A process in which two or more crystals, or parts of crystals, assume orientations such that one may be brought to coincidence with the other byreflection across a plane or by rotation about an axis. Crystal twins represent a particularly symmetric kind of grain boundary; however, the Energy of the twin boundary is much lower than that of the general grain boundary because some of the atoms in the twin interface are in the correct positions relative to each other.
The formation in a single crystal of regions of regularly changed orientation of the crystal structure. The structures of twin formations are eithermirror images of the atomic structure of the parent crystal (matrix) in a certain plane (the twinning plane) or are formed by rotation of the matrixstructure about the crystallographic axis (twinning axis) to an angle that is constant for a given material or by other symmetry transformations. The pair made up of the matrix and the twin formation is called the twin.
Twinning takes place during crystal growth because of violations in the packing of atoms during the growth of the atom layer on the nucleus or onthe formed crystal (stacking faults) and the intergrowth of neighboring nuclei (growth twins; see Figure l). It also takes place because ofdeformation upon mechanical action on the crystal, such as the impact of an indentor, tension, compression, twisting, bending (mechanical twins);rapid thermal expansion and contraction; heating of deformed crystals (recrystallization twins); and transition from one crystal modification toanother.
Crystal twinning is inter grown crystals that are formed in a symmetrical fashion.
It is caused by stress during the growth or temperature/pressure. These inter grown crystals are called twinned crystals.
Twinning is important to recognize cause it's helps identification of minerals in nature.
A crystal structure may contain a change in repeat-direction, to form a twin. Two or more parts of a twinned crystal structure are not related by the normal symmetry of that structure. They are related by a twin reflection plane or axis, or both. The twinning of a structure may be an accident of growth, or caused by stress or change in temperature during growth.
Crystal twinning occurs when two separate crystals share some of the same crystal lattice points in a symmetrical manner. The result is an inter-growth of two separate crystals in a variety of specific configurations. A twin boundary or composition surface separates the two crystals. Crystallographers classify twinned crystals by a number of twin laws. These twin laws are specific to the crystal system. The type of twinning can be a diagnostic tool in mineral identification.
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