Mineral Formation and Stability

Minerals form and persist only within the ranges of temperature, pressure, and composition where their structures are thermodynamically stable.

Definition

The study of the physical and chemical conditions, governed by thermodynamics and kinetics, under which minerals crystallize, transform, and remain stable.

Scope

This topic covers the thermodynamic and kinetic controls on mineral formation: Gibbs free energy, phase diagrams and stability fields, nucleation and crystal growth, polymorphic transitions, exsolution, and the distinction between equilibrium and metastable persistence. It explains why mineral assemblages record the conditions under which rocks formed.

Core questions

• How does Gibbs free energy determine which mineral is stable at given conditions?
• Why do metastable minerals persist outside their stability fields?
• What controls the rate of nucleation and crystal growth?
• How do polymorphic transitions and exsolution record changing conditions?

Key theories

Gibbs free energy minimization

At fixed temperature and pressure the assemblage with the lowest total Gibbs free energy is stable, so phase boundaries in pressure-temperature space mark where competing minerals have equal free energy.

Nucleation, growth, and metastability

Because forming a new phase requires overcoming a nucleation energy barrier, minerals can persist metastably outside their stability fields, which is why many rocks preserve high-temperature or high-pressure assemblages at the surface.

Clinical relevance

Understanding mineral stability lets geologists use mineral assemblages as geothermometers and geobarometers, reconstruct the pressure-temperature histories of rocks, and predict mineral reactions during metamorphism, weathering, and industrial processing.

History

Gibbs's nineteenth-century formulation of chemical thermodynamics provided the theoretical basis for mineral stability; experimental petrologists in the twentieth century, beginning with Bowen's melting studies, mapped phase relations in mineral systems, turning thermodynamics into a practical tool for reading rock histories.

Key figures

• J. Willard Gibbs
• Norman L. Bowen
• Andrew Putnis

Seminal works

• putnis1992
• anderson2005

Frequently asked questions

Why do some high-pressure minerals survive at the surface?

Their transformation to lower-pressure forms is kinetically slow at low temperatures, so they persist metastably; diamond, stable only at great depth, is the classic example.

What is a stability field?

The region of a pressure-temperature or composition diagram within which a particular mineral or assemblage has the lowest free energy and is therefore the equilibrium phase.