A crystal consists of matter that is formed from an ordered arrangement of atoms, molecules, or ions. The lattice that forms extends out in three dimensions.
Because there are repeated units, crystals have recognizable structures. Large crystals display flat regions (faces) and well-defined angles.
Crystals with obvious flat faces are called euhedral crystals, while those lacking defined faces are called anhedral crystals. Crystals consisting of ordered arrays of atoms that aren't always periodic are called quasicrystals.
The word "crystal" comes from the Ancient Greek word krustallos, which means both "rock crystal" and "ice." The scientific study of crystals is called crystallography.
Examples of everyday materials you encounter as crystals are table salt (sodium chloride or halite crystals), sugar (sucrose), and snowflakes. Many gemstones are crystals, including quartz and diamond.
There are also many materials that resemble crystals but are actually polycrystals. Polycrystals form when microscopic crystals fuse together to form a solid. These materials do not consist of ordered lattices.
Examples of polycrystals include ice, many metal samples, and ceramics. Even less structure is displayed by amorphous solids, which have disordered internal structure. An example of an amorphous solid is glass, which may resemble a crystal when faceted, yet isn't one.
The types of chemical bonds formed between atoms or groups of atoms in crystals depend on their size and electronegativity. There are four categories of crystals as grouped by their bonding:
- Covalent Crystals: Atoms in covalent crystals are linked by covalent bonds. Pure nonmetals form covalent crystals (e.g., diamond) as do covalent compounds (e.g., zinc sulfide).
- Molecular Crystals: Entire molecules are bonded to each other in an organized manner. A good example is a sugar crystal, which contains sucrose molecules.
- Metallic Crystals: Metals often form metallic crystals, where some of the valence electrons are free to move throughout the lattice. Iron, for example, can form different metallic crystals.
- Ionic Crystals: Electrostatic forces form ionic bonds. A classic example is a halite or salt crystal.
There are seven systems of crystal structures, which are also called lattices or space lattices:
- Cubic or Isometric: This shape includes octahedrons and dodecahedrons as well as cubes.
- Tetragonal: These crystals form prisms and double pyramids. The structure is like a cubic crystal, except one axis is longer than the other.
- Orthorhombic: These are rhombic prisms and dipyramids that resemble tetragons but without square cross-sections.
- Hexagonal: Six-sided prisms with a hexagon cross section.
- Trigonal: These crystals have a three-fold axis.
- Triclinic: Triclinic crystals tend not to be symmetrical.
- Monoclinic: These crystals resemble skewed tetragonal shapes.
Lattices may have one lattice point per cell or more than one, yielding a total of 14 Bravais crystal lattice types. Bravais lattices, named for physicist and crystallographer Auguste Bravais, describe the three-dimensional array made by a set of discrete points.
A substance may form more than one crystal lattice. For example, water can form hexagonal ice (such as snowflakes), cubic ice, and rhombohedral ice. It can also form amorphous ice.
Carbon can form diamond (cubic lattice) and graphite (hexagonal lattice.)
How Crystals Form
The process of forming a crystal is called crystallization. Crystallization commonly occurs when a solid crystal grows from a liquid or solution.
As a hot solution cools or a saturated solution evaporates, particles draw close enough for chemical bonds to form. Crystals can also form from deposition directly from the gas phase. Liquid crystals possess particles oriented in an organized manner, like solid crystals, yet are able to flow.