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 by reflection 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.
Crystal twinning occurs when two separate crystals share some of the same crystal lattice points in a symmetrical manner. The result is an intergrowth of two separate crystals in a variety of specific configurations. A twin boundary or composition surface separates the two crystals.
Simple twinned crystals may be contact twins or penetration twins. Contact twins share a single composition surface often appearing as mirror images across the boundary. Quartz, gypsum, and spinel often exhibit contact twinning. In penetration twins the individual crystals have the appearance of passing through each other in a symmetrical manner. Orthoclase, staurolite, pyrite and fluorite often show penetration twinning.
If several twin crystal parts are aligned by the same twin law they are referred to as multiple or repeated twins. If these multiple twins are aligned in parallel they are called polysynthetic twins. When the multiple twins are not parallel they are cyclic twins.
Albite, calcite, and pyrite often show polysynthetic twinning. Closely spaced polysynthetic twinning is often observed as striations or fine parallel lines on the crystal face. Rutile, aragonite and chrysoberyl often exhibit cyclic twinning, typically in a radiating pattern.
There are three modes of formation of twinned crystals. Growth twins are the result of an interruption or change in the lattice during formation or growth due to a possible deformation from a larger substituting ion. Annealing or Transformation twins are the result of a change in crystal system during cooling as one form becomes unstable and the crystal structure must re-organize or transform into another more stable form. Deformation or gliding twins are the result of stress on the crystal after the crystal has formed. Deformation twinning is a common result of regional metamorphism.
Of the three common crystal structures: BCC, FCC AND HCP, the HCP structure is the most likely to twin. Crystals that grow adjacent to each other may be aligned to resemble twinning. This parallel growth simply reduces system energy and is not twinning.
Twin boundaries occur when two crystals of the same type intergrow, so that only a slight misorientation exists between them. It is a highly symmetrical interface, often with one crystal the mirror image of the other; also, atoms are shared by the two crystals at regular intervals. This is also a much lower-energy interface than the grain boundaries that form when crystals of arbitrary orientation grow together.
Twin boundaries are partly responsible for shock hardening and for many of the changes that occur in cold work of metals with limited slip systems or at very low temperatures. They also occur due to martensitic transformations: the motion of twin boundaries is responsible for the pseudoelastic and shape-memory behavior of nitinol, and their presence is partly responsible for the hardness due to quenching of steel.
the formation in a single crystal of regions of regularly changed orientation of the crystal structure. The structures of twin formations are either mirror images of the atomic structure of the parent crystal (matrix) in a certain plane (the twinning plane) or are formed by rotation of the matrix structure 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 on the formed crystal (stacking faults) and the intergrowth of neighboring nuclei (growth twins; see Figure l). It also takes place because of deformation 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 to another.
In metals, the shift of a part or of the entire crystal into the twin position is accomplished by the layered glide of the atomic planes. Each plane is successively displaced by a fraction of the interatomic distance, in which case all the atoms in the twinning region are displaced by a distance proportional to their distance from the twinning plane (plane of regular reflection). In other crystals this process is more complex—for example, for calcite, CaCO3, rotation of the CO3 groups also occurs. Mechanical twins are formed when deformation by glide in the direction of the applied force is inhibited.
Twinning may be accompanied by changes in the dimensions and shape of the crystal; this is characteristic, for
Figure 1. Growth twins
example, of CaCO3. Twinning of CaCO3 may also be achieved by the pressure of a blade (see Figure 2), in which case the region of the right-hand part of the crystal shifts to the twin position. Twinning accompanied by a change in shape is observed in all metals, semiconductors (such as germanium and silicon), and many other dielectrics. Another form of twinning, which is not accompanied by changes in shape, is observed in such substances as quartz and tri-glycine sulfate.
Figure 2. Twinning of calcite by the pressure of a blade (Baumhauer’s method)
If the structural homogeneity of a single crystal is disrupted by a great number of twin formations, the crystal is called a polysynthetic twin (Figure 3). In ferroelectrics, twinning
Figure 3. Polysynthetic twin of Rochelle salt (left); polysynthetic twin of triglycine sulfate, developed by etching and photographed in reflected light (right)
formations are also ferroelectric domains, which are, however, characterized by different optical properties (Figure 4).
Figure 4. Diagram of the position of the optical indicatrix: (a) in a rhombic crystal of Rochelle salt; (b) and (c) in the components of a twin that are stretched along the axes c and b) of a monoclinic crystal
Twinning strongly affects the mechanical properties of crystals, such as strength, plasticity, and brittleness, as well as their electrical, magnetic, and optical properties. It also reduces the quality of semiconductor devices.
The principles of mechanical crystal twinning are used in geology in the diagnosis of minerals and for determining the conditions of rock formation. The distribution of twinned crystal layers in rock-forming minerals makes it possible to characterize the influences to which the rock was subjected. Mechanical twinning is taken into account by geologists and petrographers during analysis of the flow of rock after deformation.
The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.