Ginelfite, Ag2(Ag0.5Fe0.5)TlPb24.5(Sb,As)32.5S75.5, a new boxwork sulfosalt from Jas Roux, France: occurrence and crystal structure

Ginelfite, Ag2(Ag0.5Fe0.5)TlPb24.5(Sb,As)32.5S75.5 (Z = 2), was discovered in the hydrothermal deposit of Jas Roux, Hautes-Alpes, France, as dark grey, metallic, acicular crystals up to 0.5 mm long, associated with sphalerite and lopatkaite in a baryte + quartz gangue. In reflected light, ginelfite is light grey, with weak pleochroism and bireflectance. Anisotropism is moderate, with weak brownish and blue tints. Reflectance data for the four COM wavelengths in air (λ (nm): Rmin/Rmax (%)) are 470: 35.4/38.1; 546: 34.6/37.0; 589: 33.7/36.0; 650: 32.3/34.6. Electron microprobe analysis gave (in wt% - average of 35 spot analyses) Cu 0.07, Ag 2.34, Tl 1.14, Pb 45.44, Fe 0.22, As 6.17, Sb 23.46, S 21.07, total 99.91. On the basis of ΣMe = 61 apfu (atoms per formula unit), the empirical formula of ginelfite is Cu0.12Ag2.51Fe0.46Tl0.65Pb25.40Sb22.32As9.54S76.12. Ginelfite is triclinic, space group P1, with unit-cell parameters a = 8.3635(6) Å, b = 27.5481(19) Å, c = 29.199(2) Å, α = 95.335(3)°, β = 94.123(3)°, γ = 94.367(3)°, and V = 6657.7(8) Å3. The crystal structure was solved and refined by single-crystal X-ray diffraction data to a final R1 = 0.0876 on the basis of 23 779 unique reflections with Fo>4σ(Fo) and 1158 refined parameters. Ginelfite is a new rod-based Ag-Tl-Pb sulfosalt showing a boxwork organization. The name ginelfite honours Carlo Gini (born 1954) and Francesco Guelfi (born 1947), former technicians at the X-ray laboratory of the University of Pisa for their contributions to the development of the mineralogical research in Pisa during the last 40 years.