Lonsdaleite

Lonsdaleite
Crystal structure of lonsdaleite
General
CategoryMineral
Formula
(repeating unit)		C
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
Structure
Jmol (3D)Interactive image
SMILES
C1C7(CC(C6)C9)C2CC3C68C4CC5C9(C678)CC7C(C690)C5CC4C9C3CC2C0C1C7
(SMILES input only shows in Preview. SMILES is used to create the Jmol 3D image.)
Identification
ColorGray in crystals, pale yellowish to brown in broken fragments
Crystal habitCubes in fine-grained aggregates
Mohs scale hardness7–8 (for impure specimens)
LusterAdamantine
DiaphaneityTransparent
Specific gravity3.2
Optical propertiesUniaxial (+/−)
Refractive indexn = 2.404
References[2][3][4]

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]

  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. ^ Cite error: The named reference Bundy-Kasper-1967 was invoked but never defined (see the help page).
  9. ^ Cite error: The named reference He-etal-2002 was invoked but never defined (see the help page).