Beyond diamonds: Search is on for rare carbon crystals

Daniel Hummer
Daniel Hummer

Long ago, in Russia’s distant Northwest, a bird pooped. But this was not just any old splat of excrement. The feces landed on a very hot rock. It lay atop an underground coal fire. The intense heat quick-baked the poop. And that caused one of the substances previously dissolved in the feces to crystallize. With that, a new mineral was born.

Minerals are substances with a distinct chemical recipe. And the atoms of each mineral will always arrange themselves exactly the same way.

You’d expect to find crystals in rocks. You’d expect to find them in caves and mines. You would not expect to find one in baked bird poop.

The mineral tinnunculite crystallized within globs of excrement from the European kestrel (Falco tinnunculus) that had been heated by an underground coal fire.

Then again, few geologists would think to dig through bird feces. But those who dug through the Russian poop found something special. Unique even. It was a never-before-described type of crystal. The scientists named this new mineral tinnunculite (Tih-NUN-kew-lyte). That name comes, in part, from the scientific name of the European kestrel — Falco tinnunculus. It was this bird’s wastes that had rained down upon the hot Russian rock.

Tinnunculite is unusual in another way, too. It contains the element carbon.

What makes carbon minerals particularly interesting? For one thing, they’re somewhat rare. Only 8 percent of minerals contain this element; that’s 416 of all minerals known. Perhaps more importantly, carbon is a key element for life on Earth. In fact, many of the yet-to-be-discovered carbon-bearing minerals — just like tinnunculite — might be made either by living things or from compounds that they created.

There might be hundreds of undiscovered minerals on Earth, recent studies have suggested. Dozens — like diamonds — might contain carbon, geologists suspect. Now, the hunt is on for hidden relatives of the diamond. Some may be distant geological cousins. Others may seem even less related, like a nephew by marriage. Many scientists think carbon should be more heavily represented among the mineral-family tree. So they’ve begun a Carbon Mineral Challenge.

They launched it about 18 months ago. To date, scientists have already announced the discovery of eight new members of the carbon-mineral family. But there’s still plenty of time for ambitious rock hounds to make their mark on mineralogy before the search officially ends in late 2019.

Finding a new mineral means the discoverer gets to name their find. And they’ll get more than the chance to immortalize themselves in the pages of geology books. After all, they will be helping tell the story of our planet — both past and present. First, though, they’ll need to know where to search.

Fortunately, scientists have some good ideas on where to look.

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minerals graph
Of all known carbon-bearing minerals (maroon bars), most have been found in only a handful of places. Anticipated finds (tan bars) are based on statistical analyses. They offer hope of yet-to-be-found minerals.

A census of minerals

Not every mineral on Earth has been discovered. Earth is, after all, a vast place, full of unexplored reaches. How many minerals might there be? Rare or everywhere, each has its own mix of elements. More than 5,000 minerals have been identified. It is possible that only a few of them have escaped notice. Or it could be most of them. It is probably impossible to know which is true. But to understand the complexity of the issue, it may help to know that the same rocks do not typically occur everywhere.

Yes, a few minerals are (forgive the phrase) as common as dirt. Perhaps surprisingly, however, most minerals are very rare. Fewer than one in every 500 have been found at more than 1,000 sites across the planet. More than half of the world’s minerals have been found at only five or fewer locations. And at least 1,000 minerals have been reported from only a single spot on Earth, notes Robert Hazen. He’s a geophysicist at the Carnegie Institution for Science in Washington, D.C.

Robert Hazen
Robert Hazen keeps a lookout for minerals near the Chesapeake Bay.

So if one were on the hunt for crystals in the diamond’s family, where would one look? Hazen and his colleagues have a general idea. The group used statistics to analyze a large database of known minerals. The researchers noted how many sites host the common mineral. They compared these types of sites to the ones where rare minerals have…