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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 to group them according to their chemical/physical properties:

Crystal Grouped by Lattices (Shape)

There are seven crystal lattice systems. You can view examples of each type by following one of the 'Elsewhere on the Web' links I have provided.

1. Cubic or Isometric - 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, forming double pyramids and prisms.
   
   
3. Orthorhombic - like tetragonal crystals except not square in cross section (when viewing the crystal on end), forming rhombic prisms or dipyramids (two pyramids stuck together).
   
   
4. Hexagonal - six-sided prisms. When you look at the crystal on-end, the cross section is a hexagon.
   
   
5. Trigonal - possess a single 3-fold axis of rotation instead of the 6-fold axis of the hexagonal division.
6. 
   
7. Triclinic - usually not symmetrical from one side to the other, which can lead to some fairly strange shapes.
   
   
8. Monoclinic - like skewed tetragonal crystals, often forming 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).

There are four main categories of crystals, as grouped by their chemical and physical properties:Crystals Grouped by Properties

1. Covalent Crystals
   A covalent crystals 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 orhydrogen 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.

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. I'll test your knowledge by referring to these crystal shapes in crystal-growing tutorials!

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