THE CRYSTAL-STRUCTURE OF LIOTTITE, A 6-LAYER MEMBER OF THE CANCRINITEGROUP

Liottite, ideally [(Na,K)(16)Ca-8][Si18Al18O72](SO4)(5)Cl-4, is the si x-layer member of the cancrinite group. The structure of liottite has been refined in space group P (6) over bar to R = 2.7% using single-cr ystal X-ray-diffraction data. The hexagonal-system cell parameters are a 12.870(1), c 16.096(2) Angstrom. The structure consists of a perfec tly (Si,Al)-ordered framework, as shown by an average Si-O bond distan ce of 1.610(5) Angstrom, and an average Al-O bond distance of 1.730(9) Angstrom. 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 t he 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 d istribution of F and Cl. The ''losod'' and ''liottite'' cages are occu pied 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 st rongly influenced by the position of the extra-framework cations, whic h are disordered over various sites, and particularly by that of the C a atoms near the center of the six-member rings of aluminosilicate tet rahedra.