Lonsdaleite

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Lonsdaleite is an allotrope of carbon with a hexagonal lattice, believed to form when meteoric graphite falls to 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 from the Canyon Diablo meteorite at Barringer Crater (also known as Meteor Crater) in Arizona. It was first discovered in 1967. Lonsdaleite occurs as microscopic crystals associated with diamond in the Canyon Diablo meteorite; Kenna meteorite, New Mexico; and Allan Hills (ALH) 77283, Victoria Land, Antarctica meteorite. It has also been reported from the Tunguska impact site, Russia.

Lonsdaleite is also known as "hexagonal diamond." It is transparent brownish-yellow in color and has an index of refraction from 2.40 to 2.41, a specific gravity from 3.2 to 3.3, and a Mohs hardness of 7–8. The Mohs hardness of diamond is 10; the lower hardness of lonsdaleite is chiefly attributed to impurities and imperfections in the naturally occurring material. It can also be created by the thermal decomposition of a polymer, poly(hydridocarbyne), at atmospheric pressure under argon starting at 110 °C.

Lonsdaleite was named in honour of Kathleen Lonsdale.

Crystal Structure

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 considerred 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.

A diagram of the structure of lonsdaleite can be viewed here: [1]. A diagram of the structure of diamond can be viewed here: [2].

See also

• List of minerals
• List of minerals named after people

References

• Mindat.org accessed 3/13/05.
• Webmineral accessed 3/13/05.
• Anthony, J.W.; et al (1995). Mineralogy of Arizona, 3rd.ed.. 
• Frondel, C.; U.B. Marvin (1967). "Lonsdaleite, a new hexagonal polymorph of diamond". Nature 214: 587–589.
• Frondel, C.; U.B. Marvin (1967). "Lonsdaleite, a hexagonal polymorph of diamond". Am.Min. 52.
• Bianconi, P.; et al (2004). "Diamond and Diamond-like Carbon from a Preceramic Polymer". J. Am. Chem. Soc. Vol. 126 (10): 3191–3202.
• Materials Science and Technology Division, Naval Research Laboratory website accessed 5/14/2006.