The crystal structure of mordenite (ptilolite)

Mordenite, a zeolite, is orthorhombic with space group Cmcm or Cmc2t. The unit cell of the Na form has dimensions a = 18.13 A, 6 = 20.49 A, c = 7.52 A, and contains Na8Al8Si40O9e · 24H20. Its framework structure was derived by superposition of the three-dimensional Patterson function and partially refined assuming the centric symmetry Cmcm. The aluminosilicate framework of mordenite is composed of a new type of chain, in which 5-membered rings of tetrahedra prevail. Its molecular sieve properties may be attributed to a system of channels, parallel to [001], having a free diameter of 6.6 A, and being interconnected by smaller channels, parallel to * Contribution No. 2690 from the Gates and Crellin Laboratories of Chemistry. 1 Present address: Technikum des Kantons Zürich, Winterthur, Switzerland. 440 W . Μ . M E I E R [010], of 2.8 Ä free diameter. Stacking faults in the framework reduce the effective diameter of the intraerystalline channels to about 4 A. The results of the structure determination suggest an ordered Al, Si distribution reducing the symmetry of the framework to Cmc2i.