The crystal structure of mammothite, Pb6Cu4AlSbO2(SO4)2Cl4(OH)16, is monoclinic in acentric space group C2, with a 18.959(4), b 7.3398(19), c 11.363(3) Å, β 112.428(9)˚, V 1461.6(1.0) Å3, and Z = 2. It has been refined to an R index of 0.019 on the basis of 3878 observed reflections. There are three crystallographically distinct Pb sites with two different co-ordinations: [Pb1O8Cl1] is a mono-capped square antiprism polyhedron, while [Pb21O7Cl2] and [Pb22O7Cl2] are tri-capped trigonal prisms. Both Cu2+ sites have distorted [4 + 2] octahedral coordination due to the Jahn-Teller effect. The Al and Sb sites are regular-octahedral co-ordination with oxygen atoms. The [SO4] tetrahedron is quite distorted, with S–O bond lengths varying from 1.45 to 1.52 Å and subtended O–S–O angles varying from 106 to 113˚. In the structure there are eight (OH)– anions. All eight H atoms pfu were located, and it is these structure sites that reduce the symmetry from centric to acentric. Although mammothite is classified as a framework structure, it has a distinct layering. There are two layer types in the mammothite structure that parallel (001). There are three octahedrally coordinated sites; two are occupied by Cu atoms and one by an Al atom, in the octahedral layer. The tetragonal dipyramids [CuØ6] are linked forming ‘olivine-like’ chains parallel to the b-axis. The second layer, termed the cross-linked layer, has three [PbØ9] polyhedra with shared edges forming chains parallel to the b-axis, like the [CuØ6] tetragonal dipyramids. These chains are cross-linked by edge-sharing [SbO6] octahedra and decorated with [SO4] groups. The H atoms are in ‘holes’ within both layers.