Geochemical thermodynamics applies the principles of chemical thermodynamics -- Gibbs free energy, enthalpy, entropy, and chemical potential -- to predict the stability of minerals, the direction of geochemical reactions, and the composition of natural systems at equilibrium. The central quantity is the Gibbs free energy of reaction (delta-G), which determines whether a reaction proceeds spontaneously at given temperature and pressure. At equilibrium, delta-G = 0, and the equilibrium constant K relates to standard-state free energy by delta-G-naught = -RT ln K. Because geological systems operate over enormous temperature (0-1400 C) and pressure (1 atm to 30+ GPa) ranges, geochemical thermodynamics must account for T-P dependence of thermodynamic properties -- a complexity rarely encountered in benchtop chemistry.
Geochemical thermodynamics takes the abstract framework of chemical thermodynamics and applies it to the messy, heterogeneous, extreme-condition systems of the Earth. The core question is always the same: given the temperature, pressure, and composition of a system, what minerals, fluids, and gases should be present at equilibrium?
The Gibbs free energy is the master variable. For any reaction, delta-G = delta-G-naught + RT ln Q, where Q is the reaction quotient. If delta-G is negative, the reaction proceeds forward; if positive, it proceeds in reverse; at equilibrium, delta-G = 0 and Q = K. Standard-state thermodynamic data (delta-G-naught-f, delta-H-naught-f, S-naught, and heat capacity Cp) for minerals, aqueous species, and gases are tabulated in databases (Holland and Powell, SUPCRT, JANAF) and form the foundation for all equilibrium calculations.
The geological challenge is that standard-state data are typically for 25 C and 1 bar, while geological processes occur at temperatures up to 1400 C and pressures up to 30 GPa. Extrapolating thermodynamic properties requires heat capacity data (for temperature dependence), molar volume and compressibility data (for pressure dependence), and equations of state for fluids and melts. For aqueous species at hydrothermal conditions, the HKF (Helgeson-Kirkham-Flowers) model provides a framework for calculating properties up to 1000 C and 5 kbar.
A key conceptual distinction is between equilibrium and metastability. Thermodynamics predicts what should exist at equilibrium, but many geological materials persist far from equilibrium because reaction kinetics are too slow. Diamond is thermodynamically unstable at Earth's surface (graphite is the stable carbon polymorph at 1 atm), yet diamonds persist for billions of years because the activation energy for the transformation is prohibitively high at surface temperatures. This tension between thermodynamic prediction and kinetic reality pervades geochemistry.