Phase equilibrium

Phase equilibrium is the stable coexistence of distinct mineral phases under specified pressure, temperature, and bulk composition. It is constrained by thermodynamics and chemical mass balance.

Gibbs free energy: A thermodynamic potential that predicts equilibrium at constant temperature and pressure. Stable mineral assemblages minimize this potential under fixed pressure and temperature.
Mineral: A naturally occurring crystalline solid with a characteristic chemical composition and atomic structure. Each distinct mineral can constitute a separate phase in an equilibrium assemblage.
Metamorphic facies: A set of metamorphic mineral assemblages characteristic of particular pressure–temperature conditions. Assemblages define facies because stable mineral combinations vary with pressure and temperature.
Metastability: A condition in which a system persists in a local energy minimum rather than reaching global equilibrium. A metastable mineral assemblage can persist even when another assemblage is thermodynamically favored.
Chemical potential: The change in a system’s thermodynamic potential when the amount of a component changes. Coexisting phases require equal chemical potentials for each transferable component.
Phase (matter): A physically distinct, homogeneous portion of a system with uniform properties and composition. Phase equilibrium concerns the stable coexistence of such distinct portions.
Metamorphic petrology: The study of rocks transformed by changes in temperature, pressure, and chemical environment. Equilibrium relations help reconstruct the conditions recorded by metamorphic minerals.
Kinetics (chemistry): The study of reaction rates and the mechanisms by which chemical reactions proceed. Kinetics determines whether minerals can reach the equilibrium assemblage within available time.
Gibbs phase rule: A relation among degrees of freedom, components, and phases in a system at equilibrium. It counts how many conditions can vary while a mineral assemblage remains in equilibrium.
Chemical component: An independently specified chemical constituent used to describe the composition of a system. Component choice determines how mineral compositions and phase-rule constraints are counted.

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