Coparsite, ideally Cu₄O₂[(As,V)O₄]Cl, occurs in the fumarole 'Yadovitaya' (= 'Poisonous') in the North Breach of the great tissure Tolbachik eruption (1975-1976), Kamchatka Peninsula, Russia. It occurs as black or dark grey crystals, tabular on (100), elongate in the [010] direction, with well-developed {100}, {110} and {101} forms. Associated minerals are euchlorine, tolbachite, kamchatkite, ponomarevite, minerals from the klyuchevskite-alumoklyuchevskite solid-solution series, hematite, tenorite and vanadian lammerite. The grains of coparsite are found as small black inclusions in emerald-green euchlorine. The mineral has a metallic luster and a brown streak. Non-fluorescent. It is very brittle and thus does not allow measurements of hardness. Cleavage is perfect on the (100) plane. Coparsite is orthorhombic, space group Pbcm, a 5.440(1), b 11.154(2), c 10.333(2) Å, V 627.0(3) ų, Z = 4, D(calc) = 4.76(1) g/cm³ (based on the empirical formula). Diagnostic lines of the X-ray powder-diffraction pattern [d in Å(I)(hkl)] are: 5.31 (30)(010), 4.69(40)(110), 3.70(30)(012), 3.03(60)(310), 2.82(10)(113), 2.62(100)(312), 2.39(40)(022,221), 1.67(20)(324,405). Electron-microprobe analyses yielded: CuO 69.03 (67.68-70.16), Fe₂O₃ 0.17 (0-0.87), As₂O₅ 15.50 (13.13-20.23), V₂O₅ 7.72 (4.06-9.24), SO₃ 0.57 (0.20-0.99), Cl 8.47 (8.17-8.75), subtotal 101.46, O = Cl₂-1.91 (1.84-1.97), total 99.55 (98.09-101.55) wt%. The empirical formula, normalized to O + Cl = 7 apfu, is Cu_3.89Fe_0.01O_1.93 [(As_0.60V_0.38S_0.03)O₄]Cl_1.07, close to the ideal Cu₄O₂((As,V)O₄)Cl confirmed by crystal-structure analysis. The mineral is named after its chemical composition: cop(per) + ars(enic).