Understanding Twin Formation in Crystals: Causes and Characteristics
As the name suggests it is a combination of two persons or things. Here we will discuss twinning in Crystallography. Twinning in crystal occurs when two different crystals share some of the common crystal lattice points in a symmetrical way. As a result of this there is an intergrowth of two separate crystals in a variety of shape configurations. The surface along which the lattice points are shared in twinned crystals is known as a composition surface or sometimes it refers to twin planes. Twinning also occurs when reflected images along the common twinning plane,formed by repetitions of rotated planes about a common twinning axis, or both. In this article we will learn what is twinning and about crystal twinning.
What is Twinning ?
In crystallography twinning is related to regular intergrowth of two or more crystal grains so that each grain is a reflected image of its neighbour or it is rotated with respect to it. Other grains are also added to the twin in the formation of crystals that often appear symmetrically joined. Sometimes it is in the form of a starlike or crosslike shape. It mostly occurs in the beginning of crystal growth. The individual structures that form a twin have atomic structures having different orientations, but they must have certain common planes or directions. They must fit simply and it is derived from each other by a simple movement. There are different kinds of twin crystals formed.
Crystal Twinning
Twinning in crystals occurs when there are two different types of crystals that share the same crystal lattice points in a symmetrical manner. A twin boundary or composition of surfaces are separated by the crystals. Crystallographers classify twinned crystals by the number of twin laws. These twin laws are related to the crystal system.
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During the growth of a crystal, or sometimes when the crystal comes under stress, temperature or pressure conditions different from those under which it originally formed, two or more intergrown crystals are formed in a symmetrical form. These symmetrical intergrowths of crystals are known as twinned crystals. Twinning is important in crystals to recognize, because when it occurs, it is often one of the most diagnostic features that help in identification of the mineral.
Symmetry Operations that Define Twinning
Symmetry is added to a crystal by twinning, and twining can be defined by the symmetry operations that are involved. These include:
Reflection across a mirror plane- The added mirror plane is a twin plane.
Rotation about an axis or line in the crystal twinning- Here the added rotation axis is known as a twin axis.
Inversion through a point- The added center of symmetry is known as the twin center.
Types of Twinning
Merohedral twinning - This type of twinning occurs in the crystal when the lattices of the contact twins superimpose in three dimensions, by the relative rotation of one twin from the other. An example is meta zeunerite.
Penetration Twins - Here the individual crystals pass through each other in a symmetrical manner. Orthoclase, staurolite, pyrite, and fluorite often formed by penetration twinning.
Galvanized surface with macroscopic crystalline features. Twin boundaries are visible within each crystallite, that are most prominently in the bottom-left and top-right. If several twin crystal parts are aligned by the same twin law then they are known as multiple or repeated twins.
When these multiple twins are aligned in parallel they form a structure of polysynthetic twins. When the multiple twins are not aligned in parallel then they are known as cyclic twins. Albite, calcite, and pyrite often show polysynthetic twinning. They are closely kept in space with polysynthetic twinning is often observed as striations or fine parallel lines on the crystal face. Rutile, aragonite, cerussite, and chrysoberyl often exhibit cyclic twinning property, that typically occurs while radiating pattern.
Origin of Twinning
Origin of twinning originates in three different ways named as, growth twins, transformation twins, and glide or deformation twins.
Growth Twins - It occurs during the process of crystal growth and a new crystal is added to the face of an already existing crystal.
Transformation Twins - Transformation of twinning occurs when a preexisting crystal undergoes a transformation when there is change in pressure or temperature. It commonly occurs in minerals to form different crystal structures and different symmetry at different temperatures or pressures. When there is change in temperature or pressure a new crystal structure and symmetry of crystal is stable, there different parts of the crystal are arranged in different symmetrical orientations, and thus it helps in an intergrowth of one or more crystals. Dauphiné and Brazil twinning in quartz is a common example of transformation twins.
Deformation Twins - During the deformation process in the crystal we can push atoms out of place. If this happens to form a symmetrical arrangement, it produces deformation twins.
Twinned crystals may be described as follows:
Simple Twins – It consists of only two parts.
Multiple Twins – It is formed by the composition of more than two orientations.
Contact Twins – This occurs if a definite composition plane is present.
Penetration Twins – It is formed when two or more parts of a crystal appear to interpenetrate each other with the surface between the parts that is indefinable and irregular..
