Lonsdaleite: Properties and Occurrences

Lonsdaleite also called a hexagonal diamond in reference to the crystal structure, is an allotrope of carbon with a hexagonal lattice. The great heat and stress of the impact transforms the graphite into diamond but retains graphite’s hexagonal crystal lattice. It is a hexagonal-dihexagonal dipyramidal mineral containing carbon.

Lonsdaleite was first identified in 1967 from the Canyon Diablo meteorite, where it occurs as microscopic crystals associated with diamond.

General Information

  • Category: Mineral
  • Crystal system: Hexagonal
  • Crystal class: Dihexagonal dipyramidal (6/mmm)
  • Color: Gray in crystals, pale yellowish to brown in broken fragments.

Properties

The property of lonsdaleite as a discrete material has been questioned, since specimens under crystallographic inspection showed not a bulk hexagonal lattice, but instead cubic diamond dominated by structural defects that include hexagonal sequences. 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.

  • Crystal habit: Cubes in fine-grained aggregates
  • Streak: brownish yellow
  • Mohs scale hardness: 7–8 (for impure specimens)
  • Luster: Adamantine
  • Diaphaneity: Transparent
  • Specific gravity: 3.2
  • Optical properties: Uniaxial (+/-)
  • Density: 3.2 g/cm3 (Measured) and 3.51 g/cm3 (Calculated)

It is translucent, brownish-yellow, and has an index of refraction of 2.40 to 2.41 and a specific gravity of 3.2 to 3.3. Its hardness is theoretically superior to that of a cubic diamond, according to computational simulations, but natural specimens exhibited somewhat lower hardness through a large range of values.

Occurrence

Lonsdaleite occurs as microscopic crystals associated with a diamond in several meteorites: Canyon Diablo, Kenna, and Allan Hills 77283. It is also naturally occurring in non-bolide diamond placer deposits in the Sakha Republic.

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. In nature, it forms when meteorites containing graphite strike the Earth.

Association: Schreibersite, cohenite, taenite, graphite, chromite, kosmochlor, sphalerite, black diamond (Ca˜non Diablo); troilite, graphite, diamond, schreibersite, cohenite (Allan Hills 77283).

 

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