Cleavage (crystal)

  Cleavage, in mineralogy, 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 (sodium chloride).

• 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 (see below).

• Prismatic cleavage, cleavage parallel to a vertical Prism.

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This 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; however, when dicing a wafer to form chips, a procedure of scoring and breaking is often followed for greater control. Elemental semiconductors (Si, Ge, and diamond) are diamond cubic, a space group for which octahedral cleavage is observed. This means that some orientations of wafer allow near-perfect rectangles to be cleaved. Most other commercial semiconductors (GaAs, InSb, etc.) can be made in the related zinc blende structure, with similar cleavage planes.

References

• Hurlbut, Cornelius S.; Klein, Cornelis, 1985, Manual of Mineralogy, 20th ed., Wiley, pp. 200 - 202, ISBN 0-471-80580-7
• Mineral galleries: Mineral properties - Cleavage