Crystals

A crystal is a solid in which the atoms or molecules are arranged in an orderly, repeating three-dimensional pattern. The regular pattern often shows up as flat outer faces and beautiful geometric shapes. Most minerals are crystalline, although the crystals are usually tiny. Spectacular large crystals form when the right conditions exist for long periods of time, in places like cave walls, underground voids and slow-cooling magma chambers. Crystals are a fundamental feature of the universe, found from snowflakes on Earth to ice on comets to the heart of every grain of salt in your kitchen.

• Crystal systems7 majorCubic, hexagonal and 5 others
• Most abundant crystalsSnowflakesForms a quadrillion or so each year
• Largest known crystalsCave of the Crystals, MexicoSelenite beams up to 12 m long
• Famous home crystalSugarCubic monoclinic crystals from cane juice
• Hardest natural crystalDiamondPure carbon in cubic arrangement
• Used inChips, watches, lasersPlus jewellery and medicine

What makes a crystal?

Inside a crystal, atoms are arranged in a regular, repeating 3D pattern called a crystal lattice. The pattern repeats over and over in every direction. When a crystal grows freely (without being squashed against other crystals), the regular internal pattern produces flat outer surfaces called faces, with angles between them that are always exactly the same for that particular mineral.

This is why crystals of the same mineral always have the same shape, no matter where in the world they are found. A salt (halite) crystal is always a cube, whether it came from a kitchen, the sea or a mine in Pakistan. A quartz crystal is always a six-sided prism. The shape is a direct outward sign of the invisible atomic arrangement inside.

The seven crystal systems

All known crystals fit into one of seven basic geometric systems, defined by the symmetry of the lattice.

• Cubic: three equal axes at right angles. Examples: salt, pyrite, diamond, garnet. Form cubes, octahedra and other regular shapes.
• Tetragonal: like cubic but stretched along one axis. Example: zircon.
• Orthorhombic: three unequal axes at right angles. Example: olivine.
• Hexagonal: six-fold symmetry. Examples: quartz, beryl, ice (snowflakes).
• Trigonal: three-fold symmetry. Examples: corundum (ruby, sapphire), calcite.
• Monoclinic: three unequal axes, one not at right angles. Example: gypsum, mica.
• Triclinic: three unequal axes, none at right angles. Lowest symmetry. Example: plagioclase feldspar.

How crystals grow

Crystals grow by adding new atoms to the surface of an existing crystal seed, one layer at a time. The atoms have to slot into the right spots in the lattice, which is why crystal growth is normally very slow.

Crystals form in many ways:

• From a melt: as magma slowly cools, crystals start to grow. Slow cooling gives big crystals (like in granite); fast cooling gives tiny ones (like in basalt) or no crystals at all (obsidian).
• From a solution: as water with dissolved minerals slowly evaporates or cools, the minerals come out of solution and form crystals. This is how salt crystals form when sea water dries out, or how stalactites grow in caves.
• From a gas: as a gas cools, it can deposit straight into a crystal. This is how frost forms on a cold window, and how snowflakes form in clouds.
• By transformation: in metamorphic rocks, existing minerals re-crystallise into new minerals under heat and pressure.

Fact The largest natural crystals in the world were discovered in 2000, deep in the Naica Mine in Chihuahua, Mexico. The Cave of the Crystals contains enormous beams of selenite (a kind of gypsum) up to 12 metres long and weighing approximately 55 tonnes each. The crystals grew over half a million years in a hot mineral-rich water bath. The cave is so hot (47 °C) and so humid (99%) that humans can only enter for short periods wearing special cooling suits.

Famous large crystals

• Cave of the Crystals, Mexico: selenite beams up to 12 m long. The biggest natural crystals ever discovered.
• Cullinan Diamond, South Africa: a 3,106-carat rough diamond discovered in 1905. Cut into the British Crown Jewels.
• Geodes: round rocks with cavities filled with beautiful crystal-lined walls. Common in the United States, Brazil and Mexico.
• Quartz crystal balls: cut from massive single crystals of clear quartz, sometimes up to 30 cm wide.
• Aquamarine and beryl crystals: from Brazil and Madagascar can reach over a metre long.

Crystals in everyday life

You handle crystals every day without thinking about it.

• Salt (halite): every grain of table salt is a tiny cubic crystal.
• Sugar: monoclinic crystals from sugar cane juice.
• Snowflakes: hexagonal crystals of frozen water.
• Frost: thin crystals of frozen water on cold surfaces.
• Silicon chips in computers: made from one giant single crystal of silicon, sliced into wafers.
• Quartz watches: contain a tiny quartz crystal that vibrates at an exact frequency when electricity passes through it, keeping precise time.
• Lasers: many lasers use a single crystal as the amplifier.

Did you know? You can grow your own large crystals at home from sugar or salt. Mix a saturated solution in hot water, suspend a small "seed crystal" on a string in the warm solution, then leave it in a quiet warm place for weeks. As the water slowly evaporates, dissolved sugar (or salt) comes out of solution and adds to the seed crystal, growing it into a beautiful large geometric shape. Many science classrooms have a permanent crystal-growing experiment going.

Deeper dive: why every snowflake is unique

Snowflakes are one of the most beautiful crystal structures in nature. Each one starts as a single tiny ice crystal forming around a speck of dust high in a cloud. As the crystal grows, water vapour deposits onto its six points (because ice always grows in a six-fold pattern). The exact shape that develops depends on the conditions inside the cloud: temperature, humidity, the exact path the snowflake takes as it tumbles down through the air.

This is why the famous saying that "no two snowflakes are alike" is, mathematically speaking, true. A typical snowflake contains roughly 10¹⁸ water molecules, each arranged in a slightly different position from the others. The number of possible arrangements is so vast that no two snowflakes have ever (and probably will ever) be exactly identical.

The fact that snowflakes have six sides is because water ice crystals form a hexagonal lattice. Each water molecule has two hydrogen atoms and two lone electron pairs, and when frozen they line up so that each molecule is bonded to four others in a tetrahedral pattern. The resulting overall structure has six-fold symmetry, which shows up as the six-armed shape of every snowflake.

The amazing variety of snowflake shapes was first systematically catalogued by Wilson Bentley, an American farmer who photographed over 5,000 individual snowflakes between 1885 and his death in 1931. He never published a duplicate. Modern snow research has confirmed that snowflakes really do form an essentially infinite variety of patterns, although they all share the same underlying six-sided structure.

For more, see minerals and famous gems.