Lazerckerite, Ag3.75Pb4.5(Sb7.75Bi4)S24, from Kutná Hora, Czech Republic: a new Sb–Bi member of the andorite branch of the lillianite homologous series

Lazerckerite, ideally Ag3.75Pb4.5(Sb7.75Bi4)S24, is a new mineral species found in medieval mine dumps of the historic Ag–Pb–Zn Kutná Hora ore district, Czech Republic. The mineral is associated with other Sb–Bi lillianite homologues (terrywallaceite, gustavite, holubite) and Ag,Bi-bearing galena, most frequently as grain aggregates and replacement rims of earlier Ag–Pb–Bi minerals, growing together in aggregates of up to 0.6×0.3 mm. Lazerckerite is opaque, is steel grey in colour, and has a metallic lustre; the calculated density is 5.920 g cm−3. In reflected light lazerckerite is greyish white, and bireflectance and pleochroism are weak with grey tints. Anisotropy is weak to moderate with grey to bluish-grey rotation tints. Internal reflections are not observed. Electron microprobe analyses yielded the empirical formula, based on 44 apfu, (Ag3.61Cu0.04)Σ3.65(Pb4.55Fe0.01Cd0.01)Σ4.57(Sb7.87Bi3.75)Σ11.62(S24.15Se0.01)Σ24.16. Its unit cell parameters are a=13.2083(9), b=19.4595(8), c=8.4048(13), β=90.032(7)°, V=2160.3(4) Å3, space group P21/c, and Z=2. The structure of lazerckerite contains two Pb sites (Pb1 and Pb2) in bicapped trigonal prismatic coordination, 8 independent octahedral sites, and 13 distinct sulfur positions. Four of the octahedral sites are mixed (Sb,Bi) and (Bi,Sb) sites, one is a mixed (Ag,Bi) site, and one is a mixed (Sb,Pb) site. The new mineral belongs to the andorite branch of the lillianite homologous series with N=4 and is a new addition to the group of Sb–Bi mixed members of the series. Lazerckerite is defined as a lillianite homologue with the three following requirements: N=4, L (Ag++ (Bi3+, Sb3+) ↔ 2 Pb2+ substitution) ≈ 90 %–95 %, and approximately one-third of atom percentage of antimony is replaced by bismuth (Bi/(Bi+Sb) ≈ 0.30–0.38). The new mineral has been approved by the Commission on New Minerals, Nomenclature and Classification (CNMNC) of the International Mineralogical Association (IMA 2022-113) and named after Lazarus Ercker, the supreme Royal Bergmeister of the Kingdom of Bohemia and the Master of Prague Mint.