Jaszczakite, [(Bi, Pb)3 S3][AuS2], a new mineral species from Nagybörzsöny, Hungary

Abstract

Jaszczakite, [(Bi,Pb) 3 S 3 ][AuS 2 ], represents another new species from the abandoned Nagyborzsony Au deposit at Also-Rozsa, northern Hungary. The mineral is closely associated with arsenopyrite, pyrite, marcasite, sphalerite, chalcopyrite, gold, bismuth, bismuthinite, ikunolite, jonassonite, and cosalite, in a matrix of quartz. It is opaque, tin-white, has a metallic lustre and a black streak. Jaszczakite is brittle, and the fracture is irregular. Neither cleavage nor parting are observable in the section. The VHN 5 value ranges between 115 and 132 (mean 122) kg mm −2 which corresponds to a Mohs hardness of 2½–3. The calculated density is 7.327 g cm −3 . In plane-polarized light jaszczakite is weakly bireflectant and pleochroic from light grey to a slightly bluish grey. Internal reflections are absent. The mineral is distinctly anisotropic with rotation tints very similar to those of jonassonite (shades of green and purple). The reflectance COM values (air, R 1 and R 2 in %) are: 49.4, 50.9 (471.1 nm); 47.1, 48.8 (548.3 nm); 46.5, 47.7 (586.6 nm); 48.8, 49.5 (652.3 nm). The average result of five electron-microprobe analyses is Au 19.90(34), Ag 0.22(3), Bi 46.29(54), Pb 16.50(26), Cd 0.11(3), Sb 0.12(4), As 0.08(3), S 16.17(30), Se 0.16(6), total 99.55 wt%. The empirical formula (based on 9 atoms pfu ) is Au 1.00 Ag 0.02 Bi 2.18 Pb 0.78 Cd 0.01 Sb 0.01 As 0.01 S 4.97 Se 0.02 or, according to the refinement results, [(Bi 2.18 Pb 0.78 Ag 0.02 Cd 0.01 Sb 0.01 As 0.01 ) Σ=3.01 S 3 ][AuS 2 ]. The simplified formula is AuBi 3 S 5 , which requires Au 20.01, Bi 63.70, S 16.29, total 100.00 wt%. Jaszczakite is orthorhombic, space group Pmmn , with unit-cell parameters: a = 3.858(1), b = 12.552(3), c = 9.289(2) A, V = 449.83(8) A 3 , a : b : c = 0.3074:1:0.7400, Z = 2. The five strongest observed X-ray powder-diffraction lines [ d in A ( I / I 0 ) ( hkl )] are: 7.466 (100) (011), 6.2760 (90) (020), 5.2003 (24) (021), 3.0963 (68) (003), 2.7768 (26) (023). The crystal structure showed that jaszczakite is the S–Bi-analogue of buckhornite [Pb 2 BiS 3 ][AuTe 2 ]. Jaszczakite is named in honour of John A. Jaszczak, Professor of Physics at the Michigan Technological University, for his significant contributions on the complexities of natural graphite.