Twinning is a phenomenon in mineralogy whereby a single crystal of a mineral has two or more parts in which the crystal lattice is differently orientated.
The shared surface between two twins is called the composition or twin plane, and the orientation to either other is determined by symmetry through rotation or reflection; this relationship is described by a twin law.
Schematic showing the change in crystal structure across a twin plane. Atoms are shared by the two twins at regular intervals. Credit: Imperial College London