Fractional crystallization (geology)

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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, which changes the composition of the melt.

Fractional crystallization in silicate melts (magmas) is a very complex process compared to chemical systems in the laboratory because it is affected by a wide variety of phenomena. Prime amongst these is the composition, temperature and pressure of a magma during its cooling. The partial pressure of vapor phases in silicate melts is also of prime importance, especially in near-solidus crystallization of granites.

The composition of a magma is the primary control on which mineral is crystallized as the melt cools down past the liquidus. For instance, in mafic and ultramafic melts, the MgO and SiO₂ contents determine whether forsterite olivine or enstatite pyroxene is precipitated.

Two magmas of similar composition and temperature at different pressure may crystallize different minerals. An example is high-pressure fractional crystallizaion of granites to produce single-feldspar granite, and low-pressure conditions which produce two-feldspar granites.

Main article: Igneous differentiation