CARBON – From the element of life to advanced technology _ PART II (WHY DIAMOND is not the hardest natural substance?)

All forms of life on Earth are based on carbon, and although these types of carbon appear very different from graphite, they can easily be converted into its hexagonal structure through burning:

  • wood turns into charcoal when heated;
  • bread becomes burnt oast;
  • we too become black and charred when exposed to a fire.

None of these processes produce pure shiny graphite, since the hexagonal layers of carbon are not densely packed but are jumbled up. But there is a vast spectrum of black sooty materials, which all have one thing in common: they contain Carbon in its most stable form – hexagonal sheets.

In the 19th century, another form of black sooty carbon rose to ascendancy: the coal. The hexagonal planes of carbon atoms in coal are formed not through heat, as with a burnt piece of toast, but through geological processes acting on dead plant matter over millions of years. Coal starts off as a form of peat but as heat and pressure act upon it, depending on the exact conditions, it is transformed into lignite, then bituminous coal, then anthracite coal and finally graphite. What happens as it undergoes this transformation is that, step by step, it loses the volatile compounds, containing nitrogen (N), sulphur (S) and oxygen (O) that are present in the original plant matter, becoming in the process a purer and purer form of carbon. As the pure hexagonal layers get formed, so the material takes on a more metallic shine which can be seen particularly clearly in the black mirror facets of some coals such as anthracite. Nevertheless, coal is very rarely a pure form of carbon, which is why it can be quite smelly when it burns.

The type of coal most revered for its aesthetic appeal is that derived from fossilized monkey puzzle trees. It is hard, can be carved and polished to a brilliant finish, and has a beautiful dark black lustre. It is sometimes called black amber because it has similar triboelectric properties to amber: the ability to generate static charge and make hair stand on end. We know  it more commonly as jet.

The idea that diamond has anything in common with coal or graphite was pure fantasy until early chemists started to investigate what happened when you heat it. Antoine Lavoisier did just this in 1772 and found that diamond buns when it gets red hot, leaving nothing. Nothing at all. It just seems to disappear into thin air. This experiment was extremely surprising. Other gemstones such as ruby and sapphire, were found to be impervious to red heat or even white heat: they did not burn. But diamond, the kings of gems, seemed to have an Achilles heel. What Lavoisier did next makes my heart sing, such is the elegance of the experiment. He heated diamond in a vacuum so that there, with no air to react with the diamond, it might survive to higher temperatures. It’s one of those experiments that it is easy to propose but much harder to carry out, especially in the 18th century when vacuums themselves were not so easy to produce. What happened next astounded Lavoisier: the diamond still wasn’t impervious to red heat, but this time it turned into pure graphite – proof that these two materials were indeed made of the same stuff: carbon.

Armed with this knowledge, Lavoisier and countless others across Europe searched for a way to reverse the process, to turn graphite into diamond. Vast wealth would be the reward for anyone who could do it, and the race was on. But the task was a formidable one. All materials prefer to change from less stable to more stable structures, and because the diamond structure is less stable than graphite’s, it requires extremely high temperatures and pressures to persuade it to change in the opposite direction. These conditions exist inside the Earth’s crust, but it still takes billions of years to grow a big diamond crystal. Simulating the conditions in a laboratory is extremely difficult. Claim after claim was made and then retracted. None of the scientists involved got massively rich overnight, which some said was further evidence of their failure. Others suspected that those who did achieve the feat of transformation kept quiet and got rich slowly. Whatever the truth, it wasn’t until 1953 that there was reliable documented evidence of such a transformation being achieved. Now the synthetic diamond industry is indeed big business, but it does not compete head to head with the natural diamond jewellery industry. There are a few reasons for this.

The first is that although the industrial process has been mastered to the extent that small synthetic diamonds can be produced more cheaply that mining real ones, they are mostly colored and flawed, since the accelerated process of making them introduces defects which colour the diamonds. In fact, the majority of these diamonds are used in the mining industry, where they embroider drills and cutting tools, not for aesthetic effect but to enable them to cut through granite and other hard rocks. Secondly, much of the value of diamonds is derived from their authenticity. It is important when proposing marriage that the diamond you offer, although physically identical to a synthetic one, was forged in the depths of the Earth a billion years ago. Thirdly, if you are the ultra-rational sort of person who does not care about the natural history of a gemstone, then buying a synthetic diamond is still a very expensive way to embellish your loved one. There are much cheaper lustrous substitutes that will glitter and dazzle and still fool anyone except a diamond expert, such as cubic zirconia crystals or even glass.

When most people think about the hardest naturally occurring material on Earth, they think of diamonds – those pretty stones in our engagement rings that can cut steel and rock. However, the pre-eminence of natural diamond, in its fight for supremacy with graphite, was to take another blow when it was found that it was no longer the hardest known material.

Nature’s toughest material just got upgraded.

In 1967 it was discovered that there is a third way of arranging carbon atoms that produces an even harder substance than diamond. The structure is based on graphite’s hexagonal planes but modified to be 3-dimensional. This structure, called lonsdaleite, is thought to be 58% harder than diamond, although it exists in such small quantities that it is hard to test.

The first sample was found in the Canyon Diablo meteorite, where the intense heat and pressure of impact transformed graphite into lonsdaleite. Researchers have managed to make it in the lab before, but without great success. And it takes incredible temperatures of around 1,000°C to actually form the diamond.

But scientists have been gradually improving on that toughness over the past few years, and recently (in 2020) a team of Australian researchers has just created a rare type of diamond that’s even harder than diamond. This diamond is a version of Lonsdaleite, which has been found occurring naturally at the centre of a handful of meteorite impact sites around the world. The researchers have been able to make a nano-scale version of Lonsdaleite in the lab, and they predict that it’s even harder than the naturally occurring version. It’s so strong, in fact, that the team suggests its most immediate use will be in mine sites, where it can cut through ultra-solid materials, including regular diamonds. The team was able to create the new material by nanoengineering the diamond from scratch using a type of carbon that doesn’t have a set form, known as amorphous carbon.

Instead, of heating the samples at 1000°C, this team of scientists took a different approach. They´ve put this carbon into a device called a diamond anvil, which is made of two diamonds opposing each other to recreate the high pressures you’d find deep down inside Earth. Using the device, they were able to create the diamonds at temperatures of just 400°C – around half as hot as previous methods, which means it’s a lot cheaper and more efficient. And the end result is also a lot harder. The researchers now need to go through further testing of this structure to find out exactly how tough it is compared to existing materials, but if natural Lonsdaleite is anything to go off, they’re expecting it to be pretty hard. But this new diamond is not going to be on any engagement rings. You’ll more likely find it on a mining site. Any time you need a super-hard material to cut something, this new diamond has the potential to do it more easily and more quickly. An engagement ring has never been made from lonsdaleite, since the types of meteorite impacts that create it are extremely rare and produce only tiny crystals, but the discovery of this third structure of carbon led, perhaps inevitably, to the question of whether yet further carbon structures could exist, in addition to the cubic structure of diamond, the hexagons of coal, jet, charcoal  and graphite, and the 3-dimensional hexagonal structure of lonsdaleite.

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