Cleavage (crystal)

{h k ℓ} Cleavage forms parallel to crystallographic planes: • Basal, pinacoidal, or planar cleavage occurs when there is only one cleavage plane. Talc has basal cleavage. Mica (like muscovite or biotite) also has basal cleavage; this is why mica can be peeled into thin sheets. • Prismatic cleavage occurs when there are two cleavage planes in a crystal (but not three). Spodumene is an example where the planes meet at a 90 degree angles. Hornblende is an example where the planes intersect at 56° and 124°. • Cubic cleavage occurs when there are three cleavage planes intersecting at 90 degrees. Halite (or salt) has cubic cleavage, and therefore, when halite crystals are broken, they will form more cubes. • Rhombohedral cleavage occurs when there are three cleavage planes intersecting at angles that are not 90 degrees. Calcite has rhombohedral cleavage. • Octahedral cleavage occurs when there are four cleavage planes in a crystal. Fluorite exhibits perfect octahedral cleavage. Octahedral cleavage is common for semiconductors. Diamond also has octahedral cleavage. • Dodecahedral cleavage occurs when there are six cleavage planes in a crystal. Sphalerite has dodecahedral cleavage. == Parting ==

Crystal parting occurs when minerals break along planes of structural weakness due to external stress, along twin composition planes, or along planes of weakness due to the exsolution 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 of magnetite, the rhombohedral and basal parting in corundum, and the basal parting in pyroxenes. == Uses ==

, supervised by a senior cutter in the Netherlands 1946. 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 the amphiboles (56–124°) are diagnostic. Crystal cleavage is of technical importance in the electronics industry and in the cutting of gemstones. Precious stones are generally cleaved by impact, as in diamond cutting. Synthetic 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 (silicon, germanium, 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. == See also ==