Some zeolites have the ability to e

Some zeolites have the ability to exchange sodium for calcium and thereby function as water softeners
by removing Ca 2  . After the zeolite has become saturated with Ca 2  , it can be renewed by washing it
in a concentrated NaCl solution to restore the Na  ions. Zeolites are also used to prepare ion exchange
resins, as molecular sieves and as catalysts.
As are other silicates, zeolites are made up of tetrahedra that are SiO 4 or AlO 4 units. Because of the
difference in charges on Si and Al, the presence of Al 3  necessitates a  1 cation as well. The Si 6 O 18 12 
ion is a basic unit in many zeolites. The six Si (and/or Al) ions defi ne a hexagon and these hexagons
can be combined to give a structure like that shown in Figure 14.2a . This unit is known as the sodalite
structure, which is also called a β -cage. Eight sodalite units can be joined to give a cubic structure like
that shown in Figure 14.2b . In this structure, the sodalite units are joined by faces that have four members
in the ring. This type of structure contains channels that allow it to function as a molecular sieve.
In a closely related structure, shown in Figure 14.2c , the eight sodalite units are joined by bridging
oxygen atoms to give a structure known as zeolite-A . This structure contains equal numbers of Al 3  and
Si 4  ions, so the formula can be written as Na 12 (AlO 2 ) 12 (SiO 2 ) 12  27H 2 O, which can also be written as
Na 12 Al 12 Si 12 O 48  27H 2 O.
Some zeolites are useful catalysts as a result of their having very large surface area and some sites where
oxygen atoms have been converted to  OH groups. These sites are potential proton donors, so they
are referred to as Brønsted sites . Certain zeolites are particularly effective catalysts for cracking hydrocarbons.
The chemistry of silicates truly ranges from mineralogy to organic reactions.