abStRact Prewittite, ideally KPb 1.5 Cu 6 Zn(SeO 3) 2 O 2 Cl 10 , was found in the fumarole field of the second cinder cone of the North Breach of the Great fissure Tolbachik eruption (1975–1976, Kamchatka peninsula, Russia). It occurs as separate olive-green tabular crystals up to 0.2 mm in maximum dimension. It has vitreous luster and brownish-green streak. Prewittite is orthorhombic, space group Pnnm, a = 9.132(2), b = 19.415(4), c = 13.213(3) Å, V = 2342.6(9) Å 3 , Z = 4, D calc = 3.89 g/cm 3 , D meas = 3.90(2) g/cm 3 . The eight strongest lines of the powder X-ray diffraction pattern are {I [d(Å)] hkl}: 70 (8.26) 110; 60 (7.53) 101; 90 (4.111) 220, 132, 141; 100 (3.660) 212, 123; 40 (2.996) 223; 50 (2.887) 062; 40 (2.642) 322, 214; 40 (2.336) 073, 180, 244. Prewittite is biaxial (–). The optical orientation is X = a, Y = c, Z = b. The mineral has clear pleochroism: X, Y – olive green, Z – red-brown. The mineral is very brittle with the perfect cleavage on (010) and (101). The most developed crystal forms are {010}, {001}, and {101}. The chemical composition determined by the electron-microprobe is (wt%): K 2 O 1.76, PbO 21.18, CuO 33.24, ZnO 8.00, SeO 2 15.74, Cl 26.06, O=Cl –5.88, total 100.10. The empirical formula derived on the basis of O+Cl = 18 and sum of positive charges of cations equal to 26 is K 0.53 Pb 1.33 Cu 5.87 Zn 1.38 Se 1.99 O 7.67 Cl 10.33 . The crystal structure was solved by direct methods and refined to an agreement index R1 = 0.034 on the basis of 1522 independent reflections with I ≥ 2σ I . It is based upon metal oxide selenite chloride layers parallel to (010) and linked through K-Cl and Pb-Cl bonds to the K and Pb atoms located in the interlayer. The mineral name honors Charles T. Prewitt (b. 1933) in recognition of his important contributions to crystal chemistry of minerals and planetary materials.