How do Crystals Form?

Shining, sparkling crystals catch the eye, but what’s inside? How does a regular structure like crystal form in the chaos of nature?

Table of Contents

What is crystal?

Crystal is simply a regular, repeating arrangement of particles. These particles are known as atoms. Atoms are the fundamental units that make up everything that we see in the world! The device that you’re reading this on? Atoms! Tables and chairs? Atoms! Imagine you have a magical knife, with the ability to divide anything into smaller and smaller bits. At some point, you won’t be able to create a smaller bit that does not have a positive or negative charge. This smallest bit is the atom .

 

Atoms are made of three smaller particles: protons, neutrons, and electrons. Protons are positively charged, and neutrons have no charge. Protons and neutrons are packed together tightly to form a dense central core called a nucleus that has an overall positive charge. The electrons are negatively charged and arranged in rings around the nucleus. An atom looks something like our solar system. The sun is like the densely packed, positively charged nucleus. Electrons are like planets, orbiting the sun and very far apart.

The structure of the atom
The structure of the atom, Credit: Wikimedia/CNX OpenStax

When an atom loses one or more electrons, it ends up positively charged and becomes a cation. Similarly, the gain of one or more electrons creates a negatively charged anion.

 

Anions and cations usually don’t form from the same elements. Some elements have a tendency to become anions, and some elements have a tendency to become cations. Anions and cations are attracted to each other and bind together. The geometric arrangement in which they are bonded is what determines whether a substance is a crystal or not.

 

One cation surrounded by four anions forms a tetrahedron. One cation surrounded by six oxygens becomes an octahedron! These are the building blocks. Adding one identical building block to another, to another and so on is what makes a crystal. It is a fixed pattern of identical units.

A tetrahedral structure. Cations are represented by pink balls, and anions by white balls
A tetrahedral structure. Cations are represented by pink balls, and anions by white balls, Credit: Wikimedia/Benjah-bmm27
An octahedral structure. Cations are represented by pink balls, and anions by white balls.
An octahedral structure. Cations are represented by pink balls, and anions by white balls, Credit: Wikimedia/Benjah-bmm27

How do sugar and salt crystals form?

Salt is in almost everything we eat, and it is one of the most common crystals we encounter. The scientific name of salt is sodium chloride. This means salt is made up of two elements: sodium and chlorine. Sodium forms cations, and chlorine forms anions. They bond in a one-to-one ratio—one chlorine for one sodium. Alternating sodium and chlorine repeat and form the four corners of a cube.

The sodium chloride crystal
The sodium chloride crystal, Credit: Wikimedia/OpenStax College

Most of the salt on Earth is dissolved in water, which breaks down the crystal structure. When sea water is slowly dried in salt pans, or salt crystals form under the earth from ancient lakes or oceans, salt crystals form.

The spectacular halite crystal, also known as rock salt
The spectacular halite crystal, also known as rock salt, Credit: Wikimedia/Robert M. Lavinsky

Granulated sugar, the way you can buy it at the grocery store,  is usually made from sugarcane. Rarely, it is made from sugar beet. Cut cane or sugar beet is boiled and the juice is extracted. The juice is slowly thickened until hardly any water remains. The little water that is left slowly evaporates as the hot juice cools, and sugar crystals slowly form. You may have seen other forms of sugar, like rock sugar, granulated sugar, or powdered sugar. Large crystals form rock sugar. Sugar is ground down to different degrees to form granulated sugar or very fine powdered sugar.

Sugar crystals under the microscope
Sugar crystals under the microscope. Note that these are not the atoms that make up sugar, but magnified crystals that are visible to the naked eye even without a microscope, Credit: Wikimedia/Gabriela P.

How do quartz crystals form?

Quartz is a white, crystalline, shiny solid and is probably what comes to mind when you imagine a crystal. Similar to sugar, quartz also forms when a hot liquid cools. Magma from deep within the earth bubbles to the surface during a volcanic eruption. This magma most commonly cools rapidly to form igneous rocks. In special cases, the magma cools slowly, gradually forming crystals. These crystals are formed of the elements silicon and oxygen. One silicon cation is bonded to four oxygen anions, forming a tetrahedral structure. In quartz, each oxygen is shared between two tetrahedra, giving quartz crystals a hexagonal shape.

A quartz crystal
A quartz crystal, Credit: Wikimedia/JJ Harrison
The internal structure of quartz
The internal structure of quartz. White balls represent silicon, and red balls represent oxygen, Credit: Wikimedia/Andel

What are crystals used for?

One popular use of crystals is to determine the internal structure of proteins and other biological molecules. Proteins are everywhere in the natural world, but they are folded and twisted. This makes it hard to tell what exactly they are made of. Turning them into a crystal generates a repeatable pattern which is much easier to decipher. In fact, the discovery of the structure of DNA used this method!

 

Diamonds are one crystal with which we are all familiar. Beyond sparkling jewelry, it is also used as a tool since it is very hard and nearly unbreakable. Graphite is a crystal that makes up the “lead” in pencils.

 

Crystals are everywhere in our daily lives. Though their properties are varied, they are all just made up of cations and anions, in fixed patterns!

Glossary

Atom: The smallest unit which can be divided without the release of electrically charged particles

 

Anion: A negatively charged atom

 

Cation: A positively charged atom

 

Crystal: A repeated pattern made of identical units

 

Electron: A tiny, negatively charged component of an atom

 

Element: A primary constituent of matter

 

Neutron: A component of an atom present in the nucleus with no charge

 

Proton: A component of an atom present in the nucleus with a positive charge

Flesch Kincaid Grade Level: 7.2

 

Flesch Kincaid Reading Ease: 60.1

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Author

  • Yamini Srikanth

    Yamini's (he/they) interests lie in environmental education, science communication and trying to build a better world. When not languishing in front of his laptop, they can be found outside, poking at any insect, bird or plant. They love making science accessible, especially to those who aren't encouraged to pursue it. Yamini hopes that the young women who read Smore love learning from their articles and get just a little bit more excited about science!