What is a Crystal?

What are crystals?

When we hear the word "crystals," we usually think of coloured minerals. But those are not the only types of crystals. Graphite in pencils, table salt, and snow are crystals, too.

What all crystals have in common is an extremely well-organized molecular structure. In a crystal, all of the atoms (or ions) are arranged in a regular grid pattern. For example, in the case of table salt (NaCl), the crystals are made up of cubes of sodium (Na) ions and chlorine (Cl) ions. Each sodium ion is surrounded by six chlorine ions. Each chlorine ion is surrounded by six sodium ions. It's very repetitive, which is exactly what makes it a crystal!

Different minerals have different molecular compositions. These molecular compositions form structures of different shapes. Table salt naturally forms cubic crystals. Quartz crystals, which are made up of silicon dioxide (SiO₂) molecules, form hexagonal prisms with hexagonal pyramids on either end. Snow is a form of ice crystal. It is is made up of water (H₂O) molecules. These water molecules form flat hexagonal plane crystals.

Why is crystal structure important?

The crystal structure is just as important to the crystal as the molecules are. This becomes clear when we compare crystals made from carbon. For example, diamonds are transparent and so strong they can cut through glass. Meanwhile, graphite is dark, opaque and so soft that tiny bits of it are rubbed onto paper when you use a pencil. What is amazing about these two crystals? They are both made up of only carbon!

So, how can two materials that are so different be made of the same atoms? The difference is in their crystal structure. In graphite, the carbon atoms form sheets that are layered on top of each other. In diamonds, every carbon atom is bound to four other carbon atoms in the tightest possible combination. Why are these carbon atoms packed so tightly together? It is because diamonds are formed deep within the Earth, when very high pressure is applied to carbon. This causes the atoms to become packed together in the tightest crystalline state possible.

Did you know?

The hardness of minerals is measured on the Mohs scale, which ranks hardness from 1-10. Diamond is the only mineral with a strength of 10 on the Mohs scale. In comparison, graphite has a strength of 1!

How do crystals form?

Crystals can be formed in different ways. Some mineral crystals are formed when pressurized molten carbon quickly cools. That’s how diamonds are formed.

Other crystals form when the liquid from a solution evaporates. That’s how a purple quartz crystal named amethyst is formed. Amethyst starts as a highly concentrated solution of silicon dioxide (SiO₂) containing trace amounts of iron. This solution gets trapped inside a bubble of lava. As the water evaporates, the silicon and oxygen ions slowly arrange themselves to form a crystal. The iron is responsible for giving amethyst its purple colour.

But crystals can't form in the middle of a liquid. They need a starting point to grow. The crystal can begin to grow once a nucleus has formed. A nucleus is also sometimes called a seed crystal. This process is called nucleation. In the case of amethyst, its crystals form on the inside of lava bubbles. Stones with crystals on the inside form as these lava bubbles cool. This type of structure is known as a geode.

Did you know?

Amethyst is the provincial mineral of Ontario. 

It is possible to grow your own crystals at home or in a lab, but you also need to have a starting point for the crystals to form. This is why some commercial crystal growing kits come with a rock that is used as a nucleation site. Scientists who grow crystals in test tubes often make a small scratch on the inside of the tube to act as a nucleation site. That’s because crystals can't grow as easily on the smooth surface of the glass.

Once nucleation has happened, crystals can grow quite large if there is room and the growth conditions stay stable. Many of the largest crystals are found underground in giant geode caves. These caves are often discovered by miners who come across them while digging for other types of minerals.

Did you know?

In 2000, some of the largest crystals in the world were found in a silver and zinc mine in Mexico. These giant gypsum crystals were up to 11 meters long!

Summing up…

So what do fancy jewellry, pencils and snow have in common? They are all made up of crystals! The next time you see any of these objects, remember that there is some fascinating chemistry behind them.