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The new mineral challacolloite has the ideal chemical formula KPb2Cl5. The type locality of challacolloite is the Challacollo silver mine SE of Iquique, Atacama desert, northern Chile. In the year of discovery challacolloite was also found on leucotephrite lava from the Vesuvius eruption of May 1855. It occurs closely associated with cotunnite as a hydrothermal phase (Chile) or as the product of fumarole activities (Italy). Challacolloite is colourless to white with an adamantine luster. Mobs’s hardness is about 2-3. The calculated density is 4.77g/cm(3). Challacolloite is biaxial (+), 2 V-calc = 67 degrees with n alpha = 2.004 (2), n beta = 2.010 (2) and n gamma = 2.024 (3). Chemical analyses of the type material give (wt.%) K 5.45, Pb 66.30, Cl 28.69. Challacolloite is monoclinic with space group P2(1)/c. Cell parameters refined from powder diffraction data of the type material are a = 8.864 (8), b = 7.932 (8), c = 12.491 (11) (angstrom), beta = 90.153 (5)degrees, V = 878.2 (1) angstrom(3) with Z = 4. Challacolloite is isotypic with NH4Pb2Cl5 and PbU2Se5. Synthetic REE-doped equivalents of challacolloite are known as technical laser materials. The strongest reflections in the X-ray powder diffraction data of the type material are as follows [d(meas)(angstrom)(I, hkl)]: 3.686 (100, 211), 3.609 (49, 20-2), 2.669(42, 22-2), 8.855 (39, 100) and 3.961(31, 020). The mineral was named after its type locality.