Buried 300 kilometers beneath the surface of Brazil, a tiny diamond no bigger than a peppercorn has given scientists an astonishing glimpse into Earth's hidden water system. A team of Brazilian researchers has found the first evidence that goethite, a mineral that carries water molecules within it, can survive the crushing pressures and searing heat of the planet's deep interior.
The diamond, just 3 millimeters across, was pulled from the earth near the town of Juína in the state of Mato Grosso, where gold miners have been uncovering these rare stones since the late 1980s. Most diamonds form about 150 kilometers below ground, but these "super-deep" diamonds grow more than twice that distance down, carried upward by volcanic eruptions. What makes them precious to scientists is not their sparkle but what they contain inside: tiny fragments of other minerals trapped during formation, acting like time capsules from hundreds of millions of years ago.
Geologist Fernanda Gervasoni and her colleague Tiago Jalowitzki received a donation of these diamonds in 2018 from a miners' cooperative in Juína. Back at the Brazilian Synchrotron Light Laboratory in Campinas, the team examined one of the stones using the Sirius particle accelerator, an enormous machine that speeds electrons nearly as fast as light to produce incredibly bright radiation for analysis. At two specialized beamlines called Mogno and Carnaúba, researchers mapped about 100 mineral inclusions trapped inside the diamond and determined their chemical makeup.
One inclusion caught their attention: a tiny pocket of iron hydroxide, which is extremely rare at such depths because most deep rocks cannot support the chemical reactions needed to form it. Further testing revealed it was actually goethite, the same mineral that gives brown color to soil. Goethite forms where iron-rich minerals meet water, and it incorporates some of that water into its crystal structure. Scientists had suspected water could travel from Earth's surface down to depths of 660 kilometers, but this discovery suggests goethite can carry water even deeper, into the lower mantle nearly 3,000 kilometers below the surface.
"Ours is the first study conducted entirely by a Brazilian team using Brazilian instruments," Gervasoni said, noting that previous research on Juína's diamonds had always been led by foreign scientists. The findings, published in May in the journal Scientific Reports, mark a milestone for Brazilian science and hint that water may be cycling through Earth's layers in ways researchers never imagined.