The crystal structure of epifanovite, NaCaCu ₅ (PO ₄ ) ₄ [AsO ₂ (OH) ₂ ] 7H ₂ O, a new mineral from the oxidation zone of the Kester tin deposit, was determined by direct methods and refined to R ₁ = 0.087 on the basis of 2147 independent observed reflections. The mineral is monoclinic, P2 ₁ /m, a = 9.6911(8), b = 9.7547(9), c = 9.9632(14) A, β = 102.237(10)°, V = 920.46(17) A ³ , Z = 2. The crystal structure of epifanovite contains four symmetrically independent Cu sites, each coordinated by five O atoms by forming short equatorial (1.905-1.964 A) and one elongated (2.467-2.720 A) Cu-O bonds. Four CuO ₅ pyramids centered by the Cu2, Cu3 and Cu4 sites are linked at the O11 atom to form [Cu ₄ O ₁₃ ] tetramers. The [Cu ₄ O ₁₃ ] tetramers and Cu1O ₅ pyramids are linked via phosphate groups into the complex, which is incrustated by disordered [AsO ₂ (OH) ₂ ] ^- group to form fundamental building blocks in the structure. The blocks are linked through phosphate groups to form layers parallel to the (001) plane. The layers are joined into a three-dimensional framework by sharing of the apical atoms of the Cu1O5 pyramids and O atoms of disordered arsenate groups. Epifanovite is related to the group of minerals and inorganic compounds based upon the [Cu ₄ O(TO ₄ ) ₄ ] layers (T = As, P). The most closely related to epifanovite are monoclinic polytypes of the andyrobertsite-calcioandyrobertsite KMeCu ₅ (AsO ₄ ) ₄ [As(OH) ₂ O ₂ ] 2H ₂ O (Me = Cd, Ca).