Marinaite, ideally Cu2Fe3+O2(BO3), a new mineral, was found in a high-temperature (400–600°C) crack on the cooling lava field of the 2012–2013 fissure eruption at Tolbachik volcano, Kamchatka, Russia. The associated minerals are belomarinaite, tenorite, hematite, anglesite and gold. Marinaite forms prismatic, tabular, lamellar or acicular crystals up to 0.06 × 0.06 × 0.3 mm or aggregates of these forms up to 3 mm across. The mineral has a brown to black colour and submetallic lustre. Under the microscope in reflected light marinaite is grey with reddish-brown internal reflections. D(calc.)= 5.026 g/cm3. Electron microprobe analyses yielded (wt.%) 62.15 CuO, 16.71 Fe2O3, 5.11 Al2O3, 3.14 TiO2, 0.16 SiO2, 12.84 B2O3, total 100.11 wt.%. The empirical formula calculated based on 5 O apfu is Cu2.07Fe0.56Al0.27Ti0.10Si0.01B0.98O5. Marinaite is monoclinic, space group P21/c, unit-cell parameters are: a = 3.12750(18), b = 11.9690(8), c = 9.4657(5) Å, β = 97.568(6)°, V = 351.24(4) Å3 and Z = 4. The strongest lines of the powder XRD pattern [d in Å(I)(hkl)] are: 5.051(95)(021); 4.700(69)(002); 3.675(26)(031); 2.764(100)(023, $\bar 1$02, 111, 120); 2.524(94)(042); 2.444(45)(130, 102, $\bar 1$31); 2.320(31)(051, $\bar 1$13, 131); 2.037(22) (113, $\bar 1$33); 1.840(22)(044, 133, 062). Marinaite is isostructural with synthetic Cu2Fe3+O2(BO3) and has a monoclinically distorted ludwigite-type structure. The mineral is named in honour of the Russian mineralogist Marina Vsevolodovna Kuznetsova (1948–2015).