Minerals transform in many different ways as a result of changes in their physical and chemical environment. Attempts to understand transformation processes have led to classifications based on thermodynamic, structural or kinetic criteria, all of which provide a description of some aspect of the transformation. Two broad categories of transformations are those that involve changes in composition, such as unmixing of a solid solution, and polymorphic transformations in which there is a symmetry change but no change in composition. Often the latter are referred to as phase transitions, particularly when the various polymorphs are structurally related modifications.

Here we will not be overconcerned with the formalities of the various classifications, nor will we attempt to classify rigorously the mineral transformations we describe. Many aspects of transformation processes are common to different transformation types. For example in a thermodynamically first-order transformation (Section 8.4.2) two phases coexist at equilibrium, and are separated by an interface. The formation of the new phase involves a nucleation event, followed by subsequent growth. The structural relationship between the two phases controls the nature of the interface which in turn controls the way the new phase grows and its orientational and spatial relationship to the parent phase. This textural relationship between the two intergrown solid phases is referred to as the microstructure.