The thermodynamic framework links the state of chemical equilibrium to certain types of energy for a set of chosen variables, i.e. often pressure, temperature, and phase compositions. This energy is then used to calculated the equilibria at the physical and chemical conditions of choice starting from values tabulated at reference conditions, often room temperature and pressure, using a set of well defined experimentally accessible parameters. However, in many instances not only the equilibrium is of geological interest but also the chemical processes leading to it. In a macroscopic view, at equilibrium all chemical changes will come to a halt and equilibrium thermodynamics is not able to describe the actual mechanism of reaction. But other physical-chemical tools are available to deal with topics like diffusion, reaction textures, and non uniform fields and their consequences like stress and strain.

Equilibrium thermodynamics will be the main topic of this sie. Without the knowledge of the equilibrium conditions research on the path leading to it is meaningless. The theory of thermodynamics is in its fundamentals exact. It is based on mathematical rules and fundamental physical laws. Because of the mathematical background, many students in geoscience consider thermodynamics to be a difficult subject. It provides, however, a powerful tool to solve numerable questions in geochemistry, petrology, mineralogy, economic geology, hydrogeology environmental geology and even geophysics.

In the following only the thermodynamic topics which matter most in geosciences are treated whereas others are left a bit aside. It is the intention, however, to treat the relevant topics as extensively as possible. Where felt necessary full derivations for equations and methods are provided. Many of them are usually not found in texts like these and often deeply buried in the physical-chemical literature. Sometimes it seems that the reason for certain procedures were lost out of sight over time which may make understanding of these topics difficult.