What is a crystal?

CurioCity
23 January 2012

When we hear the word "crystals", we usually think of coloured minerals, but those are not the only types of crystals: Graphite (the material used to make pencils), table salt, and snow are crystals too.

What all crystals have in common is an extremely well-organized molecular structure. All the atoms or ions in a crystal are arranged in a regular grid pattern. For example, in the case of table salt, the crystal is composed of cubes of sodium ions and chlorine ions. Every sodium ion is surrounded by six chlorine ions, and every chlorine ion is surrounded by six sodium ions. It's very repetitive, and that's exactly what makes a crystal!

Different minerals have different molecular compositions and different structures called the "crystal lattice". Table salt is always made of sodium chloride (NaCl) cubes. Quartz crystal forms a six-sided prism out of silicon dioxide (silica, SiO2), and snow and ice crystals form hexagons from water molecules.

The crystal structure is just as important to the crystal as the molecules it contains. This becomes clear when we compare diamonds with graphite. One is transparent and so strong it can cut though glass, the other is black and so soft you rub off bits of it every time you use it to write or draw. Guess what: they're both made of carbon.

How can two materials that are so different be made of the same ingredient? 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. Diamonds are formed deep in the earth, under very high pressure, which causes the carbon atom to take up the most packed crystal state possible.

Did you know? The strength of minerals is measured on the Mohr scale of 1-10. Diamond is the only mineral with a strength of 10 on the Mohr scale.

Crystals can be formed in different ways. Some mineral crystals, like diamonds, are formed when molten rock slowly cools. Other crystals form from a solution when the liquid evaporates. Amethyst, a purple quartz crystal, is formed this way: it starts as a solution of a high concentration of silicon dioxide that is trapped into a bubble in lava. As the water evaporates, the silicon and oxygen ions slowly arrange as a crystal.

But crystals need a starting point to grow -- they can't form in the middle of a liquid. This is called "nucleation": once you have the nucleus, the starting point of the crystal, it can continue growing from there.

In the case of amethysts the crystals grow on the inside of the lava bubble, forming a round stone with crystals on the inside: a geode.

When you grow your own crystals at home or in a lab, 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 as nucleation site. Scientists who grow crystals in test tubes often make a small scratch on the inside of the tube as nucleation site, because crystals can't grow on the smooth surface of the glass.

Did you know? Biochemists grow crystals to study the structure of the proteins. Proteins are too small to see under a microscope. To see the three-dimensional structure of a protein it is first made into a crystal, and the structure is then calculated from the way X-rays diffract (bounce) off the crystal.

Once nucleation has occurred, crystals can grow quite big, as long as there is room and the growth conditions stay stable. Many of these huge crystals are found underground in giant geode caves. They are often discovered by miners who come across these caves while digging.

Did you know? Some of the largest crystals in the world were found in 2000 in a silver and zinc mine in Mexico. They're gypsum crystals growing up to 10 meters long in a cave at 50 degrees Celsius.

Want to learn more?

Miles Mineral Museum

Snow Crystals

Biggest Crystals

This answer was research and written by Eva Amsen. Eva studied Chemistry in Amsterdam and is now close to finishing a PhD in Biochemistry at the University of Toronto, where she studies genes

that affect skin pigmentation. Eva also plays violin and spends too much

time on YouTube.

CurioCity

This is content has that been provided for use on the CurioCity website.






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