This individual crystals composing any group may have various relationships to each other. In the majority of cases these relations are irregular and result from accidental conditions of growth. In other cases, two or more individual crystals may grow together in such a way as to yield certain crystal domains or faces parallel, whereas others are in a reverse or rotated position with respect to one another. These regular and symmetrical intergrowths of the same kind of crystal are called twins . Many minerals frequently are twinned, and include common species such as plagioclase and staurolite.
Twins often appear to consist of two or more individual crystals symmetrically united.
A twin can be described as a rational, symmetrical intergrowth of two or more individuals of the same mineral. Twins are distinctly different from the random mechanical intergrowths that are symmetrically unrelated. In the context of twinning, rational means that the two portions of the twin have a point, a line or a plane in common, and symmetrical means that the parts of the twin are related by some crystallographic symmetry operation, such as reflection, rotation or translation. The plane on which the twinned crystals are joined is called the composition plane . Figure 6-8 is a simplified attempt to illustrate how a twin might form during the growth of a crystal.