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., Banská Bystrica Region, Slovakia (type locality), Sibenicny vrch near Nová Bana, Zarnovica Co., Banská 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 about 3. It is brittle without any observable cleavage and with a conchoidal fracture. The calculated density is 6.403 g cm-3. In reflected light, argentopolybasite is grey, without any 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 (Rmin/Rmax, %): 30.3/31.0 (470 nm), 28.8/29.3 (546 nm), 28.1/28.6 (589 nm), 27.4/27.8 (650 nm). The empirical formulae (based on 29 apfu) are, Kremnica: (Ag15.94Cu0.18)16.12(Sb1.40As0.61)2.01(S10.60Se0.25Cl0.03)10.88, Nová Bana: Ag16.30(Sb1.74As0.22)1.96(S10.69Cl0.04)10.73 and Arykevaam: (Ag15.54Cu0.38)15.92(Sb1.56As0.51)2.07S11.01. The ideal end-member formula for argentopolybasite is Ag16Sb2S11. Argentopolybasite is trigonal, space group P321, a = 15.0646(5) A, c = 12.2552(5) A, V = 2408.61(15) A3 and Z = 2. The seven strongest X-ray powder diffraction lines are [dobs in A, (I), hkl]: 12.169, (40), 001; 3.162, (100), 041; 3.045, (54), 004; 2.881, (45), 042; 2.4256, (28), 421. The crystal structure of argentopolybasite from Kremnica, refined to Robs = 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.