How to build a diamond factory

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Diamonds such as this 100-carat beauty are the most prized gemstones in the world, but exactly how they form deep in the Earth is a mystery. A new study that closely examines the tiny ‘inclusions’ within a diamond shows at least some require sulfide minerals.
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They form under intense heat and pressure hundreds of kilometres underground, but what kicks off a diamond’s crystallisation from carbon in Earth’s mantle is far from clear.

The answer, at least for some diamonds, lies with sulfide minerals, according to Dorrit Jacob from Australia’s Macquarie University and colleagues from Australia and Germany. The researchers closely examined minuscule “melts” enveloped within a diamond and found oxidation of sulfide mineral pyrrhotite can trigger the gem’s growth.

They published the work in Nature Communications.

The upper part of Earth’s mantle – the 3,000-kilometre-thick layer between the crust and core – is a hodge-podge of hot silicate rock from the birth of the planet and subducted crust from the surface.

It’s also where diamonds form, around 300 kilometres into the mantle where carbon-bearing fluids swirl in hot, high-pressure environments to crystallise into a super-hard substance. But as an oyster needs a piece of shell or “seed” to build a pearl, it’s thought so too do diamonds.

One type of seed was thought to be in the form of sulfur minerals. Sulfur is rare in mantle material but relatively common in diamond “melt inclusions” – tiny pockets of material that become surrounded and trapped by a growing gem. These can be “read” like the history of the gemstone.

But there had been no direct evidence that sulfides could trigger diamond formation.

So Jacob and her colleagues examined the microstructure and composition of a polycrystalline diamond aggregate crystal, a type of diamond that forms rapidly and encases a range of solid products in the process, from the Orapa diamond mine in Botswana.

Using a technique called Transmission Kikuchi Diffraction and scanning electron microscopy let them examine two inclusions within the crystal to a resolution of five nanometres.

They found the inclusions were filled with the sulfide mineral pyrrhotite and rimmed by magnetite.

The only way this inclusion pattern could form, they write, is if pyrrhotite oxidised – that is, gained oxygen atoms – to become magnetite, at depths of around 320-330 kilometres. This process triggered diamond precipitation by “freezing” carbon atoms into a super strong, 3-D structure – which then grew around the magnetite, keeping the pyrrhotite locked inside.

Given the relative lack of sulfides in Earth’s mantle, it’s likely that plate tectonics, forcing sulfur-rich crust material into the mantle below, produced “diamond factories”, they write.

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