Fractional crystallization (geology)

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Fractional crystallization (geology)

Fractional crystallization is one of the most important geochemical and physical processes operating within the Earth's crust

Crust (geology)

In geology, a crust is the outermost solid shell of a planet or moon, which is chemically distinct from the underlying mantle . Crusts of Earth , our Moon, Mercury , Venus, and Mars have been generated largely by igneous processes, and these crusts are richer in incompatible elements than their respective mantle s....
and mantle. Fractional crystallization is the removal and segregation from a melt of mineral

Mineral

A mineral is a naturally occurring solid formed through Geology processes that has a characteristic chemical composition, a highly ordered atomic structure, and specific physical properties....
precipitates; except in special cases, removal of the crystals changes the composition of the magma. Fractional crystallization in silicate melts (magma

Magma

Magma is molten Rock that is found beneath the surface of the Earth, and may also exist on other terrestrial planets. Besides molten rock, magma may also contain suspended crystals and gas bubbles....
s) 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.

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Fractional crystallization is one of the most important geochemical and physical processes operating within the Earth's crust

Crust (geology)

Mineral

Magma

Partial pressure

In a mixture of ideal gases, each gas has a partial pressure which is the pressure which the gas would have if it alone occupied the volume. The total pressure of a gas mixture is the sum of the partial pressures of each individual gas in the mixture....
(fugacity

Fugacity

Fugacity is a measure of a chemical potential in the form of 'adjusted pressure.' It reflects the tendency of a substance to prefer one phase over another, and can be literally defined as ?the tendency to flee or escape?....
) of water in silicate melts can be of prime importance, as in near-solidus

Solidus (chemistry)

In chemistry, materials science, and physics, the solidus is the locus of temperatures below which a given substance is completely solid . The solidus is applied, among else, to metal alloys, ceramics, and natural rocks and minerals....
crystallization of magmas of granite

Granite

Granite is a common and widely occurring type of Intrusion , felsic, igneous rock rock . Granite has a medium to coarse texture, occasionally with some individual crystals larger than the groundmass forming a rock known as Porphyry ....
composition. The crystallization sequence of oxide

Oxide

An oxide is a chemical compound contaning at least one oxygen atom as well as at least one other element. Most of the Earth's crust consists of oxides....
minerals such as magnetite

Magnetite

Magnetite is a ferrimagnetism mineral with chemical formula Iron3Oxygen4, one of several iron oxides and a member of the spinel group....
and ulvospinel is sensitive to the oxygen fugacity

Mineral redox buffer

In geology, a redox buffer is an assemblage of minerals or compounds that constrains oxygen fugacity as a function of temperature. Knowledge of the redox conditions at which a rock forms and evolves can be important for interpreting the rock history....
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

Andesite

Andesite is an igneous rock, volcanic rock, of Igneous rock#Chemical classification, with aphanitic to porphyritic texture. The mineral assemblage is typically dominated by plagioclase plus pyroxene and/or hornblende....
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 crystallize to mafic

Mafic

Mafic is an adjective describing a silicate mineral or rock that is rich in magnesium and iron; the term was derived by contracting "magnesium" and "ferric"....
and ultramafic rocks. MgO and SiO₂ concentrations in melts are among the variables that determine whether forsterite

Forsterite

Forsterite is the magnesium rich end-member of the olivine solid solution series. Forsterite crystallizes in the orthorhombic system with cell parameters a 4.75 ?ngstr?m , b 10.20 ? and c 5.98 ? ....
olivine

Olivine

The mineral olivine is a magnesium iron Silicate minerals with the formula 2siliconoxygen4. It is one of the most common minerals on Earth, and has also been identified in meteorites and on the Moon, Mars, and comet Wild 2....
or enstatite

Enstatite

Enstatite is the magnesium endmember of the pyroxene silicate mineral series enstatite - ferrosilite . The magnesium rich members of the solid solution series are common rock-forming minerals found in igneous and metamorphic rock rocks....
pyroxene

Pyroxene

The pyroxenes are a group of important rock-forming silicate minerals found in many igneous and metamorphic rock rock . They share a common structure comprised of single chains of silica tetrahedra and they crystallize in the monoclinic and orthorhombic systems....
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

Granite

Feldspar

Feldspars are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's Crust .Feldspars crystallize from magma in both intrusive and extrusive igneous rocks, as veins, and are also present in many types of metamorphic rock....
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 element

Incompatible element

Incompatible element is a term used in petrology and geochemistry.During the Fractional crystallization of magma, and magma generation by the partial melting of Mantle and crust, elements that have difficulty in entering cation sites of the minerals are concentrated in the melt phase of magma ....
s. Hence, knowledge of the crystallization sequence is critical in understanding how melt compositions evolve. Textures of rocks provide insights, as documented in the early 1900's by Bowen's reaction series

Bowen's reaction series

Within the field of geology, Bowen's reaction series is the work of the petrologist, Norman L. Bowen who was able to explain why certain types of minerals tend to be found together while others are almost never associated with one another....
. Experimentally-determined phase diagram

Phase diagram

A phase diagram in physical chemistry, mineralogy, and materials science is a type of Graph of a function used to show conditions at which thermodynamically-distinct phase can occur at thermodynamic equilibrium....
s for simple mixtures provide insights into general principles. Numerical calculations with software like that referenced below have become increasing able to simulate natural processes accurately.

Main article: Igneous differentiation
Igneous differentiation

Igneous differentiation is an umbrella term for the various processes by which magmas undergo bulk chemical change during the partial melting process, cooling, emplacement of volcanic eruption....

Fractional crystallization (geology)

See also