Twinning, epitaxy, syntaxy, etc., describe the joining together or “intergrowth” of two individual crystals along a regular crystallographic orientation. Regular types of intergrowth (intergrowth relations) may also result when a crystal is decomposed or exsolved into two phases with different compositions, starting from one phase at the time of growth and forming a lamellar texture through changes of temperature and pressure. In all these intergrowths a new interface is introduced between the two individuals, and so they are in a higher energetic state than a single crystal by the amount of energy corresponding to the newly introduced interface. In some twinning relations, a re-entrant corner (edge) is also introduced between the two individuals, where preferential growth may take place. As a result, twinned crystals often show a different morphology from that of the coexisting single crystals. The aim of this chapter is to analyze the reasons for the features that result from the regular intergrowth of crystals.

Regular intergrowth relations

Crystals often intergrow with respect to a certain crystallographic orientation. The following intergrowth relations of two or more single crystals of the same species have been identified:

1. (1) twin;

2. (2) coincidence site lattice (CSL);

3. (3) parallel intergrowth.

Preferential nucleation along edges or at corners should also be mentioned. The following are examples of regular intergrowth between crystals of different species:

1. (1) epitaxy;

2. (2) syntaxy or topotaxy;

3. (3) exsolution or precipitation;

4. (4) spinodal decomposition.