Polysynthetic Twinning – It mainly occurs in crystal when three or more individuals are repeated alternately on the same twinned plane.
FAQs on Twinning Crystallography Explained: Principles, Types & Importance
1. What is twinning in crystallography?
Twinning is a crystallographic phenomenon where two or more separate crystals of the same substance share a portion of the same crystal lattice in a definite, symmetrical orientation. This intergrowth results in a 'twinned crystal', where the different segments are related to each other by a specific symmetry operation, such as a reflection across a twin plane or a rotation around a twin axis.
2. What are the primary classifications of crystal twinning?
Crystal twinning can be classified based on its formation mechanism and morphology. The main types include:
- Contact Twins: Two crystals appear to be joined at a single, shared plane, known as the composition plane.
- Penetration Twins: The individual crystals appear to grow through each other, forming an irregular composition surface.
- Polysynthetic Twins: A series of repeating, parallel contact twins are formed, giving the crystal a laminated or striated appearance.
- Cyclic Twins: Successive twinning operations on non-parallel planes result in a circular or star-shaped crystal arrangement.
3. What is a twin law in crystallography?
A twin law is the specific symmetry operation that describes the precise relationship between the two parts of a twinned crystal. It defines the twin axis (the axis of rotation) or the twin plane (the plane of reflection) that would bring one part of the crystal into the orientation of the other. For example, the Spinel Law is a common twin law in cubic crystals where the twin axis is [111].
4. Can you provide some real-world examples of twinning?
Twinning is very common in minerals and materials. Some key examples include:
- Calcite: Often exhibits simple contact twins.
- Plagioclase Feldspar: Famous for its polysynthetic twinning (specifically Albite twinning), which causes fine striations on its surface.
- Quartz: Can form both Brazil twins (intergrowth of right- and left-handed structures) and Dauphiné twins (related by a rotation).
- Rutile: Frequently forms cyclic twins, creating knee-shaped or star-like aggregates.
5. How does a twin boundary differ from a grain boundary?
A twin boundary is a highly ordered and symmetrical interface where the crystal lattice is mirrored across a plane. The atomic arrangement is specific and creates a low-energy boundary. In contrast, a grain boundary is the interface between two randomly oriented crystals (grains). It is typically a disordered, higher-energy region where the atomic arrangement does not follow a simple symmetry rule.
6. What is the significance of twinning for the mechanical properties of a material?
Twinning plays a crucial role in how materials deform under stress. It is a key deformation mechanism alongside dislocation slip. The formation of twins can increase the strength and hardness of a material by creating new boundaries that impede dislocation movement. In some materials, like magnesium alloys and shape-memory alloys, twinning is the dominant factor controlling their ductility and unique properties like the shape-memory effect.
7. How is mechanical twinning different from growth twinning?
The main difference lies in their formation. Growth twins form during the initial crystallisation process due to an 'accident' or perturbation in growth, where a new crystal layer attaches in a twinned orientation. In contrast, mechanical twins (or deformation twins) form in a pre-existing solid crystal when it is subjected to mechanical stress. This stress forces a section of the lattice to shear into a twinned orientation.
8. What does polysynthetic twinning mean?
Polysynthetic twinning describes a condition where multiple, repeating twin planes occur parallel to one another within a single crystal. This creates a stack of very thin, alternating twin lamellae, often visible as fine, parallel lines or striations on the crystal's surface. A classic example is the Albite twin law in plagioclase feldspar, which is diagnostic for the mineral.
9. Can crystal twinning be intentionally created or removed?
Yes, in many cases. Mechanical twinning can be intentionally induced through controlled deformation processes like rolling or forging to strengthen materials, a technique known as twin-boundary engineering. Conversely, in some materials like shape-memory alloys, the deformation caused by twinning can be reversed by heating the material, a process called detwinning, which allows the material to return to its original shape.
10. Why are some crystal structures more likely to twin than others?
The tendency for a crystal to twin depends heavily on its internal structure and symmetry. Twinning is more common in crystals with lower symmetry (e.g., hexagonal or monoclinic systems) because there are fewer available slip systems for deformation, making twinning a more favourable alternative under stress. Furthermore, materials with a low stacking fault energy are more prone to twinning, as the energy cost to create the twinned orientation is minimal.
