All About Crystals

What is a Crystal?

Audience: KS1&2 Curriculum Links: KS1&2 Science (Properties of Materials), KS1&2 Maths (Geometry: 2D & 3D shapes, nets)

Everything around us is made up of tiny atoms. These different types of atoms are arranged in different ways in different materials. They might be tightly packed together, like in a solid, or very far apart and able to move around each other, like in a gas. Atoms can even join together to form molecules, which are made up of two or more atoms bonded together. It is the way that these atoms or molecules are arranged that makes something we call a crystal very special.

What Does a Crystal Look Like?

Well, crystals come in a variety of different colours and shapes, but it’s not what they look like on the outside that’s interesting – it’s what’s on the inside that counts! We know that for a material to be a crystal it must have a repeating pattern of atoms inside it, but we can describe this crystal structure (also known as the crystal lattice) by looking at the shape of this repeating pattern. The smallest repeating pattern of atoms

A snowflake is a crystal

A crystal is a type of solid material. Inside a crystal, the atoms that make it up are arranged in a very structured pattern that repeats itself throughout the entire crystal. If a material does not have this repeating pattern of atoms inside of it, it is not a crystal even if it looks like one on the outside. Crystals can be found all around us, from everyday things like the salt we put on our chips to the beautiful rocks used in jewellery. Snowflakes are also an example of a crystal. Crystals can be formed naturally by various processes within the Earth or can be grown artificially in special labs. Sometimes we need very high temperatures and pressures to form crystals; for example, diamonds are made this way.

is called the unit cell and these unit cells are 3D shapes that repeat throughout the entire crystal. Different crystals have different shaped unit cells, but a unit cell can’t just be any shape – they must be able to tessellate, which means that the shapes fit together in all 3 dimensions without any gaps. For example, a cube is a 3D shape that can tessellate and is actually the unit cell shape for many crystals. Inside these 3D unit cell shapes, the atoms that make up the crystal can be placed in different locations. For example, you might find an atom at the corner of each unit cell. You might also find an atom in the centre of each face of the unit cell shape. They might even be

An example of a 3D shape that tessellates

Table salt, the type of salt we use on our food, and diamond are both crystals, but in fact they have more similarities than just that. If we look at their structure, we can see that both their unit cells are in the shape of a cube. We say that crystals with this type of shape for their unit cell have a cubic structure.

How is Salt the Same as a Diamond?

Unit cell structure of table salt (sodium chloride). Here, the red circles are the sodium atoms and the blue are the chlorine.

Unit cell structure of diamond. Each black circle represents a carbon atom.

An image of a diamond crystal

along the edges of the shape, or somewhere else within the middle of the shape. However, the point is that wherever the atoms are in one unit cell, they are in exactly the same positions in every other unit cell that make up the crystal. We have a fun activity available on our website that you can do at home all about making unit cells and building a model crystal – you can find the details here: bit.ly/crystalbuilding.

Table salt also has another name - Sodium Chloride. This just means that it is made up of two different elements: sodium (Na) and chlorine (Cl). The chemical symbol for table salt is NaCl. These two types of atoms are arranged within the cubic unit cell as shown in this picture. You can see the sodium atoms are at the corners of the cube and in the centre of each face. The chlorine atoms are arranged so they sit halfway between each pair of sodium atoms. This is also known as a face centred cubic structure because there are atoms at the centre of each face.

Diamond also has this cubic structure, but the arrangement of the atoms within the cube shaped unit cell are a bit different. Diamond is made up of carbon atoms and their arrangement in the unit cell is shown in the picture on the far left here. You can see that there are carbon atoms at each corner of the cube, like in table salt. There are also atoms at the centre of each face, meaning that diamond also has a face centred cubic structure. However, there are more carbon atoms at other places within the cube, giving

An image of table salt (sodium chloride) crystals. If you look closely, you can see the cubic structure even at this scale.

it a different structure to that of table salt. It is this different structure that gives diamond some very special properties. Not only does diamond shine beautifully, meaning that it is used in expensive jewellery, but diamond is actually the hardest naturally occurring material in the world!

Why is Amethyst Purple?

An amethyst crystal, like the one in the picture here, is another type of crystal you might have heard of and has a brilliant purple colour. Amethyst is actually a variety of quartz, which is a crystal made up of silicone (Si) and oxygen (O), also called silicone dioxide. Quartz does not have this purple colour and is in fact white or clear in colour. The reason why amethyst is purple even though it is made up of the same type of atoms is because it contains what are called impurities. This means that

Crystals can have other uses besides looking pretty. One particular crystal we are working on at the moment is iron selenide which is a crystal made up of two types of atoms: iron and selenium. This is a rather special crystal with some very interesting properties. If you cool it to a very low temperature, about -264°C, this crystal becomes a superconductor. A superconductor is a material that has zero electrical resistance. However, this only happens when we cool these materials below a

What Other Properties Can Crystals Have?

Hexagonal unit cell shape

Iron selenide crystal under a microscope.

A quartz crystal A rose quartz crystal A smoky quartz crystal

An amethyst crystal

some of the silicone or oxygen atoms within the crystal are replaced with something else, in this case iron atoms, which gives the amethyst its purple colour. There are other varieties of quartz that have different colours caused by different types of impurities. Rose quartz, for example, has a pale pink colour due to tiny amounts of titanium, iron, or manganese within the crystal. Smoky quartz is another example, its cloudy grey-black colour coming from extra silicone impurities within the crystal. All these variations of quartz are used in jewellery and ornaments because of their beautiful colours.

The crystal structure of quartz is quite different to that of diamond or table salt. Rather than a cubic unit cell, quartz crystals have a hexagonal type of structure with a unit cell in the shape of a hexagonal prism, like the shape shown in the picture here. The arrangement of the atoms within the unit cell is even more complicated, but you can still see the hexagonal shapes forming even in the pictures of whole crystals!

certain temperature, which is usually very cold! This means we have to use substances like liquid nitrogen, which has a temperature of -196°C, to keep these superconductors cold. A lot of research is currently being done to better understand these superconductors and to try to find one that works at room temperature (which is around 20°C). Superconductors also behave very strangely if we try to put them near a magnet when they are in this cooled superconducting state. In fact, a superconductor will float above a magnet! This behaviour is used to make levitating trains which can reach speeds of around 375 miles per hour.

A black superconductor, cooled using liquid nitrogen, floating over some silver magnets.

The crystal structure of iron selenide is also interesting and is shown in the picture here. As you can see, this type of shape has faces that are parallelograms with the edges in one direction being longer than the edges in the other two directions. The unit cell for this crystal has atoms placed at each corner with rows of selenium atoms above rows of iron atoms. As with any crystal, this pattern repeats itself throughout the entire crystal.

Unit cell structure of iron selenide. Here, the iron atoms are the brown circles and the selenium atoms are the green.