Iron sulfide is one of the most common minerals found in diamonds, but it is rare inside the Earth’s mantle where diamonds form. New findings published Tuesday in Nature Communications explain this paradox—for the first time sulfide has been shown to play an active role in the formation of diamond crystals. The results provide valuable new information about the depth of diamond formation and suggest that sulfide is more than a flaw introduced from the host rock, the study authors said.
Diamonds glitter in engagement rings and other jewelry, but they come from a less glamorous beginning. The gems form when carbon-rich fluid seeps into rock in the Earth’s mantle. After diamonds coalesce in the deep, high-pressure environment, they move toward the surface, carried along in pipes of volcanic rocks called kimberlites. Sometimes tiny mineral crystals are encapsulated in the diamond, giving researchers a glimpse of the composition of the mantle.
Researchers have previously debated whether such inclusions play a role in the formation of diamond crystals. Sometimes inclusions are randomly oriented, but in other diamonds they align with the crystal structure in a way that suggests a chemical interaction may have occurred between the inclusion and the diamond. Scientists have long suspected that iron sulfides donate electrons to carbon atoms during diamond formation; however, direct evidence of this process had not been found.
To better understand the chemical structure and composition of iron sulfide inclusions, Dorrit Jacob of Macquarie University in Sydney, Australia, and her colleagues used a nanoscale technique called transmission Kikuchi diffraction. The technique works much like scanning electron microscopy, but it achieves nanoscale resolution. Geochemists have used it only rarely, but it is frequently applied in the materials sciences. Jacob and her team studied ultrathin slices of diamonds from the Orapa mine in Botswana, where many samples contain iron sulfide inclusions.
Observations revealed sites within the diamond samples that contained nanoscale layers of magnetite—a magnetic mineral made of iron and oxygen that can be produced by the reaction between iron sulfide and carbon-rich fluid. In the analyzed samples, magnetite adhered to both the diamond crystals and iron sulfide inclusion fragments in rims only tens of nanometers thick, suggesting the inclusion fragments played a direct role in diamond formation. The matching alignment of the sulfide, magnetite rim, and diamond suggested the magnetite started crystallizing on the sulfide while the diamond began crystalizing on the magnetite, Jacob told Eos. The transition from carbonate to diamond requires an electron-donating partner, and it appears this role is played by iron sulfide, which gives up electrons as it transforms to magnetite.
“This is how diamonds are thought to form, but to actually see the reactants and products in the texture is quite remarkable,” said Steve Shirey, a geochemist at the Carnegie Institution of Washington in Washington, D. C. Shirey was not involved in the research.
Shirey predicts that future studies will seek to explain whether this chemistry occurs in gem-quality diamonds. Although the polycrystalline industrial-quality diamonds used in the study might develop from many centers of growth, the monocrystalline diamonds used to make jewelry could also develop by a similar chemistry, even if the inclusions are not visible after the reaction completed, he added.
The magnetite observed in the current study provides another valuable insight into diamond formation, Jacob told Eos. Researchers have long known that diamonds form deep within the Earth, but they have struggled to pinpoint the exact depth. The new laboratory experiments determined that the high-pressure phase of magnetite exists only at or below 320–330 kilometers depth inside the Earth. “This is the first time that we have had a glimpse of the depth of formation of this type of a diamond,” said Jacob.
—Amy Coombs, Editorial Intern