Erzwiesite, Ag8Pb12Bi16S40, the natural orthorhombic N =  8 member of the lillianite homologous series

Erzwiesite, ideally Ag8Pb12Bi16S40, Z= 1, is a new sulfosalt mineral discovered in the Erzwies mining area, Gastein Valley, Salzburg Province, Austria. The mineral occurs as small black, irregular needle-like crystals mixed with galena and heyrovskýite in a quartz matrix. In reflected light, erzwiesite is greyish white. Under crossed polars it is distinctly anisotropic, and the rotation tints change from pale brown to pale bluish grey to dark brown. Reflectance measurements in air yield the following Rmin/Rmax values based on the standard wavelengths (Commission on Ore Mineralogy, COM): 44.5/47.6 (470 nm), 41.9/45.0 (546 nm), 41.5/44.5 (589 nm), and 40.4/43.6 (650 nm). The Mohs' hardness is 3–3.5 (VHN50 ranges from 195 to 224, mean 210 kg mm−2). Averaged electron-microprobe analyses (n= 6) gave (in wt %) Ag 11.10(27), Cu 0.04(3), Pb 29.50(77), Cd 0.17(1), Bi 42.90(63), Te 0.21(14), Se 0.08(5), and S 16.08(11), with a total of 100.10(63). The empirical formula is Ag8.18Cu0.05Pb11.31Cd0.12Bi16.30Sb0.01S39.81Te0.13Se0.06 (based on 76 apfu). The calculated density is 7.075 g cm−3 using the empirical formula. Erzwiesite crystallises in space group Cmcm (a= 4.085(5), b= 13.462(15), c= 33.92(4) Å, and V= 1866(4) Å3). The crystal structure was determined from single-crystal X-ray diffraction data (R1= 5.24 % for 308 data with Fo > 4σ(Fo) and 51 variable parameters). The structural formula is Ag8.64Pb11.04Bi16.32S40. The seven strongest lines in the X-ray powder diagram are d in Å (intensity) hkl 3.588 (64) 028, 3.387 (98) 115, 3.349 (37) 041, 3.288 (85) 029, 2.919 (100)133, 2.846 (99)134, and 2.039(43) 157. Erzwiesite is the first natural (8 : 8) homologue of the lillianite homologous series and is named after its type locality.