Liottite, ideally [(Na,K) 16 Ca 8 ][Si 18 Al 18 O 72 ](SO 4 ) 5 Cl 4 , is the six-layer member of the cancrinite group. The structure of liottite has been refined in space group P6 to R = 2.7% using single-crystal X-ray-diffraction data. The hexagonal-system cell parameters are a 12.870(1), c 16.096(2) Aa The structure consists of a perfectly (Si,Al)-ordered framework, as shown by an average Si-O bond distance of 1.610(5) Aa, and an average Al-O bond distance of 1.730(9) Aa. Within a unit cell, three base-sharing cancrinite cages are stacked along [1/3 2/3 z], 17-hedra ("losod" cages) alternate with cancrinite cages along [2/3 1/3 z], and 23-hedra ("liottite" cages) are stacked along [0 0 z]. The small cancrinite cage contains Ca at the center of the hexagonal base and Cl within the cage, except for the cancrinite cage stacked along [2/3 1/3 z], which shows a disordered distribution of F and Cl. The "losod" and "liottite" cages are occupied by two and three sulfate groups, respectively. The sulfate groups are separated by triplets of cations (Na, K, and Ca) around the axes of symmetry. The complex distribution of anions inside the cages is strongly influenced by the position of the extra-framework cations, which are disordered over various sites, and particularly by that of the Ca atoms near the center of the six-member rings of aluminosilicate tetrahedra.