The crystal structure of mordenite was determined by Meier (1961) and refined by Gramlich (1971) on Na-exchanged natural crystals from Challis, Idaho, USA. The topology of the framework is characterized by 5-member tetrahedral rings, which are part of the mor composite building unit (see MOR). These building units are linked by edge-sharing into chains along c, which are in turn linked together by 4-rings to form a puckered sheet perforated with 8-ring holes. These permeable sheets are oriented parallel to (010). Linking these sheets together with 4-rings, 12-ring channels are formed parallel to [001]. The 8-ring holes of successive sheets do not align to make channels parallel to [010] (see fig. 2 in Simoncic and Armbruster, 2004).