What Types of Crystals Are There?

In terms of crystal systems and lattice types, There are 7 crystal systems:

   1. triclinic
   2. monoclinic
   3. orthorhombic
   4. tetragonal
   5. trigonal
   6. hexagonal
   7. cubic

Lattices can either be primitive (only one lattice point per unit cell) or non-primitive (more than one lattice point per unit cell).

If you combine the 7 crystal systems with the 2 different types of lattices, you end up with 14 Bravais Lattices (named after Auguste Bravais who figured all this out in 1850).

Another way is to categorize crystals by their physical/chemical properties. In this classification you have four types of crystals:

Covalent Crystals:

This is a crystal which has real chemical covalent between all of the atoms in the crystal. So really a single crystal of a covalent crystals is really just one big molecule. An example of this is a crystal like diamond or zinc sulfide. Covalent crystals can have extremely high melting points.

Metallic Crystals:

Individual metal atoms sit on lattice sites while the outer electrons from these atoms are able to flow freely around the lattice. Metallic crystals normally have high melting points and densities.

Ionic Crystals:

This is a crystal where the individual atoms don't have covalent bonds between them, but are held together by electrostatic forces. An example of this type of crystal is sodium chloride (NaCl). Ionic crystals are hard and have relatively high melting points.

Molecular Crystals:

This is a crystal where there are recognizable molecules in the structure and the crystal is held together by non-covalent interactions like van der Waals forces or hydrogen bonding. An example of this type of crystal would be sugar. Molecular crystals tend to be soft and have lower melting points.