Order inside solids
Look closely at grains of salt and you’ll see tiny cubes. Snowflakes have six sides. Quartz grows in six-sided columns. These are crystals: solids whose particles (atoms, ions or molecules) are arranged in a regular, repeating pattern.
The crystal lattice
The repeating 3-D arrangement of particles is called a crystal lattice. The smallest repeating piece is the unit cell. Like a single brick in a wall, the unit cell repeats in all directions to build the whole crystal.
The shape of the unit cell decides the shape of the crystal.
| Crystal system | Example |
|---|---|
| Cubic | Table salt, pyrite, diamond |
| Hexagonal | Quartz, snowflakes (ice), emerald |
| Tetragonal | Zircon |
| Orthorhombic | Sulfur, topaz |
| Monoclinic | Gypsum, mica |
| Triclinic | Feldspars like turquoise |
Crystalline vs. amorphous
| Crystalline solid | Amorphous solid |
|---|---|
| Regular, repeating pattern | No regular pattern |
| Sharp melting point | Softens gradually over a range |
| Breaks along flat faces | Breaks with curved surfaces |
| Salt, sugar, quartz, metals | Glass, plastic, rubber |
Glass is made from quartz sand, but it cools so quickly that its particles get "frozen" in a jumble before they can line up. Volcanic obsidian forms the same way.
Types of crystals
| Type | Particles | Held by | Properties | Examples |
|---|---|---|---|---|
| Ionic | Positive and negative ions | Ionic bonds | Hard, brittle, high melting point | Salt (NaCl) |
| Covalent network | Atoms | Covalent bonds | Very hard, very high melting point | Diamond, quartz |
| Molecular | Molecules | Weak attractions between molecules | Soft, low melting point | Ice, sugar |
| Metallic | Metal ions in an electron sea | Metallic bonds | Conduct, can be bent | Copper, iron |
Same atoms, different crystals
Diamond and graphite are both pure carbon, but their atoms are arranged differently.
- Diamond: each carbon is bonded to four others in a rigid 3-D lattice. It’s the hardest natural material.
- Graphite: carbon atoms form flat sheets that slide over each other. It’s soft and is used in pencils.
How crystals form
Crystallization happens when particles have time to arrange themselves in order.
| From | How | Example |
|---|---|---|
| Solution | Water evaporates or a hot saturated solution cools | Salt flats, sugar crystals, geodes |
| Melt | Molten rock or metal cools | Granite, metals |
| Vapour | Gas turns directly to solid | Snowflakes, frost |
Slow cooling makes large crystals. Fast cooling makes small ones. Granite, which cooled slowly deep underground, has large visible crystals. Basalt, which cooled quickly at the surface, has tiny ones.
The largest known crystals are in the Cave of the Crystals in Mexico: gypsum crystals up to about 12 m long, formed over hundreds of thousands of years in hot, mineral-rich water.
Crystal properties
- Cleavage: crystals often split along flat planes where bonds are weaker. Mica peels into thin sheets; salt breaks into smaller cubes.
- Hardness: depends on bond strength.
- Optical properties: some crystals bend light in special ways. Calcite makes things look doubled.
Crystals in technology and life
| Use | Crystal |
|---|---|
| Watches and electronics | Quartz vibrates at a precise frequency when electricity is applied |
| Computer chips | Silicon crystals |
| Lasers | Ruby and other crystals |
| Medicine | X-ray crystallography reveals the shapes of proteins and drugs |
| Gemstones | Diamond, ruby, emerald, sapphire |
| Kidney stones | Unwanted crystals that form in the body |
X-ray crystallography helped reveal DNA’s double-helix structure.
Common mistakes
- Thinking glass is a crystal. It’s amorphous.
- Thinking crystals are only gems. Salt, sugar, ice and metals are crystalline too.
Try it yourself
- Make a saturated sugar or salt solution in hot water (with an adult). Hang a string in it and let it sit for a week. Observe the crystals.
- Look at salt and sugar grains with a magnifying glass. Draw their shapes.
- Grow alum crystals in two jars: one cooling slowly in an insulated box, one in the fridge. Compare crystal sizes.
Quick recap
- Crystals have particles arranged in regular, repeating patterns.
- The unit cell repeats to build the crystal lattice.
- Amorphous solids like glass have no regular pattern.
- Crystals form from solutions, melts and vapours; slow cooling makes larger crystals.
- Crystal structure explains properties like hardness and cleavage.