Tolbachite occurs as one of the principal Cu minerals accompanying exhalations of the large fractures of the Tolbachin basaltic eruption in the Kurile-Kamchatka volcanic field. It was formed at 300o-350oC in incrustations of fumaroles and secondary slag cones, together with melanthallite, dolerophanite, tenorite, euchlorine, chalcokyanite and a K-Cu basic sulphate chloride. It is unstable in air, becoming hydrated to eriochalcite, CuCl₂.2H₂O. On fresh fractures of the fumarolic incrustations, tolbachite is found as a tangled fibrous deposit of golden brown colour with a pearly lustre; the average size of the crystals is 2 X 0.02 mm. In transmitted light it shows distinct pleochroism from pale green perpendlar elongation to reddish brown parallel elongation; birefringence high (refr. ind. not determined as the mineral reacts with the liquids). Indexed XRD data are tabulated; strongest lines 5.76(100), 2.915(35), 3.445(25) A; a 6.89, b 3.31, c 6.82 A, beta 122.3o; D 3.42 g/cm³. Chemical analysis of an aqueous extract indicates, after deduction of impurities, a composition corresponding with CuCl_1.96, or ideally CuCl₂. (From translation by M. Fleischer)-R.A.H.