Argentopolybasite, Ag₁₆Sb₂S₁₁, a new member of the polybasite group

Abstract The new mineral argentopolybasite, ideally Ag 16 Sb 2 S 11 , was found at the Kremnica Au–Ag epithermal deposit, Žiar nad Hronom Co., Banská Bystrica Region, Slovakia (type locality), Šibeničný vrch near Nová Baňa, Žarnovica (cotype locality) and Arykevaam Anadyr’ District, Chukotka Autonomous Okrug, Russian Federation locality). At deposit argentopolybasite as discrete, well-developed (pseudo)hexagonal tabular crystals up to 4 mm in size or complex crystalline aggregates groups 5 cavities of quartz. It is associated with pyrargyrite, polybasite, stephanite, miargyrite, rozhdestvenskayaite-(Zn), argentotetrahedrite-(Zn), naumannite, gold pyrite. Argentopolybasite dark grey black, a black streak metallic opaque lustre. Mohs hardness ~3. brittle no observable cleavage conchoidal fracture. calculated density 6.403 g⋅cm –3 . In reflected light, grey, bireflectance very weak pleochroism. shows moderate anisotropy crossed polarisers greenish green–blue tints. reflectance values for wavelengths recommended by Commission on Ore Mineralogy IMA are ( R min / max %): 30.3/31.0 (470 nm), 28.8/29.3 (546 28.1/28.6 (589 nm) 27.4/27.8 (650 nm). empirical formulae (based 29 apfu) are, Kremnica: (Ag 15.94 Cu 0.18 ) Σ16.12 (Sb 1.40 As 0.61 Σ2.01 (S 10.60 Se 0.25 Cl 0.03 Σ10.88 Baňa: 16.30 1.74 0.22 Σ1.96 10.69 0.04 Σ10.73 Arykevaam: 15.54 0.38 Σ15.92 1.56 0.51 Σ2.07 11.01 ideal end-member formula trigonal, space group P 321, = 15.0646(5) Å, c 12.2552(5) V 2408.61(15) Å 3 Z 2. seven strongest powder X-ray diffraction lines [ d obs (I), hkl ]: 12.169, (40), 001; 3.162, (100), 041; 3.045, (54), 004; 2.881, (45), 042; 2.4256, (28), 421. crystal structure from Kremnica, refined 0.0741 2804 observed reflections, confirmed that atomic arrangement isotypic other members polybasite it isostructural argentopearceite.