Lonsdaleite
| Lonsdaleite | |
|---|---|
 Crystal structure of Lonsdaleite |
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| General | |
| Category | Mineral |
| Chemical formula | C |
| Strunz classification | 01.CB.10b |
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.[1][2]
Hexagonal diamond has also been synthesized in the laboratory (1966 or earlier; published in 1967)[3] by compressing and heating graphite either in a static press or using explosives.[4] It has also been produced by chemical vapor deposition,[5][6][7] and also by the thermal decomposition of a polymer, poly(hydridocarbyne), at atmospheric pressure, under argon atmosphere, at temperature 110 °C (230 °F).[8][9]
It is translucent, brownish-yellow in color, and has an index of refraction of 2.40 to 2.41, a specific gravity of 3.2 to 3.3, and a Mohs hardness of 7–8.[10] The Mohs hardness of diamond is 10, and the lower hardness of lonsdaleite is chiefly attributed to impurities and imperfections in the naturally occurring material. A simulated pure sample has been found to be 58% harder than diamond.[11]
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[edit] Properties
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.[11] This is still below IIa diamond's <111> tip hardness of 162 GPa.
[edit] Occurrence
Lonsdaleite occurs as microscopic crystals associated with diamond in several meteorites: Canyon Diablo, Kenna, and Allan Hills 77283. It has also been reported from the Tunguska impact site.
[edit] See also
[edit] References
- ^ Frondel, C.; U.B. Marvin (1967). "Lonsdaleite, a new hexagonal polymorph of diamond". Nature 214 (5088): 587–589. Bibcode 1967Natur.214..587F. doi:10.1038/214587a0.
- ^ Frondel, C.; U.B. Marvin (1967). "Lonsdaleite, a hexagonal polymorph of diamond". Am.Min. 52.
- ^ Bundy, F. P. (1967). "Hexagonal Diamond—A New Form of Carbon". Journal of Chemical Physics 46 (9): 3437. Bibcode 1967JChPh..46.3437B. doi:10.1063/1.1841236.
- ^ He, Hongliang; Sekine, T.; Kobayashi, T. (2002). "Direct transformation of cubic diamond to hexagonal diamond". Applied Physics Letters 81 (4): 610. Bibcode 2002ApPhL..81..610H. doi:10.1063/1.1495078.
- ^ Bhargava, Sanjay; Bist, H. D.; Sahli, S.; Aslam, M.; Tripathi, H. B. (1995). "Diamond polytypes in the chemical vapor deposited diamond films". Applied Physics Letters 67 (12): 1706. Bibcode 1995ApPhL..67.1706B. doi:10.1063/1.115023.
- ^ Nishitani-Gamo, Mikka; Sakaguchi, Isao; Loh, Kian Ping; Kanda, Hisao; Ando, Toshihiro (1998). "Confocal Raman spectroscopic observation of hexagonal diamond formation from dissolved carbon in nickel under chemical vapor deposition conditions". Applied Physics Letters 73 (6): 765. Bibcode 1998ApPhL..73..765N. doi:10.1063/1.121994.
- ^ Misra, Abha; Tyagi, Pawan K.; Yadav, Brajesh S.; Rai, P.; Misra, D. S.; Pancholi, Vivek; Samajdar, I. D. (2006). "Hexagonal diamond synthesis on h-GaN strained films". Applied Physics Letters 89 (7): 071911. Bibcode 2006ApPhL..89g1911M. doi:10.1063/1.2218043.
- ^ Nur, Yusuf; Pitcher, Michael; Seyyidoğlu, Semih; Toppare, Levent (2008). "Facile Synthesis of Poly(hydridocarbyne): A Precursor to Diamond and Diamond-like Ceramics". Journal of Macromolecular Science Part A 45 (5): 358. doi:10.1080/10601320801946108.
- ^ Nur, Yusuf; Cengiz, Halime M.; Pitcher, Michael W.; Toppare, Levent K. (2009). "Electrochemical polymerizatıon of hexachloroethane to form poly(hydridocarbyne): a pre-ceramic polymer for diamond production". Journal of Materials Science 44 (11): 2774. Bibcode 2009JMatS..44.2774N. doi:10.1007/s10853-009-3364-4.
- ^ Lonsdaleite at Mindat
- ^ a b Pan, Zicheng; Sun, Hong; Zhang, Yi; and Chen, Changfeng (2009). "Harder than Diamond: Superior Indentation Strength of Wurtzite BN and Lonsdaleite". Physical Review Letters 102 (5): 055503. Bibcode 2009PhRvL.102e5503P. doi:10.1103/PhysRevLett.102.055503. PMID 19257519. Lay summary – Physorg.com (12-02-2009).
[edit] Further reading
- Anthony, J. W.; et al. (1995). Mineralogy of Arizona (3rd ed.). Tucson: University of Arizona Press. ISBN 0816515794.
[edit] External links
- Mindat.org accessed 13 March 2005.
- Webmineral accessed 13 March 2005.
- Materials Science and Technology Division, Naval Research Laboratory website accessed 14 May 2006.
- Diamond no longer nature's hardest material
- lonsdaleite 3D animation
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