What is "fractional crystallization"

Fractional \Frac"tion*al\, a.

1. Of or pertaining to fractions or a fraction; constituting a fraction; as, fractional numbers.

2. Relatively small; inconsiderable; insignificant; as, a fractional part of the population.

   Fractional crystallization (Chem.), a process of gradual and approximate purification and separation, by means of repeated solution and crystallization therefrom.

   Fractional currency, small coin, or paper notes, in circulation, of less value than the monetary unit.

   Fractional distillation (Chem.), a process of distillation so conducted that a mixture of liquids, differing considerably from each other in their boiling points, can be separated into its constituents.

n. (context physics chemistry English) the separation of two solutes from the same solution by using either a solvent or a crystallization temperature such that only one solute is supersaturated and crystallizes out

Fractional crystallization may refer to:

• Fractional crystallization (chemistry), a process to separate different solutes from a solution
• Fractional crystallization (geology), a natural process occurring in igneous rocks during which precipitation of minerals takes place

Fractional crystallization is one of the most important geochemical and physical processes operating within the Earth's crust and mantle. Fractional crystallization is the removal and segregation from a melt of mineral precipitates; except in special cases, removal of the crystals changes the composition of the magma. Fractional crystallization in silicate melts ( magmas) is complex compared to crystallization in chemical systems at constant pressure and composition, because changes in pressure and composition can have dramatic effects on magma evolution. Addition and loss of water, carbon dioxide, hydrogen, and oxygen are among the compositional changes that must be considered. For example, the partial pressure ( fugacity) of water in silicate melts can be of prime importance, as in near- solidus crystallization of magmas of granite composition. The crystallization sequence of oxide minerals such as magnetite and ulvospinel is sensitive to the oxygen fugacity of melts, and separation of the oxide phases can be an important control of silica concentration in the evolving magma, and may be important in andesite genesis.

Experiments have provided many examples of the complexities that control which mineral is crystallized first as the melt cools down past the liquidus.

One example concerns crystallization of melts that form mafic and ultramafic rocks. MgO and SiO concentrations in melts are among the variables that determine whether forsterite olivine or enstatite pyroxene is precipitated, but the water content and pressure are also important. In some compositions, at high pressures without water crystallization of enstatite is favored, but in the presence of water at high pressures, olivine is favored.

Granitic magmas provide additional examples of how melts of generally similar composition and temperature, but at different pressure, may crystallize different minerals. Pressure determines the maximum water content of a magma of granite composition. High-temperature fractional crystallization of relatively water-poor granite magmas may produce single- alkali-feldspar granite, and lower-temperature crystallization of relatively water-rich magma may produce two-feldspar granite.

During the process of fractional crystallization, melts become enriched in incompatible elements. Hence, knowledge of the crystallization sequence is critical in understanding how melt compositions evolve. Textures of rocks provide insights, as documented in the early 1900s by Bowen's reaction series. Experimentally-determined phase diagrams for simple mixtures provide insights into general principles. Numerical calculations with special software have become increasingly able to simulate natural processes accurately.

• Main article: Igneous differentiation

In chemistry, fractional crystallization is a method of refining substances based on differences in solubility. It fractionates via differences in crystallization (forming of crystals). If a mixture of two or more substances in solution is allowed to crystallize, for example by allowing the temperature of the solution to decrease, the precipitate will contain more of the least soluble substance. The proportion of components in the precipitate will depend on their solubility products. If the solubility products are very similar, a cascade process will be needed to effectuate a complete separation. This technique is often used in chemical engineering to obtain very pure substances, or to recover saleable products from waste solutions.