Unit 1.8 - The Seven Crystal Systems

Unit 1.8 of the course The Fascination of Crystals and Symmetry

A unit cell is characterized in the first instance by its so-called metric. These are the six lattice parameters: The length of the edges, a, b, and c and the angles between these edges or faces: alpha defines the angle between edge b and c, beta is the angle between a and c, and finally gamma is the angle between a and b.

Furthermore, the unit cell contains all symmetry elements of the crystal.

All the crystals of the world can be classified into not more than 7 crystal systems. Regarding the metric of the unit cells there are only 7 different unit cells, in principle!

- triclinic
- monoclinic
- orthorhombic
- tetragonal,
- trigonal
- hexagonal
- and finally cubic

However, the the classification according to crystal systems is not based upon the metric, instead it is based on the symmetry. This unit will explain these relationships.

Additonal resources at: https://crystalsymmetry.wordpress.com...

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