Crystallization is the formation of crystals that results when a solid precipitates from a liquid solution or, rarely, directly from a gas. The term is used to describe this process in nature as well as when it occurs in a laboratory or industrial setting. Mineral and organic compounds and even water can be crystallized. Most crystals are some type of compound, but crystals of pure elements are possible. Snowflakes, salt, and some gemstones are formed by this process.
Crystals form when the concentration of a solute reaches a maximum, a condition called supersaturation. Cooling or evaporation of the solution will then trigger the crystallization process. A reduction in temperature can trigger crystal formation because the ability of a solution to hold a solute depends partially on the temperature. As the temperature decreases, so does solubility. Evaporation increases the concentration of the solute in the solution, triggering crystallization.
The first stage in the formation of any crystal is called nucleation. Nucleation is the combination of two or more molecules of the solute. These molecules will attract more molecules, which will bond to the original crystals in a regular pattern. This structure of this pattern, or matrix, is dependent upon the molecular properties of the substance being crystallized but will continue to propagate in a regular way as more molecules are added. This process may continue even after the concentration of dissolved solute drops below the supersaturation point, as the crystal will continue to add more molecules to its matrix.
The most commonly known crystallization process is the formation of water crystals in the atmosphere. This occurs all over the world, on a continuous basis, as air containing water vapour is cooled. the water crystallizes into ice, and as the crystals of water grow, they form snowflakes.
Crystallization may also occur very slowly. Stalactite formation in caves is one form of crystal formation that takes place over many centuries or even millions of years. As water holding dissolved minerals flows over the surface of the stalactite, molecules of these minerals bond with other molecules, very gradually adding to the stalactite.
Ancient peoples throughout the world obtained salt by the formation of salt crystals from the evaporation of seawater. This practice is still employed today as a cheap and effective way to obtain salt. Salt crystals are mined in some areas of the world from immense deposits, sometimes thousands of feet thick, that remain from the evaporation of prehistoric seas.
Other crystals are highly sought after and mined for their value as gemstones. While not all gems are crystals, many are formed deep within the earth over a period of thousands or millions of years. Many types of crystals are artificially created by man, but all of these processes rely on the same chemical principles by which natural crystals are formed. Many foods, minerals, and industrial materials are produced by crystallization.