By Aiden Plumbe
You’ve probably heard that diamond is the hardest material on Earth and that is true. Partially. In 1967, a team of researchers discovered lonsdaleite in the Canyon Diablo meteorite as microscopic crystals very similar to diamonds. However, these seemingly innocent crystals would go on to be the hardest material known to humankind, surpassing even diamonds.
Andy Tomkins, the lead authour of this study, and his team discovered this rare substance in samples of ureleite meteors which they were researching at the time. These meteorites containing the lonsdaleite broke off from the mantle (interior layer) of a dwarf planet. After the meteorite experienced a believed-to-be catastrophic collision, the high temperatures and pressure transformed some previously existing graphite crystals into lonsdaleite.
Andy, his crew, and some scientists confirmed some shocking facts about this meteorite sample. Lonsdaleite is similar to diamond, however what separates them is their individual crystal systems. But what does this mean? Well, what differs a crystal from other naturally forming substances is their atomic arrangement. According to the International Gem Society, a crystal is defined as “a solid whose atoms are arranged in a highly ordered repeating pattern”. There are seven different crystal systems, triclinic, monoclinic, orthorhombic, tetragonal, trigonal, hexagonal, and cubic. This might just sound like a bunch of weird words but in simple terms, these crystal systems result in different varieties crystal shapes (Photo 2).
But what does this all have to do with diamonds and lonsdaleite? Well, lonsdaleite is an allotrope of carbon which has a hexagonal crystal structure. It is often called the “hexagonal diamond”, but this term is misleading since diamonds have a cubic crystal structure. This might not seem important, but to the scientists researching lonsdaleite, this is remarkably interesting news. To put it in simple terms, a diamond’s structure means there are eight atoms that form a cube, each atom forming four bonds with its neighbours. This is why it is so strong. Compared to lonsdaleite, its hexagonal structure allows each atom to bond with six neighbouring atoms each, which is why it has been proved to be 58% stronger than diamond.
However, there is a minor problem. All this information is great, but the samples scientists have found so far at meteorite sites have been so small that it has been difficult to accurately measure its hardness. Additionally, because nature is messy and not perfect, natural specimens have had defects in their crystal lattices and several impurities. So, where computer simulations have proved that pure lonsdaleite is harder than diamond, natural samples have been about a 7 to an 8 on the ‘Mohs Hardness Scale’ where diamond is 10.