Cubic AgBiS2 (schapbachite) from the Silberbrünnle mine near Gengenbach in the Central Black Forest, Germany
Walenta, Kurt; Bernhardt, Heinz-Jürgen; Theye, Thomas
published: Aug 20, 2004
Cubic AgBiS2 already known as synthetic compound for some time has been found at the Silberbrünnle mine near Gengenbach in the Central Black Forest. It is the first occurrence in nature. It forms inclusions in quartz and is associated with bismuth, acanthite, bismutite and pyromorphite. The grains of the mineral are anhedral and reach about 0.3 mm in size. They are sometimes partially altered and surrounded by the secondary minerals acanthite and bismutite. The colour is grayish black, streak black, opaque with metallic luster, hardness about 2 1/2, brittle, no cleavage observed. Density (calc.) 7.05 g/cm3. Reflectance values in air: 470 nm 45.19, 546 nm 44.73, 589 nm 44.32, 650 nm 43.94 %. Unit-cell dimension of the mineral a = 5.703 Å, Z = 2, space group Fm3m. Strongest lines of the powder pattern in Å: 3.290 (8) 111, 2.853 (10) 200, 2.014 (8) 220. Result of the chemical analysis (wt.%): Ag 21.83, Pb 18.10, Bi 42.39, S 16.15, total 98.47 %, yielding on the basis of S = 2 the empirical formula Ag0.80Pb 0.35 Bi0.81 S2 or simplified Ag(Bi, Pb)S2. This is close to the composition of a cubic phase synthesized by Hoda & Chang (1975) with 30.0 mole-% PbS, 33.8 mole-% Ag2 S and 36.2 mole-% Bi2 S3 and a unit-cell dimension a of 5.71 Å.