Epistilbite: an acentric zeolite with domain structure

The refinement of the crystal structure of Theigarhorn epistilbite showed the presence of two tetrahedral configurations, as in Elba dachiardite. These two configurations, obtained by the rotation of two tetrahedra around one edge, have both C2 symmetry, with different frequency (82% and 18% respectively), thus the resulting space group is C2. The presence of these two acentric domains is interpreted as a tendency to avoid the T 0 T angle of 1800 present in the topological symmetry C2/m of epistilbite. The amount of AI, obtained from the T 0 mean distance, has been corrected by a method which takes into account the influence of T 0 T angles on the measured T 0 distance, and resulted in good accordance with Al content from the chemical analysis. A remarkable degree of ordering in the Si/AI distribution has been found. Two extraframework cation and six water molecule sites have been located. Both cation sites have a square antiprism coordination; these polyhedra form clusters of four antiprisms, which are linked together as in a chessboard.