The crystal structure of lemanskiite is determined for the first time (R = 0.019) and the mineral is redefined. Its chemical formula, crystal system, space group and unit-cell parameters are revised. The powder X-ray diffraction pattern of lemanskiite has been correctly indexed; the IR spectrum of an impurity-free sample has been obtained. It has been shown that the mineral is not a dimorph of lavendulan NaCaCu ₅ (AsO ₄ ) ₄ Cl · 5H ₂ O and differs from the latter by a lower H ₂ O content. The studied sample was from the oxidation zone of the Perseverancia deposit, Guanako, Antofagasta, Chile. It empirical formula is Na _0.98 (Ca _0.98 Sr _0.03 )Σ _1.01 Cu _5.07 As _3.97 O _15.97 Cl _1.03 · 3H ₂ O; the idealized formula is NaCaCu5(AsO4)4Cl · 3H2O. The mineral is monoclinic, P ₂ 1/m, a = 9.250(2), b = 10.0058(10), с = 10.0412(17) Å, β = 97.37(3)°, V = 921.7(3) Å ³ , Z =2. Lemanskiite represents a new structure type in the lavendulan group. Its structure is based on heteropolyhedral layers composed of clusters consisting of four edge-shared distorted Cu-centered tetragonal pyramids joined with eight AsO ₄ tetrahedra, which are also linked with a Cu-centered plane squares not involved in the clusters. Interlayer edge-shared Na-centered trigonal prisms and Ca sevenfold polyhedra are linked with the heteropolyhedral layers from both sides.