Lonsdaleite
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, as opposed to the cubical lattice of conventional diamond. It is found in nature in meteorite debris; when meteors containing graphite strike the Earth, the immense 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 ordinary diamond.
| Lonsdaleite | |
|---|---|
Crystal structure of lonsdaleite | |
| General | |
| Category | Mineral |
| Formula (repeating unit) | C |
| IMA symbol | Lon |
| 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 |
| Structure | |
| Jmol (3D) | Interactive image |
| 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 | |
It is translucent and brownish-yellow and has an index of refraction of 2.40–2.41 and a specific gravity of 3.2–3.3 . 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 7–8 on Mohs hardness scale). The cause is speculated as being due to the samples having been riddled with lattice defects and impurities.
In addition to meteorite deposits, hexagonal diamond has been synthesized in the laboratory (1966 or earlier; published in 1967) by compressing and heating graphite either in a static press or using explosives.