Crystal and twin structure of digenite,¹ Cu₉S₅

Digenite undergoes a rapidly reversible and non-quenchable transformation between 60 and 65° C. Digenite was synthesized in octahedra modified by small cube faces. Single-crystal methods yield a cubic cell whose edge a is equal to five times the literature value: a = 27.71 Å ± 0.3%. With 100 Cu₉S₅ per cell, the calculated specific gravity is 5.715, against 5.6 observed. Only HKL reflections observed are of the type 10m ± L, 10n ±L, L, with m and n integers. The numerous structural absences are explained by twinning; the octahedron consists of four rhombohedral crystals, oriented with their hexagonal c axes along the body diagonals and their hexagonal a axes along the face diagonals of the simulated cubic cell. The twin axis is [337] in rhombohedral notation; twin index 5; twin obliquity 0. The rhombohedral cell (a = 16.16 Å, α = 13° 56′) contains one Cu₉S₅. The diffraction aspect is R**. The pronounced pseudo-cube, a′ = a/5 = 5.54 Å, is explained as follows. Only those reflections whose cubic indices HKL are multiples of 5 receive contributions from all four crystals of the twin; they are the only reflections, moreover, to which sulfur atoms contribute. Space group R3m leads to a tentative structure in which all atoms lie on the 3-fold axis of the rhombohedral cell, in positions xxx. For the five sulfur atoms, the values of x are: 0, ±1/5, ±2/5; for the nine copper atoms: 1/2, ±0.060, ±0.133, ±0.250, ±0.350.