In this chapter we discuss various aspects of the mechanism of transformations in minerals in which there is no change in chemical composition. Such polymorphic transformations are often described in terms of the degree of similarity between the structures, which in turn can be used to define the structural changes required to transform one to the other. Often a kinetic classification is also implied in this approach. For example a transformation between two very similar structures may only involve a distortion of the bonds such as that between the high and low silica polymorphs (Section 6.8.1). Such displacive transitions are generally fast and cannot be prevented from occurring even with very rapid cooling rates (i.e. they are unquenchable). Displacive transitions may be thermodynamically first- or second-order (Section 8.4.2).

On the other hand, a reconstructive transition involves a major reorganisation of the structure, with bonds being broken and new bonds formed. Transformations between the silica polymorphs (quartz ⇔ tridymite ⇔ cristobalite) are typical reconstructive transitions which have high activation energies and are kinetically very sluggish. Rapid cooling can easily quench the high temperature form which may persist indefinitely at low temperatures. The other classic example is the persistence of the diamond structure at atmospheric temperatures and pressures where graphite is the stable form of carbon. Reconstructive transformations are always thermodynamically first-order.

Order-disorder transitions may be slow, as in the case of substitutional disorder (e.g. Si,Al disorder in aluminosilicates) or fast, as in orientational disorder (e.g. the orientation of the CO3 group in calcite – see Section 8.13.2 and Figure 8.38).