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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 debris; when 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 mixed in with ordinary diamond.
It is translucent and brownish-yellow and has an refractive index of 2.40–2.41 and a specific gravity of 3.2–3.3. Its hardness is theoretically superior to that of Diamond cubic (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 to be due to the samples having been riddled with lattice defects and impurities.
(2026). 9781845645403, WIT Press. ISBN 9781845645403
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.
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 Miller index face, and to resist indentation pressures of 152 GPa, whereas diamond would break at 97 GPa.
The extrapolated properties of lonsdaleite have been questioned, particularly its superior hardness, since specimens under crystallography inspection have not shown a bulk hexagonal lattice structure, but instead a conventional cubic diamond dominated by structural defects that include hexagonal sequences.
In 2020, researchers at Australian National University found by accident they were able to produce lonsdaleite at room temperatures using a diamond anvil cell.
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.
In July 2025, Chinese researchers reported the successful synthesis of high-purity lonsdaleite crystals, ranging from micrometre to millimetre in size, by compressing ultrapure graphite single crystals under precisely controlled high-pressure, high-temperature, and quasi-hydrostatic conditions. The work, published in Nature, is regarded as the first clear laboratory production of bulk hexagonal diamond, which is predicted to have greater hardness and thermal stability than conventional cubic diamond.
Hardness
According to the conventional interpretation of the results of examining the meagre samples collected from 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-packing 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 while hexagonal diamond is represented by wurtzoids.
This is yet exceeded by diamond type diamond's <111> tip hardness of 162 GPa.
Occurrence
Lonsdaleite occurs as microscopic crystals associated with diamond in several meteorites: Canyon Diablo, Kenna meteorite, and Allan Hills 77283. It is also naturally occurring in non-bolide diamond in the Sakha Republic.
Manufacture
In addition to laboratory synthesis by compressing and heating graphite either in a static press or using explosives,
Scams
Since the characteristics of lonsdaleite are unknown to most people outside of scientists trained in geology and mineralogy, the names "lonsdaleite" and "hexagonal diamond" have frequently been used in the fraudulent sale of ceramic artifacts passed off as meteorites on online e-commerce sites and at Street fair and street markets, with prices ranging from a few dollars to thousands of dollars.
See also
Further reading
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