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.
There are 6 (or 7) 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.