Adventist Youth Honors Answer Book/Nature/Rocks & Minerals/Cleavage

Cleavage is the tendency of crystalline materials to split along definite planes, creating smooth surfaces, of which there are several named types:

• Basal cleavage: cleavage parallel to the base of a crystal, or to the plane of the lateral axes. This occurs quite easily in graphite, making the material feel slippery.
• Cubic cleavage: cleavage parallel to the faces of a cube. This is the source of the cubic shape seen in crystals of ground table salt.
• Diagonal cleavage: cleavage parallel to a diagonal plane.
• Lateral cleavage: cleavage parallel to the lateral planes.
• Octahedral, Dodecahedral, or Rhombohedral cleavage: cleavage parallel to the faces of an octahedron, dodecahedron, or rhombohedron (respectively). Octahedral cleavage is seen in common semiconductors used in electronics.
• Prismatic cleavage, cleavage parallel to a vertical prism.

Green fluorite with prominent cleavage.

Cleavage is of technical importance in the electronics industry and in the cutting of gemstones. While precious stones are generally cleaved by impact, man-made single crystals of semiconductor materials are generally sold as thin wafers which are much easier to cleave. Simply pressing a silicon wafer against a soft surface and scratching its edge with a diamond scribe is usually enough to cause cleavage.