Zeolite is an ore, first discovered in 1756. Swedish mineralogist Axel Fredrik Cronstedt discovered that a type of natural aluminosilicate ore will boil when it is burned, so it is named “zeolite” (zeolit in Swedish).
Zeolite structural features
Up to 36 kinds of zeolites have been discovered so far.
Their common feature is that they have a shelf structure, that is, in their crystals, molecules are connected together like a shelf, and many cavities are formed in the middle. Because there are still many water molecules in these cavities, they are hydrous minerals.
Different zeolites have different morphologies, such as cristobalite and chabazite are generally axial crystals, flake zeolite and pyrozeolite are plate-shaped, and mordenite is needle-shaped or fibrous.
All kinds of zeolites should be colorless or white if they are pure inside, but if other impurities are mixed inside, they will show a variety of light colors.
Zeolite also has a glass-like luster. We know that the water in the zeolite can come out, but this does not destroy the crystal structure inside the zeolite.
Zeolite crystal structure
The crystal structure of zeolite can be divided into three components:
- Aluminosilicate framework,
- The skeleton contains pores and voids that can exchange cations M,
- The water molecules of the latent phase, namely zeolite water.
The structure of zeolite is somewhat different from the framework 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 empty, with a specific gravity of 2.0~2.2. The cavity after dehydration 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 voids of the crystal skeleton composed of O ions. Unless the crystal is destroyed, these metal cations are difficult to move freely. The exchange of Na or K by Ca must be performed simultaneously with the substitution of Si and Al. That is, the substitution in pairs will inevitably cause the Si/AI ratio to change.
In the feldspar-like structure, the metal cations are located in relatively open interconnected voids with a specific gravity of 2.14~2.45. The cations can be exchanged through the structural channels without damaging their crystal skeleton. Sodalite and cabernet were once considered zeolite minerals.
In the zeolite structure, the metal cations are located in pores or cavities with large crystal structures and interconnected with each other. Therefore, cations can freely exchange through the pores without affecting the crystal structure. Exchanges like 2(Na,K)(Ca2+) are easy to occur in zeolites but not in feldspars. This form of exchange may be an extreme form of ion exchange, limited to zeolites and similar minerals.
The relationship between the water molecules of zeolite and framework ions and exchangeable metal cations is generally weak and weak. These water molecules can move and enter and exit channels more freely than cations. Under the tendency of heat, it can be freely detached and attached without affecting its skeleton structure.