Crystal fractionation

The separation of crystals from melt in a cooling magma, which drives the residual liquid toward compositions increasingly different from the original rock.

Bowen's reaction series: The ordered sequence in which minerals crystallize from cooling magma, from olivine and calcium plagioclase toward quartz. Fixes which minerals settle out at each temperature, setting the melt's changing path.
Magmatic differentiation: The family of processes by which a single magma produces rocks of varied composition. Crystal fractionation is the dominant member of this family; the result is the diversity of igneous rock types.
Partial melting: The process where a rock melts incompletely, generating a melt enriched in low-melting-point components. Creates compositional variety before any crystallization; the two are often confused as origins of diverse magmas.
Bushveld Complex: A vast South African layered intrusion hosting most of the world's platinum and chromium ores. Shows fractionation operating at a scale of tens of thousands of cubic kilometres.
Eutectic crystallization: Crystallization behavior where melts solidify at a fixed temperature and composition until one component is exhausted. The phase-rule logic that makes fractional removal of crystals change melt composition predictably.
Settling of crystals: Dense crystals sinking through magma under gravity, the simplest way crystals and melt separate. The physical act of separation; without it crystals stay in equilibrium with the melt.
Felsic magma: Silica-rich magma, such as rhyolite, typically produced as the final residual liquid of extensive fractionation. The end product when basaltic melt loses iron and magnesium minerals to settling.
Assimilation: The incorporation and digestion of surrounding country rock into rising or stored magma. Alters melt composition by adding material rather than removing crystals.
Palisades Sill: A New Jersey diabase sill whose olivine-rich basal layer was explained by crystal settling. An early, accessible demonstration that crystals sank within a single simple intrusion.
Phase diagrams: Graphs mapping which minerals are stable at given temperature, pressure and composition. The graphical tool used to predict and interpret crystallization sequences.

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