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Introduction to Crystallography

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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. 

FAQ (Frequently Asked Questions)

1. Name Common Twin Laws.

Ans: Following are the common twin laws:

  • Triclinic System

  • Monoclinic System 

  • Orthorhombic System

  • Tetragonal System

  • Hexagonal System 

  • Isometric System 

2. What are Pseudo Twins?

Ans: Pseudo-twins is formed in ordered structures under high stress conditions. These twins are formed with the help of lattice sites and are placed at twin positions but are incorrectly occupied by different species of atoms.

3. Define Monohedral Twin?

Ans: Merohedral twins have a twin symmetry element which is not a symmetry element of the space group of the sample but it is a symmetry element of the Laue group. Both the direct and reciprocal lattices of the different domains coincide with one another, that makes the crystal appear to be single until near the end of refinement. This type of twinning occurs sometimes described as twin lattice symmetry.