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Crystal structure of lonsdaleite
(repeating unit)
IMA symbolLon[1]
Strunz classification1.CB.10b
Crystal systemHexagonal
Crystal classDihexagonal dipyramidal (6/mmm)
H-M symbol: (6/m 2/m 2/m)
Space groupP63/mmc
Unit cella = 2.51 Å, c = 4.12 Å; Z = 4
Jmol (3D)Interactive image
ColorGray in crystals, pale yellowish to brown in broken fragments
Crystal habitCubes in fine-grained aggregates
Mohs scale hardness7–8 (for impure specimens)
Specific gravity3.2
Optical propertiesUniaxial (+/-)
Refractive indexn = 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, 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.[5][6]

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.[7]

In addition to meteorite deposits, hexagonal diamond has been synthesized in the laboratory (1966 or earlier; published in 1967)[8] by compressing and heating graphite either in a static press or using explosives.[9]


According to the conventional interpretation of the results of examining the meagre samples collected from meteorites or manufactured in the lab, 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. Its diamond structure can be considered to be made up of interlocking rings of six carbon atoms, in the chair conformation. In lonsdaleite, some rings are in the boat conformation instead. At nanoscale dimensions, cubic diamond is represented by diamondoids while hexagonal diamond is represented by wurtzoids.[10]

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; whereas in lonsdaleite the bonds between layers are in the eclipsed conformation, which defines the axis of hexagonal symmetry.

Mineralogical simulation predicts lonsdaleite 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 yet exceeded by IIa diamond's <111> tip hardness of 162 GPa.

The extrapolated properties of lonsdaleite have been questioned, particularly its superior hardness, since specimens under crystallographic inspection have not shown a bulk hexagonal lattice structure, but instead a conventional cubic diamond dominated by structural defects that include hexagonal sequences.[12] 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.[13] 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 comparable to meteorite impacts.[14][15]


Diamond samples from the Popigai impact structure: (a) is pure diamond, while (b) is diamond with some lonsdaleite impurities.

Lonsdaleite occurs as microscopic crystals associated with diamond in several meteorites: Canyon Diablo,[16] Kenna, and Allan Hills 77283. It is also naturally occurring in non-bolide diamond placer deposits in the Sakha Republic.[17] 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.[18] Claims of Lonsdaleite and other nanodiamonds in a layer of the Greenland ice sheet that could be of Younger Dryas age have not been confirmed and are now disputed.[19] 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.[20][21]


In addition to laboratory synthesis by compressing and heating graphite either in a static press or using explosives,[8][9] lonsdaleite has also been produced by chemical vapor deposition,[22][23][24] and also by the thermal decomposition of a polymer, poly(hydridocarbyne), at atmospheric pressure, under argon atmosphere, at 1,000 °C (1,832 °F).[25][26]

In 2020, researchers at Australian National University found by accident they were able to produce lonsdaleite at room temperatures using a diamond anvil cell.[27][28]

In 2021, Washington State University's Institute for Shock Physics published a paper stating that they created lonsdaleite crystals large enough to measure their stiffness, confirming that they are stiffer than common cubic diamonds. However, the explosion used to create these crystals also destroys them nanoseconds later, providing just enough time to measure stiffness with lasers.[29]

See also[edit]


