A new mineral feodosiyite Cu₁₁Mg₂Cl₁₈(pH)₈·16H₂O is found in the Glavnaya Tenoritovaya fumarole at the Second scoria cone of the Northern Breakthrough of the Great Tolbachik Fissure Eruption, Tolbachik volcano, Kamchatka, Russia. Closely associated minerals are belloite, avdoninite, sylvite, carnallite, chlorothionite, and dioskouriite. Feodosiyite forms tabular or prismatic crystals up to 0.02 × 0.1 × 0.1 mm³ combined in interrupted incrustations up to 0.1 × 0.5 × 1 cm³ overgrowing basalt scoria. The mineral is transparent, bright green, with vitreous lustre. Feodosiyite is brittle. Its Mohs' hardness is ca 3. One direction of imperfect cleavage was observed, the fracture is uneven. D_meas = 2.57(1), D_calc = 2.563 g cm^-3. Feodosiyite is optically biaxial (-), α = 1.660(3), β = 1.690(5), γ = 1.718(5), 2V_meas = 90(5)°. The Raman spectrum is reported. The chemical composition (wt.%, electron-microprobe data, H₂O calculated by total difference) is: MgO 5.39 (4.84-5.91), Cu 46.98, Cl 35.42, H₂O_calc 20.21,-O = Cl-8.00, total 100.00. The empirical formula calculated based on 42 atoms O+Cl pfu is: Cu_10.58Mg₂₄₀Cl_17.90(OH)_8.06 · 16.04H₂O. Feodosiyite is monoclinic, P2₁/c, a = 12.9010(6), b = 16.4193(5), c= 11.9614(5) Å,β = 113.691(6)°, V= 2320.20(17) ų and Z = 2. The strongest reflections of the powder XRD pattern [d,Å(I)(hkl)] are: 11.87(100)(100), 6.585(15)(021), 5.969(25)(-102), 5.905(16)(200), 5.231(13)(121), 3.135(8) (222,-412,-151) and 2.924(11)(-333,-251). The crystal structure, solved from single-crystal XRD data [R1(F) = 0.0867], is unique. It is based on layers of Cu²⁺-centred polyhedra. Cu²⁺ cations occupy six crystallographically non-equivalent sites that are placed in distorted octahedra Cu(1,3,5)(OH)₂Cl₄, Cu(2)(OH)₃Cl₃, and Cu(6)(OH)₂(H₂O)Cl₃ and distorted tetragonal pyramids Cu(4)(OH)₂Cl₃. Isolated almost regular Mg(H₂O)₆ octahedra, linked with the layers of Cu-centred polyhedra by H-bonds, occur in the interlayer space, as well as separate H₂O molecules. The mineral is named in honour of the outstanding Russian geologist Feodosiy Nikolaevich Chernyshev (1856-1914).