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Meet the mineral known as the time lord

SARAH MCCAMMON, HOST:

The Earth is really old - about 4 1/2 billion years old. And when scientists want to learn about its earliest history, they turn to a mineral that serves as an almost perfect geologic clock. As part of our series Finding Time, NPR's Nell Greenfieldboyce looks into a mineral that's been called the time lord.

NELL GREENFIELDBOYCE, BYLINE: The mineral is zircon. Not to be confused with cubic zirconia, the synthetic diamond look-alike, zircon forms naturally. It can be reddish or golden. There's some big, polished ones in the Smithsonian's National Museum of Natural History. They're in a case just past the Hope Diamond.

MICHAEL ACKERSON: So these are pretty zircons. These are zircons that are clear enough, have enough color that they were actually faceted as gemstones.

GREENFIELDBOYCE: Michael Ackerson is a geologist who studies zircons. The ones back in his office are teeny tiny. To the naked eye, they look like crud or sand. He sticks them under the microscope and lets me have a peek.

So here's this microscope. Let me see if I can do this. Geez, I hate looking through microscopes. OK. Oh, my goodness. How pretty is that?

ACKERSON: Yeah.

GREENFIELDBOYCE: They look like little diamonds or something. You said they weren't going to be pretty.

ACKERSON: (Laughter).

GREENFIELDBOYCE: He tells me that one of the crystals I'm looking at is 4.32 billion years old. Zircons are the oldest known pieces of earth that still exist on the surface today. The oldest go back as far as 4.37 billion years.

ACKERSON: They are really the best markers of Earth's time or the history of the Earth.

GREENFIELDBOYCE: Ackerson says a zircon crystal originally forms in magma - hot molten rock. Other minerals do too. Together, they'll make up, say, a granite mountain that slowly weathers away.

ACKERSON: Most of the minerals don't survive. So things like quartz, things like feldspar, they're chemically or physically weathered and eroded to a point where they're no longer quartz and feldspar. Zircon - and one of the main reasons that zircon is so useful - is that zircon is very resilient.

GREENFIELDBOYCE: The hardy crystals persist and eventually end up incorporated into another rock as it's forming. That means scientists can crush up the Earth's oldest rocks and pick through the debris to find little grains of zircon that are even older. To know a zircon's exact age, they zap it with a laser.

JOSHUA GARBER: So welcome to our lab. This corner is all of the instruments that we use to date zircon.

GREENFIELDBOYCE: Joshua Garber at Penn State University showed me how he sticks a little zircon into this machine that blasts off minuscule pieces.

GARBER: So then I, you know, torture them in an argon plasma to break them down to their smallest constituents. And then I very precisely measure the number of atoms that are hitting a detector at the end.

GREENFIELDBOYCE: The important atoms are uranium and lead. A growing crystal of zircon loves uranium and will take it in. But zircon hates lead. That means if you find lead inside, it pretty surely came from the decay of uranium, which happens at a steady rate like the ticking of a clock.

Jesse Reimink is a geologist at Penn State. He says it's almost as if zircons were designed to be a timekeeper.

JESSE REIMINK: If you believed in a higher power, you'd say, oh, the higher power created this mineral with this specific system because it is so perfect for Earth.

GREENFIELDBOYCE: But looking at the chemical makeup of a zircon can do more than just reveal its age. It can also give scientists clues about the conditions that existed when that zircon got created. Ackerson says people used to think the early Earth was a hot, glowing hellscape for its first 500 million years. But the oldest zircons found on Earth show that's not so.

ACKERSON: We know from just this one collection of zircon crystals that the Earth had continents, which we didn't think was possible, that were interacting with liquid water oceans.

GREENFIELDBOYCE: He says tiny zircons provide a unique window into deep time, letting us explore big philosophical questions about how our planet's early years set the stage for life.

Nell Greenfieldboyce, NPR News. Transcript provided by NPR, Copyright NPR.

Nell Greenfieldboyce is a NPR science correspondent.

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