Zeolite Pebble Stone, Crushed Stone

Zeolites, zeolites or zeolites [from the Greek terms zein (to boil) + lithos (stone)] constitute a numerous group of minerals that have a porous structure. The term was first applied by the Swedish mineralogist Axel Fredrik Cronstedt after he observed that after rapidly heating a natural mineral, the stones began to bounce as the water evaporated. Using the Greek words meaning "stone that boils", he called this material zeolite.

Description
More than 80 species of natural zeolites and more than 150 artificial ones are known. Basically, they are hydrated aluminosilicates[1] that have an open structure that can accommodate a wide variety of positive ions, such as Na+, K+, Ca2+, Mg2+, and others. These positive ions are weakly bound to the structure and can be readily replaced by others in contact solution. Some of the most common minerals in the zeolite group are: analcime, chabazite, heulandite, natrolite, phillipsite and stilbite. An example of the chemical formula for one of these minerals is Na2Al2Si3O10.2H2O, the formula for natrolite.

Naturally occurring zeolites form at sites where volcanic rocks and volcanic ash react with alkaline water; they also occur in post-depositional environments where they have crystallized over thousands or even millions of years in shallow marine basins. Naturally occurring zeolites are very rarely pure, being contaminated to varying degrees by other minerals, metals, quartz or other zeolites. For this reason naturally occurring zeolites are excluded from many of their commercial applications where purity and uniformity are essential.

Their crystals are generally small but well formed, which is why they are highly prized by collectors. Well known are those from India and southern Brazil.

Zeolites as molecular sieves
Zeolites are the aluminosilicate members of the family of microporous solids known as molecular sieves. The term molecular sieve refers to the particular property of these materials that consists of the ability to selectively retain molecules by an exclusion process based on their size. This ability is due to their highly regular porous structure of molecular dimensions. The maximum size of the molecular or ionic species that can enter the pores of a zeolite is limited by the tube diameters.

The terms "zeolites", "microporous materials" or "molecular sieves" are often used as synonyms, albeit mistakenly (a molecular sieve or microporous materials are not necessarily zeolitic materials). The combination of a stable crystalline arrangement, the possibility to modulate surface properties and a regular configuration of micropores with well-defined dimensions are some of the main attractions for catalysis and numerous other applications. The term molecular sieve was coined by McBain in 1932 to define porous materials that exhibit the property of acting like sieves at the molecular scale.

The mode of formation of the structure of zeolites can be conveniently addressed by starting from a silicate. The replacement of a silicon atom (oxidation number +4) with an aluminum atom (oxidation number +3) results in a negative residual charge on the crystal lattice. This charge is neutralized by so-called counterions, which can be protons, alkali metal cations and cationic complexes whose volumes are compatible with the cavities of the zeolitic structure. These counterions are generally exchangeable.

The high ion exchange capacity, a large surface area distributed across pores with variable diameters, high thermal stability and high acidity make zeolites materials of extreme potential for numerous applications, having shown prominence as adsorbents in gas purification, as ion exchangers in detergents,[2] in catalysis in petroleum refining, in petrochemistry and in biotechnological applications.