Lonsdaleite

Lonsdaleite
Crystal structure of lonsdaleite
General
Category	Mineral
Formula (repeating unit)	C
Strunz classification	1.CB.10b
Crystal system	Hexagonal
Crystal class	Dihexagonal dipyramidal (6/mmm) H-M symbol: (6/m 2/m 2/m)
Space group	P63/mmc
Unit cell	a = 2.51 Å, c = 4.12 Å; Z = 4
Identification
Color	Gray in crystals, pale yellowish to brown in broken fragments
Crystal habit	Cubes in fine-grained aggregates
Mohs scale hardness	7–8 (for impure specimens)
Luster	Adamantine
Diaphaneity	Transparent
Specific gravity	3.2
Optical properties	Uniaxial (+/-)
Refractive index	n = 2.404
References

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 1,000 °C (1,832 °F).

It is translucent, brownish-yellow, 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). The cause is speculated as being due to the samples having been riddled with lattice defects and impurities.

The property of lonsdaleite as a discrete material has been questioned, since specimens under crystallographic inspection showed not a bulk hexagonal lattice, but instead cubic diamond dominated by structural defects that include hexagonal sequences. A quantitative analysis of the X-ray diffraction data of lonsdaleite has shown that about equal amounts of hexagonal and cubic stacking sequences are present. Consequently, it has been suggested that "stacking disordered diamond" is the most accurate structural description of lonsdaleite. On the other hand, recent shock experiments with in situ X-ray diffraction show strong evidence for creation of relatively pure lonsdaleite in dynamic high-pressure environments such as meteor impacts.