General Description of the Zeolites

Natural zeolites described by Gottardi and Galli1 are grouped in first line according to the old mineralogical morphological-structural system which has now been fully adapted to the secondary building unit (SBU)5 classification. The SBUs are build up by spokes which symbolize the connection between the centers of two (Si,Al)O4 tetrahedrons. An integral number of SBUs is present in a unit cell, but in some cases a given zeolite framework type can be represented by more than one SBU type. The concept is used in order to clarify the complex framework topology of these types of tecto- silicates to which alumino-silicate zeolites belong. They are only theoretical topological building units and should not be considered to be or equated with species that may be in the solution/gel during the crystallization of a zeolitic material5. The majority of todays known zeolites (natural and synthetic) can be grouped according to the following eighteen SBU types of figure 2.1.