Cleavage, inmineralogy andmaterials science, is the tendency ofcrystalline materials tosplit along definitecrystallographic structural planes. These planes of relative weakness are a result of the regular locations ofatoms andions in the crystal, which create smooth repeating surfaces that are visible both in the microscope and to the naked eye. Ifbonds in certain directions are weaker than others, the crystal will tend to split along the weakly bonded planes. These flat breaks are termed "cleavage".[1] The classic example of cleavage ismica, which cleaves in a single direction along thebasal pinacoid, making the layers seem like pages in a book. In fact, mineralogists often refer to "books of mica".
Diamond andgraphite provide examples of cleavage. Each is composed solely of a singleelement,carbon. In diamond, each carbon atom is bonded to four others in atetrahedral pattern with shortcovalent bonds. The planes of weakness (cleavage planes) in a diamond are in four directions, following the faces of theoctahedron. In graphite, carbon atoms are contained in layers in ahexagonal pattern where the covalent bonds are shorter (and thus even stronger) than those of diamond. However, each layer is connected to the other with a longer and much weakervan der Waals bond. This gives graphite a single direction of cleavage, parallel to the basal pinacoid. So weak is this bond that it is broken with little force, giving graphite a slippery feel as layersshear apart. As a result, graphite makes an excellentdry lubricant.[2]
While allsingle crystals will show some tendency to split along atomic planes in theircrystal structure, if the differences between one direction or another are not large enough, the mineral will not display cleavage.Corundum, for example, displays no cleavage.
Cleavage forms parallel to crystallographic planes:[1]
Crystalparting occurs when minerals break along planes of structural weakness due to external stress, alongtwin composition planes, or along planes of weakness due to theexsolution of another mineral. Parting breaks are very similar in appearance to cleavage, but the cause is different. Cleavage occurs because of design weakness while parting results from growth defects (deviations from the basic crystallographic design). Thus, cleavage will occur in all samples of a particular mineral, while parting is only found in samples with structural defects. Examples of parting include the octahedral parting ofmagnetite, the rhombohedral and basal parting incorundum,[3] and the basal parting inpyroxenes.[1]
Cleavage is a physical property traditionally used in mineral identification, both in hand-sized specimen and microscopic examination of rock and mineral studies. As an example, the angles between the prismatic cleavage planes for the pyroxenes (88–92°) and theamphiboles (56–124°) are diagnostic.[1]
Crystal cleavage is of technical importance in theelectronics industry and in the cutting ofgemstones.
Precious stones are generally cleaved by impact, as indiamond cutting.
Synthetic single crystals of semiconductor materials are generally sold as thinwafers which are much easier to cleave. Simply pressing asilicon wafer against a soft surface and scratching its edge with adiamond 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 (silicon,germanium, and diamond) arediamond cubic, aspace 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 relatedzinc blende structure, with similar cleavage planes.
