crystal system

Any of the seven crystal systems defined by symmetry, into which all known crystalline susbtances crystallize. The elements of symmetry used for this purpose are: (1) planes of mirror symmetry, across which a mirror image is seen, and (2) axes of rotational symmetry, about which, in a 360° rotation of the crystal, equivalent faces are seen two, three, four, or six times. To be assigned to a particular crystal system, a mineral must possess a certain minimum symmetry, but it may also possess additional symmetry elements. Since crystal symmetry is related to internal structure, a given mineral will always crystallize in the same system, although the crystals may not always grow into precisely the same shape. In cases where two minerals have the same chemical composition but different internal structures (for example graphite and diamond, or quartz and cristobalite), they will generally have different crystal systems.