Of 32 crystal classes (based on 32 point groups, the possible combinations
of symmetry elements intersecting at a point) assigned to 7 crystal
systems, only 11 are found in common minerals. Each system may be
described in terms of three noncoplanar vectors (crystallographic axes)
that are generally nonorthogonal as well. Although mineral assignment to a
crystal system may require only examination of external crystal
morphology, assignment of crystal class commonly requires X-ray
diffraction analysis. The crystal systems are triclinic, monoclinic,
orthorhombic, tetragonal, trigonal, hexagonal, and isometric.
See also: crystallographic axes; crystal systems.
Dictionary of Mining, Mineral, and Related Terms