So, diamond is the hardest material on Earth, right? Not really, although the element carbon still forms the building blocks of one of the contenders for the crown.
Lonsdaleite (named after Kathleen Lonsdale) is also a form of carbon crystal but one that is assembled in a hexagonal lattice rather than the cubic arrangement of diamond. It has been produced in the lab by compressing and heating graphite, either in a static press or using explosives, and also by chemical vapor deposition. In nature, its formation is altogether more spectacular, occurring when meteorites containing graphite (also a type of carbon) strike the Earth. It was first identified in 1967 from the Canyon Diablo meteorite, where it occurs as microscopic crystals associated with diamond.
The translucent, brownish-yellow-coloured crystal is theoretically harder than cubic diamond, according to computational simulations. However, natural specimens have exhibited somewhat lower hardness through a large range of values (from 7 to 8 on Mohs hardness scale). Mineralogists speculate that this is because the test samples were riddled with lattice defects and impurities.
Nevertheless, the material is a good demonstration that the arrangement of atoms within a material composed of the same element can lead to widely varying properties – check out graphite and diamond. The importance of structure is also important in a relatively new material called ACNR (aggregated carbon nanorods).
This lab-created, carbon-based material is harder than diamond and was created by packing together tiny carbon “nanorods”. It derives its strength from the interlocking arrangement of the nanorods, rather than a diamond’s strong molecular bonds between the atoms.
ACNR was produced by compressing and heating super-strong carbon molecules called buckyballs or carbon-60 – these molecules consist of 60 atoms that interlock in hexagonal or pentagonal shapes and resemble tiny soccer balls. Extreme pressure and temperature are needed: the scientists compressed the buckyballs to 200 times normal atmospheric pressure and heated them to 2,226°C. They found that ACNR was 0.3% denser than ordinary diamond and more resistant to pressure than any other known material.
Another contender for the world’s hardest substance is a mineral called Wurtzite Boron Nitride, a relatively new “superabrasive” manufactured by detonation. Its polycrystalline structure means more chemical bonds work together at the same time compared with a cubic structure. This rare mineral has a number of industrial applications for manufacturing cutting and grinding tools and also polishing steel.
Diamond fact file
The word “diamond” derives from the ancient Greek αδάμας – adámas, meaning “unbreakable”?
Most natural diamonds are formed at high temperature and pressure at depths of 140 to 190 kilometers in the Earth’s mantle.
Sometimes so-called space diamonds are formed during the course of a meteorite impact; in contrast, carbonado diamonds found in South America and Africa may actually have been carried to Earth and deposited there by an asteroid impact.
Evidence indicates that white dwarf stars have a core of crystallized carbon and oxygen nuclei.
Diamonds are thought to have first been recognised and mined in India, where they have been known for at least 3,000 years but probably some 6,000 years.