Mahnertite, (Na0.88Ca0.12)Cu2.75[AsO4]2Cl0.62·3.63 H2O, M = 565.17, is tetragonal, with space group I4/mmm, a = 10.037(1), c = 23.739(1) Å, V = 2391.5(4) Å3, Z = 8, ρo = 3.32, ρc = 3.14 g/cm3, μ = 9.78 mm-1. The crystal structure of mahnertite was solved and refined against data collected at the Daresbury Synchrotron Radiation Source, using a 20 × 20 × 2 μm3 single crystal, Λ = 0.6898 Å, F(000) = 2150, R = 0.049 for 639 reflections with I > 2σ(I). The structure consists of sheets parallel to (001), which comprise two symmetrically independent distorted Cu 5-vertex square pyramids Cuϕ5 (ϕ =O, Cl or H2O) as well as AsO4 tetrahedra. Four Cuϕ5 pyramids located around the 4-fold axis share edges to form clusters linked by AsO4 tetrahedra. The sheets are linked by the common apical vertex of the Cu(2)ϕ5 polyhedra. As a result, a heterogeneous framework is formed. (Na,Ca) atoms, additional Cu(3) cations and H2O molecules are located in the interlayers. The mineral is structurally related to zdenekite, NaPbCu5(AsO4)4Cl·5H2O.