Sinnerite Cu6As4S9, is triclinic, a = 9.064 Å, b = 9.830 Å, c = 9.078 Å, α = 90°00′, β = 109°30′, γ = 107°48′, Z = 2, and space group Pl. The structureis based on a sphalerite-like substructure and contains two kinds of twisted, chain-like groups of [AsS3] pyramids; one group contains 3 pyramids, the other 5 pyramids. Every S atom in the structure belongs to one of the groups and is bound to one or two As atoms, leading to the structural formula Cu12[As3S7][As5S11]. The structure of sinnerite may be viewed as a close packing of the two groups stacked parallel to . Each kind of group forms indistinct layers parallel to (010). Disorder in the packing sequence of the two groups, duet to their internal symmetry and configurational similarities, leads to the twin laws n ∥(10), n∥(010), m∥(414), and 2∥. Four As atoms, each belonging to a distinct group, occur around each of the two vacant S positions per unit cell, giving mutually interconnected [As4S12] clusters of [AsS3] pyramids. All Cu atoms are tetrahedrally coordinated. The twinning produces twin aggregates which contain 24 individuals and simulate 43m cubic symmetry.