Beyond Earth's Hardness: Discovering the Origins of Lonsdaleite
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Diamond, with its tough-to-break carbon lattice of interlocking cubes, is traditionally considered the hardest material on Earth. Yet a rare form of diamond known as lonsdaleite – a crystal with carbon atoms arranged in flexing three-dimensional hexagons – may be even harder. To date, natural lonsdaleite has been found only in impact craters, where it has formed by the intense pressure of meteorites crashing to Earth. But now researchers say that they’ve identified lonsdaleite crystals that formed billions of years before the meteorites carrying them ever reached the planet.
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… The research team examined 18 different meteorite samples from a family known as “ureilites.” Because ureilites are relatively homogeneous in their chemical composition – which is uniquely rich in carbon – scientists theorize they originate from the same parent body. “There was this dwarf planet just after the start of our solar system – so 4.5 billion years ago – and the planet got hit by an asteroid,” says Alan Salek, a graduate student in applied physics at the Royal Melbourne Institute of Technology in Australia and co-author of the new study. This cataclysmic impact tore the dwarf planet apart, sparking a chemical reaction that could have turned pieces of the planet’s graphite into lonsdaleite, he adds.
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Graphite is made up of flat layers of carbon atoms bonded together as hexagons. These stacked layers are weakly attracted to each other and relatively easy to pull apart. On Earth, high heat and pressure can rearrange these carbon atoms into a 3-D lattice of cubes, thereby creating the traditional kind of diamond. But a brief period of extremely intense pressure – such as that of a meteorite impact – can preserve graphite’s original hexagonal arrangement while its layers bond into the strong 3-D lattice of lonsdaleite.
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The researchers propose that rather than the rapid impact pressure known to produce tiny lonsdaleite crystals on Earth, these samples instead formed through a rapid release of pressure. They claim that a fluid mix of carbon, hydrogen, oxygen and sulfur was heated and pressurized in the dwarf planet’s mantle until an asteroid impact smashed that mantle into pieces. Study co-author Andrew Tomkins says that the rapidly depressurizing mix of chemicals could have interacted with the dwarf planet’s graphite to transform it into lonsdaleite.
In this particular reaction, graphite crystals would have been essentially torn apart and rebuilt into lonsdaleite. “It’s called ‘coupled dissolution-reprecipitation’ because it’s kind of dissolving this thing and replacing it at the same time,” Tomkins says. This fluid-driven reaction took place in chunks of the dwarf planet as they went flying into space… Tomkins explains that the structure of these meteorites’ minerals indicates a rapid cooling process that points to a dramatic collision. Looking at particular radioactive signatures of the minerals, the researchers estimated a date for this collision – roughly 4.5 billion years ago. Plus, the ureilite samples contain interlocking layers of lonsdaleite, cubic diamond and graphite in a pattern that points to the fluid-driven transformation Tomkins’s team describes. The specks of lonsdaleite that Salek and Tomkins’ research group identified were up to a micron in size – still extremely small but roughly 1,000 times larger than any lonsdaleite crystals previously known. This suggests that a fluid-driven transformation of graphite into lonsdaleite might produce bigger crystals than the impact method…
