Abstract: The crystal structure of reyerite, (Na,K)₂Ca₁₄Si₂₂Al₂O₅₈(OH)₈.6H₂O, Z = 1, was refined in the space group P3¯, a = 9.765, c = 19.067Å, to R = 0.064 for 1540 reflections. The structure is composed of the following structural units: (a) tetrahedral sheets S₁, with composition (Si₈O₂₀)⁸⁻, characterized by six-membered rings of tetrahedra; (b) tetrahedral sheets S₂, characterized by six-membered rings of tetrahedra, with six tetrahedra pointing in one direction and two pointing in the other direction—the apical oxygens of these two tetrahedra connect two inversion-related S₂ sheets to build S2S¯2 double sheets, with composition (Si₁₄Al₂O₃₈)¹⁴⁻ and ordered distribution of aluminum cations; (c) sheets O of edge-sharing calcium octahedra. The various structural units are connected through corner sharing according to the schematic sequence …O¯S1OS2S¯2O¯…; the corresponding composition is [Ca₁₄Si₂₂Al₂O₅₈(OH)₈]²⁻. The charge balance is restored by alkali cations which are placed, together with water molecules, in the cavities of the structure at the level of the double tetrahedral sheet.