The zeolites are framework silicates consisting of interlocking tetrahedrons of SiO4 and AlO4. In order to be a zeolite the ratio (Si +Al)/O must equal 1/2. The alumino-silicate structure is negatively charged and attracts the positive cations that reside within. Unlike most other tectosilicates, zeolites have large vacant spaces or cages in their structures that allow space for large cations such as sodium, potassium, barium and calcium and even relatively large molecules and cation groups such as water, ammonia, carbonate ions and nitrate ions. In the more useful zeolites, the spaces are interconnected and form long wide channels of varying sizes depending on the mineral. These channels allow the easy movement of the resident ions and molecules into and out of the structure. Zeolites are characterized by their ability to lose and absorb water without damage to their crystal structures. The large channels explain the consistent low specific gravity of these minerals.
Zeolites have basically three different structural variations.
1. There are chain-like structures whose minerals form acicular or needle-like prismatic crystals, ie natrolite.
2. Sheet-like structures where the crystals are flattened platy or tabular with usually good basal cleavages, ie heulandite.
3. And framework structures where the crystals are more equant in dimensions, ie Chabazite.
Zeolite-based oxygen generation systems are used in producing medical grade oxygen. It is used as a molecular sieve for extracting oxygen from air, in a process which involves adsorption of atmospheric nitrogen. The usage of zeolite is also being explored for a quick clotting of severe bleeding.
Heating and Refrigeration
Zeolites can also be used as solar thermal collectors and for adsorption refrigeration. Their high heat of adsorption and ability to hydrate and dehydrate is exploited while maintaining the structural stability. This hygroscopic property along with an inherent exothermic reaction, while transitioning from dehydrated to a hydrated form, make the natural zeolites effective in the storage of solar and the waste heat energy.
Synthetic zeolite is widely used as the global laundry detergent.
Thomsonites is collected as gemstones from a series of lava flows along the Lake Superior (Minnesota) and in a lesser degree in Michigan (USA).
The biggest differences between natural and synthetic zeolites are:
Synthetics are manufactured from energy consuming chemicals and naturals are processed from natural ore bodies.
Synthetic zeolites have a silica to alumina ratio of 1 to 1 and clinoptotilite (clino) zeolites have a 5 to 1 ratio.
Clino natural zeolites do not break down in a mildly acid environment, where synthetic zeolites do. The natural zeolite structure has more acid resistant resistant silica to hold its structure together. The clino natural zeolite is broadly accepted for use in the agriculutral industry as a soil amendment and as a feed additive.
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