Recrystallization

Recrystallization is an essentially physical process that has meanings in chemistry and geology.

In chemistry, recrystallization is a procedure for purifying compounds. A typical situation is that a desired compound X is contaminated by a small amount of compound Y. A chemist can prepare a saturated solution of the mixture X+Y in a warm solvent and subsequently lower the temperature. For most compounds, the solubility decreases with decreasing temperature. If X and Y have approximately equal solubilities, then X will precipitate while Y stays in solution. The precitipate now has a much higher purity than the original mixture. The cost of this purification method is the loss of the part of compound X that stays in solution.

In geology, solid-state recrystallization is a metamorphic process that occurs under situations of intense temperature and pressure where grains, atoms or molecules of a rock or mineral are packed closer together, creating a new crystal structure. The basic composition remains the same. This process can be illustrated by observing how snow recrystallizes to ice without melting. As opposed to metasomatism, which is a chemical change caused by metamorphism, recrystallization is a physical process. However, recrystallization can occur when a local migration of chemicals results in the chemical change of the rock or mineral with no external addition of materials.

Limestone is a sedimentary rock that undergoes metamorphic recrystallization to form marble, and clays can recrystallize to muscovite mica.

In metallurgy, recrystallization is the growth of particular grain fragments in a metal or alloy at the expense of others. This occurs when the metal or alloy is severely worked, as by cold rolling. Recrystallization results in greater, strain-free grains.

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