One of the central themes in mineralogy is the study of mineral behaviour, which describes the response of a mineral structure to a change in the physical or chemical environment. The structure responds so that its free energy always tends to a minimum value. In the process the structure may distort and change its symmetry; it may undergo a major reconstructive transformation to a new polymorph, or alternatively, occupancies of the cation sites may become more ordered or less ordered. Whatever the nature of the response, the direction of change is always in the direction of a free energy minimum, i.e. towards an equilibrium state, and in this chapter we will outline the thermodynamic concepts involved in defining mineral equilibrium in simple systems.

On a larger scale, when we consider an assemblage of minerals in a rock, it too responds to changes in the physical and chemical environment by reactions between the minerals. Cation interchange may alter their relative compositions, or alternatively completely new mineral assemblages may form. For any given bulk composition, changing the temperature and pressure conditions defines a sequence of different equilibrium states consisting of different assemblages of coexisting minerals. The study of the way mineral assemblages in rocks respond to changing conditions in the Earth is a central theme in the field of metamorphic petrology, and in having to deal with a large number of possible mineral reactions in various rock compositions, the thermodynamic procedures used there are somewhat different to those we will describe in this chapter.