Another step toward the solution of the real structure of zinkenite

The crystal structure of Cu-bearing zinkenite from Saint-Pons (Alpes Maritimes department, France), having idealized chemical composition Cu0.7Pb9.7Sb21.3S42, has been studied. It has a pronounced hexagonal sub-cell, with unit-cell parameters a = 22.1219(11), c = 4.3207(3) Å, V = 1831.2(2) Å3, space group P63, Z = 1. The sub-cell crystal structure was refined to R1 = 0.072 on the basis of 3905 reflections with Fo > 4σ(Fo) and 133 refined parameters. It can be described as formed by one kind of rod, with walls of columns of (Pb/Sb)-centered polyhedra flanking both the rods and the sites located along the 63 screw axis. Minor Cu is hosted in the tetrahedral voids between the rods and the walls of polyhedra. Alternatively, the crystal structure of zinkenite can be described as formed by trigonal rods, delimited by lone electron-pair micelles, and tunnels hosting (Pb/Sb) atoms. The occurrence of weak superstructure reflections points to a triclinic unit cell with parameters a = 38.271(2), b = 22.1219(13), c = 8.6475(5) Å, α = 89.931(3), β = 90.030(3), γ = 89.957(3)°, V = 7323.6(7) Å3, space group P1, Z = 4. The twin laws making the twin lattice hexagonal have been taken into account and the crystal structure has been solved and refined. Notwithstanding the very low R1 value (R1 = 0.038 on the basis of 22563 reflections with Fo > 4σ(Fo) and 1194 refined parameters), several shortcomings, mainly due to the low diffraction quality of the available crystals, allow only the description of the main structural features of the superstructure of zinkenite, indicating the correctness of the triclinic model hypothesized by previous authors.