Cossaite, ideally (Mg0.5,□)Al6(SO4)6(HSO4)F6·36H2O, was found in the altered pyroclastic breccia of an active fumarole (T about 350°C) located at the rim of the La Fossa crater, Vulcano Island, Aeolian archipelago, Sicily, Italy. Cossaite is trigonal, space group R3İ, with a = 22.010(2), c = 9.238(1) Å, V = 3875.6(6) Å3, Z = 3. It forms stout prismatic hexagonal crystals up to 100 μm in size, terminated by rhombohedral faces, and is associated with thermessaite, vlodavetsite, sassolite and salammoniac. Cossaite is colourless to white, the streak is white and the lustre vitreous. It is not fluorescent in either long-wave or short-wave ultraviolet radiation. The calculated density is 2.075 g cm−3. The mean refractive index nobs is 1.49(1) (589 nm). Chemical analysis gave MgO 1.4, Al2O3 19.5, SO3 34.7, F 5.7, (H2O 40.85, from structure refinement), O=F −2.4, total 99.75 wt.%, corresponding to the empirical formula Mg0.56Al6.19S7.01H73.37F4.85O65.15 calculated on the basis of 70 oxygen plus fluorine atoms. The strongest six lines in the X-ray powder diffraction pattern [dobs(Å) (I) (hkl)] are: 4.15 (100) (140), 3.87 (70) (32İ2), 11.00 (50) (110), 4.58 (25) (131), 2.770 (20) (3İ33), 2.166 (20) (1İ8İ1). The crystal structure was refined to a final R index of 0.0349. It contains octahedral [Al(H2O)5F]2+ cations and sulphate anions interacting via hydrogen bonds to form channels running along , where disordered [Mg(H2O)6]2+ cations and hydrogensulphate anions are hosted.