# Crystal system

From Conservapedia

**Crystal systems** are a way of characterising the three-dimensional shape shape of mineral crystals by their symmetry and thus aiding in 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

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