Argentopolybasite, Ag₁₆Sb₂S₁₁, a new member of the polybasite group

The new mineral argentopolybasite, ideally Ag16Sb2S11, was found at the Kremnica Au-Ag epithermal deposit, Ziar nad Hronom Co., Banska Bystrica Region, Slovakia (type locality), Sibenicny vrch near Nova Bana, Zarnovica Co., Banska Bystrica Region, Slovakia (cotype locality) and the Arykevaam epithermal Au-Ag deposit, Anadyr' District, Chukotka Autonomous Okrug, Russian Federation (cotype locality). At the Kremnica deposit argentopolybasite was found as discrete, well-developed (pseudo)hexagonal tabular crystals up to 4 mm in size or as complex crystalline aggregates and groups up to 5 mm in size in cavities of quartz. It is associated with pyrargyrite, polybasite, stephanite, miargyrite, rozhdestvenskayaite-(Zn), argentotetrahedrite-(Zn), naumannite, gold and pyrite. Argentopolybasite is dark grey to black, with a black streak and metallic to opaque lustre. The Mohs hardness is similar to 3. It is brittle with no observable cleavage and with a conchoidal fracture. The calculated density is 6.403 g center dot cm(-3). In reflected light, argentopolybasite is grey, with no observable bireflectance and very weak pleochroism. It shows moderate anisotropy in crossed polarisers with weak greenish and green-blue tints. The reflectance values for wavelengths recommended by the Commission on Ore Mineralogy of the IMA are (R-min/R-max, %): 30.3/31.0 (470 nm), 28.8/29.3 (546 nm), 28.1/28.6 (589 nm) and 27.4/27.8 (650 nm). The empirical formulae (based on 29 apfu) are, Kremnica: (Ag15.94Cu0.18)(Sigma 16.12)(Sb1.40As0.61)(Sigma 2.01)(S10.60Se0.25Cl0.03)(Sigma 10.88), Nova Bana: Ag-16.30(Sb1.74As0.22)(Sigma 1.96)(S10.69Cl0.04)(Sigma 10.73) and Arykevaam: (Ag15.54Cu0.38)(Sigma 15.92)(Sb1.56As0.51)(Sigma 2.07)S-11.01. The ideal end-member formula for argentopolybasite is Ag16Sb2S11. Argentopolybasite is trigonal, space group P321, a = 15.0646(5) angstrom, c = 12.2552(5) angstrom, V = 2408.61(15) angstrom(3) and Z = 2. The seven strongest powder X-ray diffraction lines are [d(obs) in angstrom, (I), hkl]: 12.169, (40), 001; 3.162, (100), 041; 3.045, (54), 004; 2.881, (45), 042; and 2.4256, (28), 421. The crystal structure of argentopolybasite from Kremnica, refined to R-obs = 0.0741 for 2804 observed reflections, confirmed that the atomic arrangement is isotypic to that of the other members of the polybasite group and it is isostructural with argentopearceite.