Single phases and their assemblages are able to react, melt, crystallize, evaporate, condense, sublimate, dissolve, precipitate, mix, exsolve, order, disorder or rearrange. Quantification of such chemical interactions might give valuable insight into the systematics of geological processes. The key of the quantification is the knowledge of the chemical equilibrium.

The equilibrium conditions of chemical interactions can be approached by two means. The first uses direct experimental investigations, while the second applies thermodynamic calculations. Performing and evaluating experiments is straightforward but is often a tedious procedure and by far not all systems can be studied at the required physical and chemical conditions, considering all of the possible variables in a reasonable time frame. Therefore experimental investigations are in many instances only cornerstones and sometimes just case studies in simplified systems. But the thermodynamic framework provides a powerful tool to interpolate or even to extrapolate complex phase equilibria at least in principal to any desired condition.