Crystal system

Crystal systems are a way of characterising the three-dimensional shape of mineral crystals by their symmetry, thus aiding their classification. Mineral crystals come in a seemingly infinite range of shapes, but behind this tremendous diversity of form there is a strict order, because crystals always grow according to simple mathematical laws.[1]

There are 6 (or 7)[2] crystal systems:

• Hexagonal, requires 1 sixfold axis of rotation.
• Isometric or cubic, requires 4 threefold axes of rotation.
• Monoclinic, requires either 1 twofold axis of rotation or 1 mirror plane.
• Orthorhombic, requires either 3 twofold axes of rotation or 1 twofold axis of rotation and two mirror planes.
• Trigonal or rhombohedral, requires 1 threefold axis of rotation.
• Tetragonal, requires 1 fourfold axis of rotation.
• Triclinic, all cases not satisfying the requirements of any other system; thus there is no other symmetry than translational symmetry, or the only extra kind is inversion.

Notes & references

1. Chesterman, Charles W. The Audubon Society Field Guide to North American Rocks and Minerals, Alfred A. Knopf: New York (1987) p. 33
2. U.S. classification usually put trigonal as a sub-set of the hexagonal system, whereas British practice is to make it a separate class.