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Crystal Twinning: Definition & Symmetry

Instructor: Julie Zundel

Julie has taught high school Zoology, Biology, Physical Science and Chem Tech. She has a Bachelor of Science in Biology and a Master of Education.

There are many things that can have twins, including crystals! This lesson will explore crystal twinning, including the twinning operations associated with symmetry and different mechanisms that can lead to twinning.

What is Crystal Twinning?

You show up to a party wearing the same shirt as your friend. Twinning! Or you notice that there are three sets of twins at your school. Twinning! Or the rock on your windowsill is made up of two crystals. Twinning!

While we could go on and on about ways we can use the word ''twinning,'' the focus here will be on our last example. Specifically, crystal twinning is when two or more crystals are formed in a symmetrical way in a single sample.

Twinning in pyrite
pyrite

A crystal is a material that has a specific structure, including symmetrical surfaces and three-dimensional patterns. Crystals are all around us, from the salt on your French fries to the quartz rock on the bottom of a river.

If you were able to zoom in on a crystal and look at its atoms, you'd notice that they are in symmetrical arrangements that repeat. These repeating patterns in a crystal are called crystal lattices.

Imagine a lattice point as any point within the crystal lattice where you could put an atom. During twinning, lattice points of one crystal are shared with another crystal. The area where the two crystals are separated is termed the twin boundary. Finally, the surface that the twinned crystals share and the lattice points shared between the crystals is called the composition surface.

The striations show the twin boundaries between twinned crystals
boundary point

Twinning Operations and Types

The way twins can occur is based on a few factors called twinning operations, which are associated with symmetry. Let's get these terms under our belt before we move on.

  • Mirror images: Sometimes, twins are mirror images of one another separated by a twin boundary. The added twin is referred to as the twin plane.
  • Rotational: Here, there's a rotation around an axis, and the added rotational axis is termed a twin axis.
  • Inversion: Sometimes, a newly added point of symmetry occurs, which is termed a twin center.

The type of twins is, in part, determined by the twinning operations. For example, contact twins have a well-defined composition surface and are mirror images of each other. Penetration twins, on the other hand, do not have a definitive composition surface, and the twinned crystals can penetrate one another (hence the name). Penetration twins have a twin center or a twin axis.

Formation

Crystal twinning can occur over different periods of a crystal's life, and it is often due to stress, like temperature or pressure changes. Let's explore three mechanisms for crystal twinning: growth, transformation, and deformation.

Growth

Most crystal twinning results from growth twinning, which occurs when there is a change that occurs during the formation and growth of the crystal. During normal crystal formation, atoms are layered on top of one another in a pattern.


A salt crystal. Note how the atoms are layered in a pattern.
NaCl

In growth twinning, however, when the crystal is being formed, there may be a different or missing atom in the lattice structure, which changes the crystal-forming pattern. This difference could be due to a temperature or pressure change.

Transformation

Transformation twinning happens after the crystal is formed and is the result of a change in temperature or pressure that alters the way the atoms are oriented. For example, while quartz is made up of silicon and oxygen, the way in which it is configured can vary depending upon the temperature. Alpha quartz, for example, changes into beta quartz at 573 degrees Celsius. This change in the configuration of quartz can lead to twinning.

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