A new mineral, dioskouriite, CaCu₄Cl₆(OH)₄ 4H₂O, represented by two polytypes, monocline (2M) and orthorhombic (2O), which occur together, was found in moderately hot zones of two active fumaroles, Glavnaya Tenoritovaya and Arsenatnaya, at the Second scoria cone of the Northern Breakthrough of the Great Tolbachik Fissure Eruption, Tolbachik volcano, Kamchatka, Russia. Dioskouriite seems to be a product of the interactions involving high-temperature sublimate minerals, fumarolic gas and atmospheric water vapor at temperatures not higher than 150 ° C. It is associated with avdoninite, belloite, chlorothionite, eriochalcite, sylvite, halite, carnal-lite, mitscherlichite, chrysothallite, sanguite, romanorlovite, feodosiyite, mellizinkalite, flinteite, kainite, gypsum, sellaite and earlier hematite, tenorite and chalcocyanite in Glavnaya Tenorito-vaya and with avdoninite and earlier hematite, tenorite, fluorophlogopite, diopside, clinoenstatite, sanidine, halite, aphthitalite-group sulfates, anhydrite, pseudobrookite, powellite and baryte in Arsenatnaya. Dioskouriite forms tabular, lamellar or flattened prismatic, typically sword-like crystals up to 0.01 mm × 0.04 mm × 0.1 mm combined in groups or crusts up to 1 × 2 mm² in area. The mineral is transparent, bright green with vitreous luster. It is brittle; cleavage is distinct. The Mohs hardness is ca. 3. D_meas is 2.75(1) and D_calc is 2.765 for dioskouriite-2O and 2.820 g cm^-3 for dioskouriite-2M. Dioskouriite-2O is optically biaxial (+), α = 1.695(4), β = 1.715(8), γ = 1.750(6) and 2V_meas = 70(10)°. The Raman spectrum is reported. The chemical composition (wt%, electron microprobe data, H₂O calculated by total difference; dioskouriite-2O/dioskouriite-2M) is: K₂O 0.03/0.21; MgO 0.08/0.47; CaO 8.99/8.60; CuO 49.24/49.06; Cl 32.53/32.66; H₂O(calc.) 16.48/16.38; -O=Cl -7.35/-7.38; total 100/100. The empirical formulae based on 14 O + Cl apfu are: Dioskouriite-2O: Ca_1.04(Cu_4.02Mg_0.01)_∑4.03[Cl_5.96(OH)_{3 90}O_0.14]_∑10-4H₂O; dioskouriite-2M: (Ca_1.00K_0.03)_∑4.03(Cu_4.01Mg_0.08)_∑4.09[Cl_5.99(OH)_3.83O0.18]_∑10-4H₂O. Dioskouriite-2M has the space group P2₁/c, a = 7.2792(8), b = 10.3000(7), c = 20.758(2) Â, β = 100.238(11)°, V = 1531.6(2) Â3 and Z = 4; dioskouriite-2O: P212121, a = 7.3193(7), b = 10.3710(10), c = 20.560(3) Å, V = 1560.6(3) ³ and Z = 4. The crystal structure (solved from single-crystal XRD data, R = 0.104 and 0.081 for dioskouriite-2M and -2O, respectively) is unique. The structures of both polytypes are based upon identical BAB layers parallel to (001) and composed from Cu²⁺-centered polyhedra. The core of each layer is formed by a sheet A of edge-sharing mixed-ligand octahedra centered by Cu(1), Cu(2), Cu(3), Cu(5) and Cu(6) atoms, whereas distorted Cu(4)(OH)₂Cl₃ tetragonal pyramids are attached to the A sheet on both sides, along with the Ca(OH)₂(H₂O)4Cl₂ eight-cornered polyhedra, which provide the linkage of the two adjacent layers via long Ca—Cl bonds. The Cu(4) and Ca polyhedra form the B sheet. The difference between the 2M and 2O polytypes arises as a result of different stacking of layers along the c axis. The cation array of the layer corresponds to the capped kagomé lattice that is also observed in several other natural Cu hydroxychlorides: Atacamite, clinoatacamite, bobkingite and avdoninite. The mineral is named after Dioskouri, the famous inseparable twin brothers of ancient Greek mythology, Castor and Polydeuces, the same in face but different in exercises and achievements; the name is given in allusion to the existence of two polytypes that are indistinguishable in appearance but different in symmetry, unit cell configuration and XRD pattern.