The crystal structure of keyite, ideally (Cux,Pb1-x)Cu2Zn4Cd2(AsO4)6 · (H2O)2x, has been refined in space group symmetries C2/c and C1-. The keyite structure is compared with the alluaudite structure type, confirming that keyite belongs to the alluaudite group of minerals. While keyite is metrically monoclinic, ordering of Cu atoms and associated H2O within one of the channels of the alluaudite heteropolyhedral framework reduces the symmetry to triclinic and gives rise to weak X-ray diffraction maxima forbidden in space group C2/c. Streaks of diffuse scattering parallel b* may indicate additional ordering in layers normal to this direction. Pb2+ partially replaces Cu2+ within the channels, thus also decreasing the concentration of the water ligands coordinating Cu.