Abstract
Thermodynamically, a solution is a special description of a homogeneous mixture for which it is convenient to distinguish between the solvent A on the one hand and the solutes B, C, ... , on the other hand. This asymmetrical description arises from a different choice of the standard states, i.e. the thermodynamic functions of solvent and solutes are referred to the states of the pure substance and infinite dilute solution, respectively. A solid solution, however, usually consists of components which are thermodynamically treated in the same way, i.e. with the states of the pure substances as reference states. In some textbooks on chemical thermodynamics (e.g. McGlashan, 1979) they are therefore referred to as homogeneous solid mixtures. The term solid solution, which implicitly suggests mixing at the scale of single crystals, is nevertheless the common expression used in geochemical literature.
This article focuses on physicochemical fundamentals and comparatively simple models of solid solutions. Several other, more complicated models have been described in literature. Related concepts are ordering models or the thermodynamics of defect structures (see Mineral defects).