Nicksobolevite, ideally Cu7(SeO3)2O2Cl6, was found in a fumarole of the second cinder cone of the North Breach of the Great Fissure Tolbachik volcano eruption (1975–1976), Kamchatka Peninsula, Russia. The mineral occurs as aggregates of red needle-like crystals up to 0.4 mm in maximal dimension, elongated along [001]. Associated minerals are chloromenite, prewittite, melanothallite, sophiite, ralstonite, ponomarevite, and native gold. Nicksobolevite is monoclinic, P21/c, a = 10.958(9), b = 14.483(5), c = 10.494(14) Å, β = 113.61(7)°, V = 1526(3) Å3, Z = 4 (from powder diffraction data). The eight strongest lines of the X-ray powder diffraction pattern are (I-d-hkl): 77-8.25-(110); 100-5.877-(120); 26-4.239-(112); 37-3.619-(040); 95-3.257-(310), (321); 50-2.715-(402); 26-2.668-(033), (411); and 40-2.278-(242), 134). Nicksobolevite is dark red, with vitreous luster and orange-red streak. The mineral is very brittle and transparent. Cleavage is perfect on {010} and {–101} and good on {120}. The Mohs hardness measured by microindentation is 2–2½. The calculated density is 4.18 g/cm3 (based on the empirical chemical formula). Nicksobolevite is optically positive, with α = 2.00(1), β = 2.01(1), γ = 2.08(1), 2Vcalc. = 43°, and the orientation is X = b, Z∧c = 36 (in obtuse β). Pleochroism: X , Y= red, Z= brownish red. The chemical composition determined by the electron-microprobe analysis is (wt. %): CuO 56.17, ZnO 2.34, SeO 223.29, Cl 22.69, O = Cl –5.13, total 99.36. The empirical formula of nicksobolevite, calculated on the basis of 14 anions per formula unit, is (Cu6.71Zn0.27)Σ6.98Se1.99O7.92Cl6.08. The simplified formula is Cu7(SeO3)2O2Cl6, which requires CuO 59.02, SeO 223.52, Cl 22.54, O = Cl –5.09, total 100.00 wt%. The crystal structure was solved by direct methods and refined to an agreement index R 1 = 0.075 on the basis of 3590 independent observed reflections. The structure contains eight symmetrically independent Cu2+ positions with coordination numbers varying from 5 to 6. The crystal structure is based upon [O4Cu13] tetramers consisting of four corner-sharing OCu4 tetrahedra. The tetrahedra are surrounded by the Se1O3 and Se2O3 selenite triangular pyramids to form linear {[O4Cu13](SeO3)4} complexes oriented parallel to [210] and [–210] and linked to each other into layers with a ladder-like configuration. The layers are perpendicular to the a axis and are surrounded by Cl- anions, which provide their linkage in the [100] direction. The mineral is named in honor of Academician Nikolay (Nick) Vladimirovich Sobolev (b. 1935), in recognition of his important contributions to mineralogy and petrology. Nicksobolevite is the most structurally complex and Cl-rich copper selenite chloride known so far.