The new mineral zubkovaite, Ca ₃Cu ₃(ASO ₄) ₄, was found in the Arsenatnaya fumarole at the Second scoria cone of the Northern Breakthrough of the Great Tolbachik Fissure Eruption, Tolbachik volcano, Kamchatka, Russia. It is associated with anhydrite, svabite, hematite, johillerite, tilasite, fluorophlogopite, sanidine and aphthitalite. Zubkovaite occurs as coarse prismatic crystals up to 0.01 mm × 0.01 mm × 0.2 mm combined in radiating aggregates or crusts. The mineral is transparent, bright sky-blue, turquoise-coloured or light bluish-green, with vitreous lustre. It is brittle, with imperfect cleavage. The Mohs' hardness is ca 3. Dcalc is 4.161 g cm ^-3. Zubkovaite is optically biaxial (-), α = 1.747(5), β = 1.774(5), γ = 1.792(5) and 2Vmeas = 75(10)°. Chemical composition (wt.%, electron microprobe) is: CaO 19.22, CuO 27.37, As2O5 52.54, SO3 0.67, total 99.80. The empirical formula based on 16 O apfu is Ca _2.96Cu _2.97(As _3.945S _0.07) _Σ4.015O ₁₆. Zubkovaite is monoclinic, C2, a = 16.836(3), b = 5.0405(8), c = 9.1173(17) Å, β = 117.388(13)°, V = 687.0(2) Å ³ and Z = 2. The strongest reflections of the powder XRD pattern [d,Å (I) (hkl)] are: 7.44 (100) (01), 3.727 (79) (400, 02, 11), 3.334 (92) (12), 2.914 (73) (311), 2.765 (50) (01, 02), 2.591 (96) (13) and 2.521 (53) (020). The crystal structure is unique for minerals. It was solved from single-crystal X-ray diffraction data to R = 7.19%. The structure contains trimers of Cu ²⁺-centred polyhedra (consisting of one distorted square CuO ₄ in the core and two distorted square pyramids CuO ₅) and two crystallographically independent As ⁵⁺O ₄ tetrahedra playing different roles: As(2)O ₄ tetrahedra link neighbouring trimers into ribbons whereas As(1)O4 tetrahedra link adjacent ribbons into heteropolyhedral layers; Ca cations are located in the interlayer space. The mineral is named in honour of the Russian crystallographer and crystal chemist Natalia Vital'evna Zubkova (born 1976).