Fractional crystallization, or crystal fractionation, is one of the most important geochemical and physical processes operating within
crust and
mantle
A mantle is a piece of clothing, a type of cloak. Several other meanings are derived from that.
Mantle may refer to:
*Mantle (clothing), a cloak-like garment worn mainly by women as fashionable outerwear
**Mantle (vesture), an Eastern Orthodox ve ...
of a rocky planetary body, such as the Earth. It is important in the formation of
igneous rocks because it is one of the main processes of
magmatic differentiation
In geology, igneous differentiation, or magmatic differentiation, is an umbrella term for the various processes by which magmas undergo bulk chemical change during the partial melting process, cooling, emplacement, or eruption. The sequence of (u ...
. Fractional crystallization is also important in the formation of
sedimentary evaporite rocks.
Igneous rocks
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.
In essence, fractional crystallization is the removal of early formed crystals from an originally homogeneous magma (for example, by gravity settling) so that these crystals are prevented from further reaction with the residual melt. The composition of the remaining melt becomes relatively depleted in some components and enriched in others, resulting in the precipitation of a sequence of different minerals.
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, and oxygen are among the compositional changes that must be considered.
For example, the
partial pressure
In a mixture of gases, each constituent gas has a partial pressure which is the notional pressure of that constituent gas as if it alone occupied the entire volume of the original mixture at the same temperature. The total pressure of an ideal gas ...
(
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
An oxide () is a chemical compound that contains at least one oxygen atom and one other element in its chemical formula. "Oxide" itself is the dianion of oxygen, an O2– (molecular) ion. with oxygen in the oxidation state of −2. Most of the E ...
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
The liquidus temperature, TL or Tliq, specifies the temperature above which a material is completely liquid, and the maximum temperature at which crystals can co-exist with the melt in thermodynamic equilibrium. It is mostly used for impure subst ...
.
One example concerns crystallization of melts that form
mafic and
ultramafic rocks. MgO and SiO
2 concentrations in melts are among the variables that determine whether
forsterite
Forsterite (Mg2SiO4; commonly abbreviated as Fo; also known as white olivine) is the magnesium-rich end-member of the olivine solid solution series. It is isomorphous with the iron-rich end-member, fayalite. Forsterite crystallizes in the orthor ...
olivine or
enstatite
Enstatite is a mineral; the magnesium endmember of the pyroxene silicate mineral series enstatite (MgSiO3) – ferrosilite (FeSiO3). The magnesium rich members of the solid solution series are common rock-forming minerals found in igneous and m ...
pyroxene
The pyroxenes (commonly abbreviated to ''Px'') are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. Pyroxenes have the general formula , where X represents calcium (Ca), sodium (Na), iron (Fe II) ...
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 diagram
A phase diagram in physical chemistry, engineering, mineralogy, and materials science is a type of chart used to show conditions (pressure, temperature, volume, etc.) at which thermodynamically distinct phases (such as solid, liquid or gaseous ...
s for simple mixtures provide insights into general principles. Numerical calculations with special software have become increasingly able to simulate natural processes accurately.
Sedimentary rocks
Fractional crystallization is important in the formation of sedimentary evaporite rocks.
See also
*
*
*
*
*
References
{{DEFAULTSORT:Fractional Crystallization (Geology)
Igneous rocks
Igneous petrology
he:התגבשות הדרגתית