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

Green fluorite with prominent 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.

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.