Cullinan diamond's amazing journey from the centre of the Earth
An astonishing treasure inside a tiny sliver of diamond has ascended from the mine east of Pretoria
More than a century after it produced the largest diamond yet found, the Cullinan mine east of Pretoria has produced a tiny treasure of untold value.
While the 3,106.75-carat Cullinan diamond was dug up just 5.5m below the surface in 1905, the mine’s latest gem was retrieved less than 1,000m down — but formed an astonishing 700km below the Earth’s surface.
Compared with the 621g Cullinan it is microscopic — just 3.1mm across — but even more remarkably it contains a mineral that has never before been seen, because above a depth of about 650km it becomes unstable..
Calcium silicate perovskite is thought to be the fourth-most-abundant mineral on Earth, but remained hypothetical until it ascended inside the tiny sliver of diamond from the Cullinan mine.Announcing the find on Thursday, geochemist Graham Pearson from the University of Alberta said diamonds from the mine are among the most scientifically valuable in the world as well as the most commercially valuable.
“Nobody has ever managed to keep this mineral stable at Earth’s surface,” said Pearson, from the university’s earth and atmospheric sciences department.
“The only possible way of preserving this mineral at Earth’s surface is when it’s trapped in an unyielding container like a diamond.”
The find, revealed in the journal Nature, provides an important clue to the puzzle of how Earth’s inner structure behaves. Scientists examining the calcium silicate perovskite — chemical formula CaSiO3 — say they expect it to yield a host of discoveries.
Most diamonds are born between 150km and 200km deep, but at 700km the Cullinan sliver was formed under pressure equivalent to around 240,000 atmospheric pressures at sea level.It was this intense force that created a stable home for the CaSiO3, preventing it from deforming as the diamond moved towards the surface over aeons.
While rare to human eyes, calcium silicate perovskite is common deep inside the Earth — the number of tons there is estimated at 10 to the power of 21, also known as zetta tons. It is thought to be especially prevalent in slabs of oceanic crust that have plunged into the planet’s mantle at tectonic boundaries.The chunk of CaSiO3 within the gemstone was visible with the naked eye after the diamond was polished, the researchers wrote, but proper analysis and imaging required an international effort involving X-rays and spectroscopy tests.
“Diamonds are really unique ways of seeing what’s in the Earth,” said Pearson. “And the specific composition of the perovskite inclusion in this particular diamond very clearly indicates the recycling of oceanic crust into Earth’s lower mantle. It provides fundamental proof of what happens to oceanic plates as they descend into the depths of the Earth.”He said the discovery highlighted the uniqueness of diamonds being able to preserve things that otherwise we would not be able to see.
“And it’s a nice illustration of how science works. That you build on theoretical predictions, in this case from seismology, and that once in a while you’re able to make a clinching observation that really proves that the theory works,” said Pearson.
One of the best-known diamond researchers in the world, Pearson was also behind the 2014 discovery of ringwoodite — Earth’s fifth-most-abundant mineral — in a diamond that pointed to a vast reservoir of water bound to silicate rocks in the Earth’s mantle.
He came across a seemingly worthless 3mm piece of brown diamond from Mato Grosso, Brazil, while he was researching another type of mineral. Within it, he and his team found ringwoodite — and discovered that roughly 1.5% of its weight was made up of trapped water.