Crystal structure of lonsdaleite
|Crystal symmetry||Hexagonal dihexagonal dipyramidal
H-M symbol: (6/m 2/m 2/m)
Space group: P 63/mmc
|Unit cell||a = 2.51 Å, c = 4.12 Å; Z=4|
Gray in crystals, pale yellowish to brown inbroken fragments
|Crystal habit||Cubes in fine-grained aggregates|
|Mohs scale hardness||7–8 (for impure specimens)|
|Optical properties||Uniaxial (+/-)|
|Refractive index||n = 2.404|
Lonsdaleite (named in honour of Kathleen Lonsdale), also called hexagonal diamond in reference to the crystal structure, is an allotrope of carbon with a hexagonal lattice. In nature, it forms when meteorites containing graphite strike the Earth. The great heat and stress of the impact transforms the graphite into diamond, but retains graphite's hexagonal crystal lattice. Lonsdaleite was first identified in 1967 from the Canyon Diablo meteorite, where it occurs as microscopic crystals associated with diamond.
Hexagonal diamond has also been synthesized in the laboratory (1966 or earlier; published in 1967) by compressing and heating graphite either in a static press or using explosives. It has also been produced by chemical vapor deposition, and also by the thermal decomposition of a polymer, poly(hydridocarbyne), at atmospheric pressure, under argon atmosphere, at temperature 110 °C (230 °F).
It is translucent, brownish-yellow in color, and has an index of refraction of 2.40 to 2.41 and a specific gravity of 3.2 to 3.3. Its hardness is theoretically superior to that of cubic diamond (up to 58% more) according to computational simulations but natural specimens exhibited somewhat lower hardness through a large range of values (from 7 to 8 on Mohs hardness scale), a speculation based on the fact that the samples were ridden with lattice defects and impurities.
Lonsdaleite has a hexagonal unit cell, related to the diamond unit cell in the same way that the hexagonal and cubic close packed crystal systems are related. The diamond structure can be considered to be made up of interlocking rings of six carbon atoms, in the chair conformation. In lonsdaleite, some of the rings are in the boat conformation instead. In diamond, all the carbon-to-carbon bonds, both within a layer of rings and between them, are in the staggered conformation, thus causing all four cubic-diagonal directions to be equivalent; while in lonsdaleite the bonds between layers are in the eclipsed conformation, which defines the axis of hexagonal symmetry.
Lonsdaleite is simulated to be 58% harder than diamond on the <100> face and to resist indentation pressures of 152 GPa, whereas diamond would break at 97 GPa. This is still below IIa diamond's <111> tip hardness of 162 GPa.
Lonsdaleite occurs as microscopic crystals associated with diamond in several meteorites: Canyon Diablo, Kenna, and Allan Hills 77283. It is also naturally occurring in non-bolide diamond placer deposits in the Sakha Republic. Material with d-spacings consistent with Lonsdaleite has been found in sediments with highly uncertain dates at Lake Cuitzeo, in the state of Guanajuato, Mexico, by proponents of the controversial Younger Dryas impact hypothesis. Its presence in local peat deposits is claimed as evidence for the Tunguska event being caused by a meteor rather than by a cometary fragment.
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- Diamond no longer nature's hardest material
- lonsdaleite 3D animation