Minerals do not always grow under ideal conditions to form perfect crystals. During growth (or after), some crystals may form twins.
There are basically two types of twins:
- contact twins
- penetration twins
There are typically 3 causes of twinning:
- during crystal growth
- from transformation
- by deformation
Types of twins
Contact twins share a single plane (or face).
Penetration twins share a single axes (usually a rotation axis).
Causes of twinning
During the growth of a crystal there may be changes inside the magma (like temperature, pressure and flow) which cause elements of the magma to alter a crystal's orientation.
Most twinning occurs under these circumstances.
When a fully grown crystal experiences sudden change in temperature and pressure, the internal arrangement of atoms can shift. A good example of this is quartz.
At temperatures above 573°C, quartz is hexagonal and is termed beta-quartz. Below that temperature, beta-quartz will transform into trigonal alpha-quartz, and twinning will occur during this transformation.
When a fully grown crystal undergoes mechanical stress (such as pressure), the crystal lattice may be distorted. One can imagine this by laying the shell of a matchbox on a table and applying slight pressure to it. The matchbox will shear, distorting its form. When two matchbox shells are placed next to each other, the same pressure will cause them to shear into an arrow head.
An example of this is calcite.