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Close-up of an amethyst geode

Close-up of an amethyst geode (dlinca, iStockphoto)

STEM in Context

What is a Crystal?

Eva Amsen

Summary

Crystals aren’t just rocks and minerals. Crystals have a well-organized molecular structure, just as important to a crystal as the molecules it contains.

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.

Molecular structures and photographs of quartz, table salt and ice crystals
Molecular structures and photographs of quartz, salt crystals and ice crystals (Let’s Talk Science using images: Quartz structure by Ben Mills via Wikimedia Commons , Quartz crystal: JJ Harrison [CC BY-SA 2.5] via Wikimedia Commons, Salt crystal by Ravodina Anastasia [CC BY 4.0] via Wikimedia Commons and Ice crystal by Thomas Bresson [CC BY 3.0] via Wikimedia Commons).

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.

This video explains how the molecular structure of diamonds and graphite determines their properties (4:19 min.)

 

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.

Amethyst geode
Amethyst geode. Notice the layer of rock on the outside and the crystals growing towards the centre (Source: Haluk Köhserli via iStockphoto).

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.

Cave of the Crystals or Giant Crystal Cave in Chihuahua, Mexico (2016) by Geology Page (4:53 min.)

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.

Starting Points

Connecting and Relating
  • Do you use crystals in your daily life? Identify some of the common uses for crystals.
  • Have you ever grown crystals at home or at school? If so, what kind of crystals were they?
     
Connecting and Relating
  • Do you use crystals in your daily life? Identify some of the common uses for crystals.
  • Have you ever grown crystals at home or at school? If so, what kind of crystals were they?
     
Relating Science and Technology to Society and the Environment
  • Synthetic crystal, such as cubic zirconia, has been manufactured for decades. Should the use of these synthetic crystals be expanded as we continue to use more of Earth’s resources? Why or why not?
  • How is crystalline structure related to different industrial applications of crystals? Provide specific examples.
     
Relating Science and Technology to Society and the Environment
  • Synthetic crystal, such as cubic zirconia, has been manufactured for decades. Should the use of these synthetic crystals be expanded as we continue to use more of Earth’s resources? Why or why not?
  • How is crystalline structure related to different industrial applications of crystals? Provide specific examples.
     
Exploring Concepts
  • How is it possible that two different mineral crystals can exist that are made from the same molecules? Explain.
  • Explain how nucleation leads to crystal growth. 
  • How is strength or hardness of a mineral related to the crystalline structure of that mineral?
  • Explain the process that forms geodes. 
     
Exploring Concepts
  • How is it possible that two different mineral crystals can exist that are made from the same molecules? Explain.
  • Explain how nucleation leads to crystal growth. 
  • How is strength or hardness of a mineral related to the crystalline structure of that mineral?
  • Explain the process that forms geodes. 
     
Nature of Science/Nature of Technology
  • There are many different types of naturally-occuring crystals in the world. Why would scientists be interested creating new types of crystals? What applications could new synthetic crystals have? (Note: This question requires additional research)
Nature of Science/Nature of Technology
  • There are many different types of naturally-occuring crystals in the world. Why would scientists be interested creating new types of crystals? What applications could new synthetic crystals have? (Note: This question requires additional research)
Media Literacy
Media Literacy
Teaching Suggestions
  • This article can be used in Chemistry and Earth & Environment for teaching and learning related to compounds, crystals, and rocks and minerals. Concepts introduced include minerals, molecular structure, atoms, solution, nucleation and geode. 
  • Before students read this article, teachers could have students watch the video Minerals. This video defines minerals and outlines the characteristics that are used to  identify minerals. Students can then complete a Print-Video Venn Diagram comparing the article and video. Ready-to-use reproducibles for this strategy can be downloaded in [Google doc] and [PDF] formats. 
  • After reading the article and viewing the video, teachers could have students use a Key Ideas Round Robin learning strategy to to help consolidate recently-learned information. Ready-to-use reproducibles for the Key Ideas Round Robin are available [Google doc] and [PDF] formats.
  • To further consolidate, teachers could have students complete a Concept Definition Web learning strategy for the concept: crystals. Ready-to-use Concept Definition Web reproducibles are available in [Google doc] and [PDF] formats. 
  • This topic lends itself to the observation of real crystals and geodes. Students could be asked to bring in examples of crystals or geodes they may have at home to share with the class. 
  • To go further, teachers could have students in groups research the various uses and applications of different types of crystals. To summarize and present their research, students could create graphic organizers or infographics
     
Teaching Suggestions
  • This article can be used in Chemistry and Earth & Environment for teaching and learning related to compounds, crystals, and rocks and minerals. Concepts introduced include minerals, molecular structure, atoms, solution, nucleation and geode. 
  • Before students read this article, teachers could have students watch the video Minerals. This video defines minerals and outlines the characteristics that are used to  identify minerals. Students can then complete a Print-Video Venn Diagram comparing the article and video. Ready-to-use reproducibles for this strategy can be downloaded in [Google doc] and [PDF] formats. 
  • After reading the article and viewing the video, teachers could have students use a Key Ideas Round Robin learning strategy to to help consolidate recently-learned information. Ready-to-use reproducibles for the Key Ideas Round Robin are available [Google doc] and [PDF] formats.
  • To further consolidate, teachers could have students complete a Concept Definition Web learning strategy for the concept: crystals. Ready-to-use Concept Definition Web reproducibles are available in [Google doc] and [PDF] formats. 
  • This topic lends itself to the observation of real crystals and geodes. Students could be asked to bring in examples of crystals or geodes they may have at home to share with the class. 
  • To go further, teachers could have students in groups research the various uses and applications of different types of crystals. To summarize and present their research, students could create graphic organizers or infographics
     

Learn more

A Brief Introduction to Minerals (2013)

In this video (10:21 min.), Mike Sammartano explains the properties of minerals, and their uses in everyday life.

Weird Places: Mexico's Giant Crystal Cave (2014)

SciShow explores the Giant Crystal Cave with this video (3:59 min.), where chemistry has created the world’s largest crystals in an environment so hot and humid that humans can only survive a few minutes.

Snow Crystals

This website by Prof. Kenneth G. Libbrecht contains images of and information about snow crystals and how they form.

It rains solid diamonds on Uranus and Neptune (2017)

In this article, Sarah Kaplan explains how scientists have proven the existence of diamond rain by creating it in the lab, using lasers and plastic to simulate conditions on gas giant planets. 

References

Aldrich, K. (2018, March 13). How are amethyst geodes formed? Sciencing.

Baggaley, K. (2012, November 20). Where do geodes come from? Scienceline.

Berkeley University. (n.d.). What is a crystal?

Clark, J. (2012, October). Ionic structures. Chemguide.

Gem Select. (2019, January 22). Diamond and graphite.

Helmenstine, A. M. (2019, May 6). How to grow crystals - Tips and techniques. ThoughtCo.

Eva Amsen

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.