# Types of Crystals: Shapes and Structures

Shapes and Structures of Crystals

There's more than one way to categorize a crystal. The two most common methods are to group them according to their crystalline structure and to group them according to their chemical/physical properties.

## Crystals Grouped by Lattices (Shape)

There are seven crystal lattice systems.

1. Cubic or Isometric: These are not always cube-shaped. You'll also find octahedrons (eight faces) and dodecahedrons (10 faces).
2. Tetragonal: Similar to cubic crystals, but longer along one axis than the other, these crystals forming double pyramids and prisms.
3. Orthorhombic: Like tetragonal crystals except not square in cross-section (when viewing the crystal on end), these crystals form rhombic prisms or dipyramids (two pyramids stuck together).
4. Hexagonal: When you look at the crystal on end, the cross-section is a six-sided prism or hexagon.
5. Trigonal: These crystals possess a single 3-fold axis of rotation instead of the 6-fold axis of the hexagonal division.
6. Triclinic: These crystals are not usually symmetrical from one side to the other, which can lead to some fairly strange shapes.
7. Monoclinic: Like skewed tetragonal crystals, these crystals often form prisms and double pyramids.

This is a very simplified view of crystal structures. In addition, the lattices can be primitive (only one lattice point per unit cell) or non-primitive (more than one lattice point per unit cell). Combining the 7 crystal systems with the 2 lattice types yields the 14 Bravais Lattices (named after Auguste Bravais, who worked out lattice structures in 1850).

## Crystals Grouped by Properties

There are four main categories of crystals, as grouped by their chemical and physical properties.

1. Covalent Crystals: A covalent crystal has true covalent bonds between all of the atoms in the crystal. You can think of a covalent crystal as one big molecule. Many covalent crystals have extremely high melting points. Examples of covalent crystals include diamond and zinc sulfide crystals.
2. Metallic Crystals: Individual metal atoms of metallic crystals sit on lattice sites. This leaves the outer electrons of these atoms free to float around the lattice. Metallic crystals tend to be very dense and have high melting points.
3. Ionic Crystals: The atoms of ionic crystals are held together by electrostatic forces (ionic bonds). Ionic crystals are hard and have relatively high melting points. Table salt (NaCl) is an example of this type of crystal.
4. Molecular Crystals: These crystals contain recognizable molecules within their structures. A molecular crystal is held together by non-covalent interactions, like van der Waals forces or hydrogen bonding. Molecular crystals tend to be soft with relatively low melting points. Rock candy, the crystalline form of table sugar or sucrose, is an example of a molecular crystal.

Crystals may also be classified as piezoelectric or ferroelectric. Piezoelectric crystals develop dielectric polarization upon exposure to an electric field. Ferroelectric crystals become permanently polarized upon exposure of a sufficiently large electric field, much like ferromagnetic materials in a magnetic field.

As with the lattice classification system, this system isn't completely cut-and-dried. Sometimes it's hard to categorize crystals as belonging to one class as opposed to another. However, these broad groupings will provide you with some understanding of structures.

## Sources

• Pauling, Linus (1929). "The principles determining the structure of complex ionic crystals." J. Am. Chem. Soc. 51 (4): 1010–1026. doi:10.1021/ja01379a006
• Petrenko, V. F.; Whitworth, R. W. (1999). Physics of Ice. Oxford University Press. ISBN 9780198518945.
• West, Anthony R. (1999). Basic Solid State Chemistry (2nd ed.). Wiley. ISBN 978-0-471-98756-7.
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