The crystal structure of zeolite

The crystal structure of zeolite can be divided into three components: (1) aluminosilicate skeleton, (2) pores and cavities containing exchangeable cations M in the skeleton, (3) water molecules in the potential phase, that is, zeolite water.
The structure of zeolite is somewhat different from the skeleton of quartz and feldspar. The framework structure of quartz and feldspar is relatively tight, with a specific gravity of 2.6~2.7, while the framework structure of zeolite is relatively sparse, with a specific gravity of 2.0~2.2. The dehydrated cavity can be as large as 47%, such as chabazite, or even 50%, such as synthetic zeolite.
In the feldspar structure, metal cations are confined in the gaps of the crystal skeleton composed of O ions. Unless the crystal is destroyed, it is difficult for these metal cations to move freely. The replacement of Na or K by Ca must be carried out at the same time as the replacement of Si and Al, that is, a paired replacement, which will inevitably cause a change in the Si/AI ratio.
In the feldspar-like structure, metal cations are located in relatively open interconnected gaps with a specific gravity of 2.14~2.45. The cations can be exchanged with each other through the structural pathways without damaging the crystal skeleton. Sodalite and hydronepheline were once considered to be minerals of the zeolite group.
In the zeolite structure, metal cations are located between larger interconnected pores or cavities in the crystal structure. Therefore, cations can freely exchange through the pores without affecting its crystal skeleton. Exchanges such as 2(Na,K)(Ca2+) are easy to occur in zeolites, but not in feldspars. This form of exchange, probably an extreme form of ion exchange, is limited to zeolites and similar minerals.
The connections between zeolite water molecules, framework ions and exchangeable metal cations are generally loose and weak. These water molecules can move in and out of the pores more freely than cations. Under the influence of heat, it can be freely detached and attached without affecting its skeleton structure.