# Crystal System

Atomic arrangement of the atoms of an element when it is in its solid state. In mineralogy, these are  best classified in terms of their symmetry and correspond to the seven fundamental shapes for unit cells consistent with the 14 Bravais lattices. Six systems are recognized: isometric (cubic), hexagonal, tetragonal, orthorhombic, monoclinic, and triclinic.

1. The isometric system (also known as the cubic system) has mutually perpendicular crystallographic axes of equal length. All crystals have four 3-fold axes of symmetry diagonally from corner to corner through the center of the unit cell. They may also have up to three separate 4-fold axes of rotational symmetry from the center of each face through the origin to the center of the opposite face; these correspond to the crystallographic axes.
2. The hexagonal system has three crystallographic axes which intersect at 120 ° and a fourth which is perpendicular to the other three. This fourth axis is usually depicted vertically. The hexagonal system may be subdivided into hexagonal or trigonal divisions based upon whether the vertical axis has 6-fold or 3-fold rotational symmetry.
3. The tetragonal crystal system has three mutually perpendicular axes. The two horizontal axes are of equal length, while the vertical axis is of different length and may be either shorter or longer than the other two. Crystals all have a single 4-fold symmetry axis.
4. The orthorhombic system has three mutually perpendicular axes, each of which is of a different length than the others.  Crystals of this system possess three 2-fold rotation axes and/or three mirror planes.
5. The monoclinic system has three unequal axes. Two axes are inclined toward each other at an oblique angle; these are usually depicted vertically. The third axis is perpendicular to the other two. The two vertical axes do not intersect one another at right angles, although both are perpendicular to the horizontal axis. Monoclinic crystals demonstrate a single 2-fold rotation axis and/or a single mirror plane.
6. The triclinic system has three unequal axes, all of which intersect at oblique angles. None of the axes are perpendicular to any other axis. Crystals possess no symmetry at all.
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