Diamonds are forever? So is lonsdaleite

Diamond, an allotropic form of carbon, has for long captured the imagination of human beings. This hexagonal glittering stone known for its hardness has been crowned as the ‘world’s hardest material’.

Nanoparticles have replaced diamonds as the world’s toughest material, but are not natural forms. Now, a new natural substance has literally stripped the diamond of its crown. This material is known as lonsdaleite. This is also a hexagonal diamond and is its close cousin.

This allotropic form of carbon has been named after the famous British crystallographer, Kathleen Lonsdale. Lonsdaleite is formed from graphite present in meteorites following their impact on earth.

It is formed 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. The material is available in certain parts of Northern America.

The material was first identified from the Canyon Diablo meteorite at Barringer Crater at Arizona in 1967. This occurs as a microscopic crystal associated with diamond in the meteorite.

Lonsdaleite is present naturally in a very small form. This has been synthesised in the laboratory by compressing and heating the graphite either in static pressure or using explosion techinques. This translucent substance is brownish-yellow in colour. When the material is available in its purest form, it is 58 per cent harder than diamond. This has been reported by scientists at Shanghai Jiao Tong University in the New Scientist magazine.

Wurtzite boron nitride

Another naturally available substance that has been identified is wurtzite boron nitride. Its molecular structure is similar to that of the diamond, but it is made of different atoms. This natural substance comes together from volcanic eruptions and has flexible atomic bonds. When under stress, this material is 18 per cent harder than the diamond. Scientists at the University of Nevada and University of Shanghai are carrying out research on this material.

If confirmed, this will be most useful as a super abrasive in the cutting tool industry. Another important use of this material is that it can be used as a protective layer on space vehicles to protect shuttles during their re-entry to earth.
There are many meteorite minerals available in nature. Some of them are in the form of metallic iron (kumalite, tacnite), silicate minerals (pyroxenes, eustalite, bronzide, etc.)