Abstract: Caratiite is a sulphate-chloride of potassium and copper with ideal formula K₄Cu₄O₂(SO₄)₄MeCl (where Me = Na and/or Cu); it formed as fine green acicular crystals in lava of the 1869 eruption of Mt. Vesuvius, Naples, Italy. Caratiite is tetragonal, space group I4; a 13.60(2), c 4.98(1) Å, Z = 2. The strongest lines of the powder pattern are [d Å, I, hkl]: 9.61 100 (110); 6.80 80 (200); 4.296 60 (310); 3.015 100b (420,321); 2.747 70 (411); 2.673 60 (510); 2.478 60 (002); 2.388 70 (431,501); 2.281 60 (600). The mineral is uniaxial positive, ω 1.598, ɛ 1.711; it does not fluoresce under either short- or long-wavelength ultra-violet light. The specific gravity is 3.0 (meas.) and 3.22 (calc.).