The crystal structure of burnsite, KCdCu₇O₂(SeO_{3 2}Cl₉, a new mineral species from fumaroles of the Tolbachik volcano, Kamchatka, Russia, has been solved by direct methods and refined on the basis of F² for all unique data collected with monochromatic MoKα X-radiation and a CCD (charge-coupled device) detector to an agreement factor (R1) of 1.9%, calculated using the 460 unique observed reflections (\F_o\ = 4σ_F). The mineral is hexagonal, space group P6₃/mmc. a 8.7805(8), c 15.521(2) Å, V 1036.3(2) ų, Z = 2. There are two non-equivalent Cu²⁺ cations in the structure of burnsite. The Cu(I) cation is coordinated by three atoms of O and three of Cl; this is the first example of this type of mixed-ligand Cu²⁺Φ₆ (Φ: O^2-, Cl^-) octahedron in a mineral. The octahedron is (2 + 4 -distorted owing to the Jahn-Teller effect, in contrast to the vast majority of Cu²⁺ oxysalt minerals that contain (4 + 2)- distorted Cu²⁺ octahedra. The Cu(2) cation is coordinated by two O and three Cl anions arranged at the vertices of a trigonal bipyramid; this is the first occurrence of a mixed-ligand Cu²⁺O₂Cl₃ trigonal bipyramid in a mineral. The single symmetrically independent Cd²⁺ cation is coordinated by six Cl anions located at the vertices of a regular octahedron. The structure contains one symmetrically independent K cation that is coordinated by six Cl anions in a trigonal prismatic arrangement. There is one unique Se⁴⁺ cation that is strongly bonded to three O anions on one side of the cation, owing to the presence of a s² lone-electron pair. The structure is described both in terms of cation-centered polyhedra and oxocentered OCu₄ tetrahedra. The structure of burnsite is closely related to the structures of other natural copper oxide chloride selenites discovered in the Tolbachik fumaroles (chloromenite, georgbokiite, ilinskite).