Cleavage (crystal)

Cleavage, in mineralogy, is the tendency of crystalline materials to split along definite crystallographic structural planes. These planes of relative weakness are a result of the regular locations of atoms and ions in the crystal, which create smooth repeating surfaces that are visible both in the microscope and to the naked eye.[1]

Fluorita green.jpeg
Green fluorite with prominent cleavage.
Biotita1.jpeg
Biotite with basal cleavage.

Types of cleavage

Miller Indices Felix Kling
Miller indices {h k ℓ}

Cleavage forms parallel to crystallographic planes:[1]

  • Basal or pinacoidal cleavage occurs when there is only one cleavage plane. Graphite has basal cleavage. Mica (like muscovite or biotite) also has basal cleavage; this is why mica can be peeled into thin sheets.
  • Cubic cleavage occurs on 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.
  • 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.
  • Rhombohedral cleavage occurs when there are three cleavage planes intersecting at angles that are not 90 degrees. Calcite has rhombohedral cleavage.
  • Prismatic cleavage occurs when there are two cleavage planes in a crystal. Spodumene exhibits prismatic 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 or along twin composition planes. Parting breaks are very similar in appearance to cleavage, but only occur due to stress. Examples include magnetite which shows octahedral parting, the rhombohedral parting of corundum and basal parting in pyroxenes.[1]

Uses

Cleavage is a physical property traditionally used in mineral identification, both in hand 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.[1]

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 (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.

See also

References

  1. ^ a b c d * Hurlbut, Cornelius S.; Klein, Cornelis, 1985, Manual of Mineralogy, 20th ed., Wiley, ISBN 0-471-80580-7

External links

Frankdicksonite

Frankdicksonite is a halide mineral with the chemical formula BaF2 which corresponds to the chemical compound barium fluoride. It occurs in the Carlin gold deposit of Eureka County, Nevada as cubic crystals sized between 0.1 and 4 mm, and is of hydrothermal origin. Its only associated mineral is quartz and the frankdicksonite crystals are always completely encapsulated in it. Frankdicksonite has fluorite crystal structure with a cubic symmetry and the lattice constant a = 619.64 pm. Its Vickers hardness on the {111} cleavage crystal faces varies between 88 and 94 kg/mm2 and is close to that of the synthetic barium fluoride (95 kg/mm2). Its refractive index (1.475) is almost identical to that of BaF2 (1.474). Under electron irradiation, it emits strong blue cathodoluminescence. The major impurity in frankdicksonite is strontium with concentrations up to 0.5% by weight. Also present are silicon (0.02%) and magnesium (0.0015%); other impurities have concentrations below 0.0015%.Although synthetic barium fluoride has been commonly known from at least 1846, it had not been found in nature. The natural existence of barium fluoride was predicted by Michael Fleischer in 1970. Later in the same year, the mineral was discovered by Arthur S. Radtke and named after Frank W. Dickson (born 1922), professor of Geochemistry at Stanford University in recognition of his contributions to geology and geochemistry of low-temperature ore deposits. Frankdicksonite was officially recognized by the Commission of New Minerals and Mineral Names in 1974.

Parting

Parting may refer to:

Parting (film), a 2016 Afghan-Iranian film

Parting.com, a funeral home directory

Parting tradition

Cleavage (crystal)#Parting

Side-parting, a common male hairstyle: see Regular haircut

Won Lee-sak, known as PartinG, a South Korean StarCraft II player

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