Crystals are a form of matter in which atoms, molecules, or ions are arranged in highly ordered three-dimensional lattices. While liquid crystals are possible, most crystals are solid. Examples include diamond (carbon), salt (sodium chloride), quartz (silicon dioxide), and snowflakes (frozen water). Many gems are crystals, including emerald, citrine, ruby, and sapphire. The word “crystal” comes from the Greek word "krustallos," which means both “rock crystal” and “ice.” The study of crystals is named crystallography.
The organized structure of crystals has repeating patterns throughout. The smallest repeating pattern of the crystal is known as a unit cell. The symmetry within crystal structures can be described by space groups. Space groups describe the three-dimensional symmetry within the crystal related to the unit cell and overall shape.
Example of a unit cell and crystal lattice.
The properties of a crystal are primarily determined by its chemical bonds. Crystals can be classified by the type of chemical bonds formed between their atoms or ions:
- Covalent crystals
- Ionic crystals
- Metallic crystals
- Molecular crystals
For example, ionic and metallic crystals tend to have high melting and boiling points. Ionic crystals often dissolve in polar solvents, like water.
There are seven types of crystalline structures, with each distinguished by the geometric parameters of its unit cell. The following are the seven crystalline structures:
- Cubic or Isometric—a shape including octahedrons and dodecahedrons as well as cubes
- Tetragonal—crystals forming prisms and double pyramids, similar to a cubic crystal, except one axis is longer than the other
- Orthorhombic—rhombic prisms and dipyramids that resemble tetragons but without square cross-sections
- Hexagonal—six-sided prisms with a hexagon cross-section
- Trigonal—crystals that have a three-fold axis
- Triclinic—the least symmetric lattice among all crystal systems and the only type that does not have any mirror planes within its structure
- Monoclinic—crystals resembling skewed tetragonal shapes
Lattices may have one or more lattice point per cell; therefore, structures may be expanded to a total of fourteen Bravais crystal lattices, named after crystallographer and physicist Auguste Bravais, who described three-dimensional arrays in terms of points. Also, many substances crystallize into more than one type of lattice. For example, water can form hexagonal ice, rhombohedral ice, or cubic ice. It can also form amorphous ice, which is not crystalline. Carbon can form both graphite (hexagonal) and diamond (cubic).
