Research Diamonds Are a Geologist’s BF
Scientists solve a secret of the Earth’s crust with the help of a gem
william blake saw the world
in a grain of sand. Graham Pearson, who teaches Earth and atmospheric sciences at the University of Alberta, sees proof of a long-suspected geological theory in a mineral. But not just any mineral: Pearson has wrestled a revelation from a crystal found near the surface of the Earth.
The source of Pearson’s fascination is calcium silicate perovskite, which scientists estimate is the fourth most abundant mineral on the planet. That bountifulness means this compound is crucial to the Earth’s lower mantle, making it a source of great interest to geologists. But despite its prevalence, researchers like Pearson had never been able to study the compound. In fact, no one had seen it before.
That’s because at the surface of the Earth, calcium silicate perovskite is entirely unstable. The only place this essential mineral normally exists is buried 400 miles below ground, where pressure squeezes the components together. There was just one way to keep it stable enough for scientists to study, and that would require preservation in an “unyielding container,” says Pearson, “like a diamond.”
That’s exactly where he and a team of international researchers found it: in a diamond from South Africa’s Cullinan Mine, famous for producing two of the largest diamonds in the British crown jewels.
Finding the unusual mineral was, says Nester Korolev, postdoctoral fellow at the University of British Columbia, “a complete surprise.” It also, the authors noted in a paper published in a March issue of Nature, verified a theory regarding how the Earth’s crust moves.
Diamonds, Pearson said in a statement about the discovery, “are really unique ways of seeing what’s in the Earth.” A close look at this particular gem and its calcium silicate perovskite inclusion revealed its unusual past. The gem, the researchers deduced, formed when a piece of oceanic crust sank from the surface of the planet to about 500 miles below, into the lower mantle, where it was subjected to more than 24 billion pascals of pressure—roughly equivalent to the pressure created by a stack of paper 240 billion sheets tall, enough to crush steel. Gradually, the diamond ascended with a bit of lower mantle encased inside, eventually ending up half a mile below the Earth’s surface.
Such recycling—a piece of the Earth’s crust descending into the mantle and then rising back up again—has been suspected but never confirmed. But the specific composition of the perovskite found in the diamond, says Pearson, “provides fundamental proof of what happens to the fate of oceanic plates as they descend into the depths of the Earth.” The same goes for the existence of calcium silicate perovskite, which researchers believe makes up more than 90 percent of the lower mantle but has never been seen. Now, they are sure it exists.
Pearson calls the discovery “a nice illustration of how science works.” You could also call it a gem of a find.