-polarized reflected light, selenopoly-basite is light grey, weakly to moderately bireflectant, and weakly pleochroic from grey to a violet-blue grey. Between crossed polars, it is anisotropic, with no characteristic rotation-tints and no internal reflections. Reflectance values (R_min and R_max) for the four standard COM wavelengths are 32.8, 34.1 (471.1 nm), 31.0, 32.9 (548.3 nm), 30.2, 31.8 (586.6 nm), and 29.3, 30.0% (652.3 nm), respectively. Selenopoly-basite is trigonal, space group Pm1, with unit-cell parameters: a 7.5950(4), c 12.0731(6) Å, V 603.12(5) ų, c: a 1.5896, Z = 1. The strongest eight X-ray powder-diffraction lines [d in Å(I/I₀)(hkl)] are: 3.1731(48)(201), 3.0183(84)(004), 2.8880(48)(022), 2.8880(100)(202), 2.5466(23)(023), 2.3629(34)(114), 2.2237(28)(024) and 1.8987(31)(220). Average results of 10 electron-microprobe analyses gave the chemical formula [(Ag_5.67Cu_0.20Bi_0.01Pb_0.01Zn_0.01Fe_0.03)_5.93(Sb_1.86As_0.19)_2.05(S_6.68Se_0.34)_7.02] [Ag₉Cu(S_1.79Se_0.21)_2.00Se₂], on the basis of 29 atoms and according to results of the structure refinement. The name chosen, selenopolybasite, recalls its status as the Se-dominant analogue of polybasite. Moreover, as it shows the 111 unit-cell type, the full name for the polytype is selenopolybasite-Tac, the Se-dominant analogue of polybasite-Tac.

Keywords: selenopolybasite, new mineral species, polytypes, electron-microprobe data, reflectance data, X-ray diffraction data, De Lamar mine, Owyhee County, Idaho.

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