What Is a Crystal? Definition and Examples


Quartz crystals commonly occur in nature.
Quartz crystals commonly occur in nature. (Ken Hammond, USDA)

A crystal is a form of matter in which the atoms, molecules, or ions are arranged in a highly ordered three-dimensional lattice. Crystals are also called crystalline solids because most crystals are solid. However, liquid crystals also exist. The word “crystal” comes from the Greek word krustallos, which means both “rock crystal” and “ice.” The study of crystals is named crystallography.

Examples of Crystals

Crystals are familiar in everyday life. Examples of crystals include diamond (crystal carbon), salt (sodium chloride crystals), quartz (silicon dioxide crystals), and snowflakes (water ice crystals). Many gems are crystals, including emerald, citrine, ruby, and sapphire.

Other materials look like crystals but don’t consist entirely of ordered lattices. For example, polycrystals form when crystals fuse together. Polycrystals include ice, many metals, and ceramics.

Chemical Bonds in Crystals

One way to classify crystals is by the type of chemical bonds formed between their atoms or ions:

  1. 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).
  2. Ionic Crystals: Electrostatic forces form ionic bonds between atoms with different electronegativity values. A classic example of an ionic crystal is a halite or salt crystal.
  3. Metallic Crystals: Metals often form metallic crystals, where some of the valence electrons are free to move throughout the lattice. A single metal can form multiple types of metallic crystals. Iron, for example, can form different metallic crystals, including a body-centered cubic and face-centered cubic.
  4. Molecular Crystals: Entire molecules are bonded to each other in an organized manner. A good example is a sugar crystal, which contains sucrose molecules.

Crystal properties are largely determined by their chemical bonds. For example, ionic and metallic crystals tend to have high melting and boiling points. Ionic crystals often dissolve in polar solvents, like water.

7 Types of Crystal Lattices

Crystals can be classified according to their lattice structures. The lattice structures are also called space lattices.

  1. Cubic or Isometric: This shape includes octahedrons and dodecahedrons as well as cubes.
  2. Tetragonal: These crystals form prisms and double pyramids. The structure is like a cubic crystal, except one axis is longer than the other.
  3. Orthorhombic: These are rhombic prisms and dipyramids that resemble tetragons but without square cross-sections.
  4. Hexagonal: Six-sided prisms with a hexagon cross section.
  5. Trigonal: These crystals have a three-fold axis.
  6. Triclinic: Triclinic crystals tend not to be symmetrical.
  7. Monoclinic: These crystals resemble skewed tetragonal shapes.

Because a lattice may have one lattice point per cell or more than one lattice point, the structures may be expanded to a total of 14 Bravais crystal lattices. Bravais lattices are named for crystallographer and physicist Auguste Bravais, who described three-dimensional arrays in terms of points.

Many substances crystallize into more than one type of lattice. For example, water can form hexagonal ice, rhomohedral ice, or cubic ice. It can also form amorphous ice, which is not crystalline. Carbon can form graphic (hexagonal) and diamond (cubic).

How Crystals Form

Crystals grow via a process called crystallization. Basically, one particle bonds to another and so on until a structure forms. The beginning of the process is called nucleation. Most crystals people grow form from a liquid solution. As the solution cools or the liquid evaporates, the particles draw closer together. Eventually, chemical bonds form. Other crystals grow as solids deposited from the gas phase or from a melted pure solid (e.g., bismuth).

What Is Not a Crystal?

Despite the names, leaded crystal and crystal glass aren’t actually crystals. They are glass, which is an amorphous solid, that has been cut to resemble the sharp faces of crystals. Many gemstones are crystals, but not all of them. For example, turquoise is cryptocrystalline. This means it contains many tiny crystals, but is not crystalline overall. Similarly, pearl forms from concentric layers of crystalline calcium carbonate, but the gem is not a single crystal. Any material that has to be cut to look like a crystal isn’t usually a crystal.

References

  • Cressey, G.; Mercer, I.F. (1999). Crystals. London. Natural History Museum.
  • Green, D.; Smithsonian Institution (2016). The Rock and Gem Book: And Other Treasures of the Natural World. DK Children. ISBN: 978-1465450708 .
  • Pellant, Chris (2002). Smithsonian Handbooks: Rocks & Minerals. DK Smithsonian Handbook. ISBN: 978-0789491060.

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