Silicate and zeolite structures

Further structural variety is possible when Al atoms replace some Si atoms. This leads to the formation of aluminosilicates. The Al atom can substitute directly for an Si atom in a tetrahedral site, insert into an octahedral environment outside of the silicate framework, or, very occasionally occur with different coordination numbers.

Due to aluminium being Al(III), when it replaces Si(IV) in an aluminosilicate you get an overall charge negative by one unit. Therefore the framework contains exchangeable metal cations to balance the change in overall charge (H+, Na+, K+, 1/2Ca2+, etc) and can also hold removable and replaceable guest molecules (water in naturally occurring zeolites).

Molecular sieves are crystalline aluminosilicates which have open structures with gaps of molecular dimensions. The reason they are known as ‘molecular sieves’ is that these molecules only absorb molecules smaller than the dimensions of the gaps in their structure and so can be used to separate different sized molecules.

The zeolites are a subclass of molecular sieves. They are a class of crystalline aluminosilicates based on rigid anionic frameworks with cations trapped inside tunnels or cages.

Aswell as acting as molecular sieves, zeolites can also exchage their ions for those in a surrounding solution.