Chemical analysis, by T. A. Cherepova, of black plates from fumaroles of the 1975-76 eruption of the Tolbachin volcano, Kamchatka, gave Cu 59.41, Cl 28.33, Pb 0.01, Zn 0.07, Na 0.04, K 0.06, Li 0.04, SO₄ 1.20, H₂O⁺ 0.05, H₂O^- 1.75, O (calc.) 8.55, = 99.51. This is interpreted as 85.50% Cu₂OCl₂, 10.79% CuO and 3.22% 'others'. XRD data agree well with artificial Cu₂OCl₂ (ASTM card 23-954); tenorite lines are masked by the principal constituent. The mineral is termed melanothallite, following the identification on the ASTM card, and revised cell dimensions are given: a 9.595, b 9.693, c 7.461 A; D_calc. 4.08 g/cm³. (The data on ASTM card 23-954 are from L. Walther-Levy et al., Bulletin de la Societe Chimique de France, 1970, p 2789; their synthetic material differs from the melanothallite of A. Scacchi (1870), E. Scacchi (1884) and F. Zambonini (1910), which rapidly absorbs water from the air, turning to yellow-green hydromelanothallite. The Tolbachin mineral is clearly Cu₂OCl₂, but is almost certainly not melanothallite.)-M.H.H.