crystal structure of zeolite
Jan 03, 2022
The crystal structure of zeolite can be divided into three components: (1) aluminosilicate framework, (2) pores and voids containing exchangeable cations M in the framework, (3) latent phase water molecules, 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 sparse, 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 interstices of the crystal framework composed of O ions, and it is difficult for these metal cations to move freely unless the crystal is destroyed. The exchange of Na or K by Ca must be carried out at the same time as the replacement of Si and Al, that is, paired replacement, which will inevitably lead to the change of the Si/AI ratio.
In the feldspar-like structure, the metal cations are located in the relatively open interconnected gaps, with a specific gravity of 2.14~2.45, and the cations can exchange with each other through the structural pathways without destroying the crystal framework. Sodalite and hydronepheline were once considered zeolite minerals.
In the zeolite structure, the metal cations are located in the larger and interconnected pores or cavities of the crystal structure. Therefore, cations can freely exchange through the pores without affecting the crystal framework. Exchanges like 2(Na,K)(Ca2+) are easy to occur in zeolite, but not in feldspar. This form of exchange, possibly an extreme form of ion exchange, is limited to zeolites and similar minerals.
The connection between the water molecules of zeolite and the framework ions and exchangeable metal cations is generally relaxed and weak. These water molecules can move in and out of pores more freely than cations. Under the influence of heat, it can be freely detached and attached without affecting its skeleton structure.