  1. ^ Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
  2. ^ "Lonsdaleite". Mindat.org.
  3. ^ "Lonsdaleite" (PDF). Handbook of Mineralogy – via University of Arizona, Department of Geology.
  4. ^ "Lonsdaleite data". Webmineral.
  5. ^ Frondel, C.; Marvin, U.B. (1967). "Lonsdaleite, a new hexagonal polymorph of diamond". Nature. 214 (5088): 587–589. Bibcode:1967Natur.214..587F. doi:10.1038/214587a0. S2CID 4184812.
  6. ^ Frondel, C.; Marvin, U.B. (1967). "Lonsdaleite, a hexagonal polymorph of diamond". American Mineralogist. 52 (5088): 587. Bibcode:1967Natur.214..587F. doi:10.1038/214587a0. S2CID 4184812.
  7. ^ Carlomagno, G.M.; Brebbia, C.A. (2011). Computational Methods and Experimental Measurements. Vol. XV. WIT Press. ISBN 978-1-84564-540-3.
  8. ^ a b Bundy, F.P.; Kasper, J.S. (1967). "Hexagonal diamond — a new form of carbon". Journal of Chemical Physics. 46 (9): 3437. Bibcode:1967JChPh..46.3437B. doi:10.1063/1.1841236.
  9. ^ a b 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.
  10. ^ Abdulsattar, M. (2015). "Molecular approach to hexagonal and cubic diamond nanocrystals". Carbon Letters. 16 (3): 192–197. doi:10.5714/CL.2015.16.3.192.
  11. ^ Pan, Zicheng; Sun, Hong; Zhang, Yi & 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.
  12. ^ Nemeth, P.; Garvie, L.A.J.; Aoki, T.; Natalia, D.; Dubrovinsky, L.; Buseck, P.R. (2014). "Lonsdaleite is faulted and twinned cubic diamond and does not exist as a discrete material". Nature Communications. 5: 5447. Bibcode:2014NatCo...5.5447N. doi:10.1038/ncomms6447. PMID 25410324.
  13. ^ Salzmann, C.G.; Murray, B.J.; Shephard, J.J. (2015). "Extent of stacking disorder in diamond". Diamond and Related Materials. 59: 69–72. arXiv:1505.02561. Bibcode:2015DRM....59...69S. doi:10.1016/j.diamond.2015.09.007. S2CID 53416525.
  14. ^ Kraus, D.; Ravasio, A.; Gauthier, M.; Gericke, D.O.; Vorberger, J.; Frydrych, S.; Helfrich, J.; Fletcher, L.B.; Schaumann, G.; Nagler, B.; Barbrel, B.; Bachmann, B.; Gamboa, E.J.; Goede, S.; Granados, E.; Gregori, G.; Lee, H.J.; Neumayer, P.; Schumaker, W.; Doeppner, T.; Falcone, R.W.; Glenzer, S.H.; Roth, M. (2016). "Nanosecond formation of diamond and lonsdaleite by shock compression of graphite". Nature Communications. 7: 10970. Bibcode:2016NatCo...710970K. doi:10.1038/ncomms10970. PMC 4793081. PMID 26972122.
  15. ^ Turneaure, Stefan J.; Sharma, Surinder M.; Volz, Travis J.; Winey, J.M.; Gupta, Yogendra M. (1 October 2017). "Transformation of shock-compressed graphite to hexagonal diamond in nanoseconds". Science Advances. 3 (10): eaao3561. Bibcode:2017SciA....3O3561T. doi:10.1126/sciadv.aao3561. ISSN 2375-2548. PMC 5659656. PMID 29098183.
  16. ^ Lea, Robert (12 September 2022). "Dwarf planet collision may have sent strange ultra-hard diamonds to Earth". Space.com. Retrieved 13 September 2022.
  17. ^ Kaminskii, F.V.; G.K. Blinova; E.M. Galimov; G.A. Gurkina; Y.A. Klyuev; L.A. Kodina; V.I. Koptil; V.F. Krivonos; L.N. Frolova; A.Y. Khrenov (1985). "Polycrystalline aggregates of diamond with lonsdaleite from Yakutian [Sakhan] placers". Mineral. Zhurnal. 7: 27–36.
  18. ^ Israde-Alcantara, I.; Bischoff, J.L.; Dominguez-Vazquez, G.; Li, H.-C.; Decarli, P.S.; Bunch, T.E.; et al. (2012). "Evidence from central Mexico supporting the Younger Dryas extraterrestrial impact hypothesis". Proceedings of the National Academy of Sciences. 109 (13): E:738–747. Bibcode:2012PNAS..109E.738I. doi:10.1073/pnas.1110614109. PMC 3324006. PMID 22392980.
  19. ^ Kurbatov, Andrei V.; Mayewski, Paul A.; Steffensen, Jorgen P.; West, Allen; Kennett, Douglas J.; Kennett, James P.; Bunch, Ted E.; Handley, Mike; Introne, Douglas S.; Hee, Shane S. Que; Mercer, Christopher; Sellers, Marilee; Shen, Feng; Sneed, Sharon B.; Weaver, James C.; Wittke, James H.; Stafford, Thomas W.; Donovan, John J.; Xie, Sujing; Razink, Joshua J.; Stich, Adrienne; Kinzie, Charles R.; Wolbach, Wendy S. (20 September 2022). "Discovery of a nanodiamond-rich layer in the Greenland ice sheet". PubPeer. Retrieved 28 September 2022.
  20. ^ Kvasnytsya, Victor; Wirth; Dobrzhinetskaya; Matzel; Jacobsend; Hutcheon; Tappero; Kovalyukh (August 2013). "New evidence of meteoritic origin of the Tunguska cosmic body". Planetary and Space Science. 84: 131–140. Bibcode:2013P&SS...84..131K. doi:10.1016/j.pss.2013.05.003.
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  24. ^ 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.
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