Watanabeite Cu₄(As,Sb)₂S₅, in which As>Sb, is a new copper sulphosalt that occurs with quartz in a hydrothermal vein at the Teine mine, Sapporo, Hokkaido, Japan. It is silvery lead-grey in colour with lead-grey streak. VHN₁₀₀ = 253–306 kg/mm², brittle. It has no cleavage and the fracture is uneven. The measured density = 4.66(2) g/cm³. The mean of six microprobe analyses gave Cu 41.1, Ag 0.1, Mn 0.3, As 15.4, Sb 14.3, Bi 2.4, S 26.2, a total of 99.8 wt.%, corresponding to: (Cu_3.94Mn_0.03-Ag_0.01)_Σ3.98 (As_1.25Sb_0.72Bi_0.07)_Σ2.04S_4.98 (basis: total atoms = 11), or ideally, Cu₄(As,Sb)₂S₅, with As > Sb. The X-ray powder pattern resembles that of tetrahedrite but has subsidiary diffractions and is similar to that of synthetic Cu₂₄As₁₂S₃₁ (Maske and Skinner, 1971). It is indexed on an orthorhombic cell with a = 14.51 Å, b = 13.30 Å, c = 17.96 Å (all ±0.01 Å), and Z = 16. Calculated density is 4.66 g/cm³. It is optically similar to tetrahedrite but is grey and weakly bireflectant. No internal reflections were observed. The maximum and minimum reflectance values in air and in oil (n_D = 1.515) for the COM wavelengths are: 470 nm−32.5, 31.5; 17.7, 17.0, 546 nm−32.0, 31.1; 17.0, 16.3, 589 nm−31.1, 30.3; 16.1 m 15.5, 650 nm−30.0, 29.3; 15.0, 14.5%, respectively. Watanabeite forms masses composed of aggregates of minute grains up to 50 µm in diameter. Apart from some minute inclusions of emplectite, native bismuth and tennantite, it is almost monominerallic.