Earth grows large crystals, rare elements in just minutes

Oct. 7 (UPI) — Scientists have gained new insights into crystal growth rates inside pegmatites, veinlike formations that host some of the planet’s biggest crystals, as well as valuable elements such as tantalum, niobium and lithium.

Magma cooling time typically controls the size of crystals — when magma cools quickly, crystals remain microscopic, and when it cools slowly, crystals have time to grow.

But pegmatite crystals appear to upend this logic, researchers said in a study published this week in the journal Nature Communications.

“Pegmatites cool relatively quickly, sometimes in just a few years, and yet they feature some of the largest crystals on Earth,” Cin-Ty Lee, professor of geology at Rice University, said in a news release. “The big question is really, ‘How can that be?'”

To determine the growth rates of pegmatite crystals, scientists turned to the rare elements that are often found inside pegmatites.

“It was more a question of, ‘Can we figure out how fast they actually grow?'” said Rice graduate student Patrick Phelps. “Can we use trace elements — elements that don’t belong in quartz crystals — to figure out the growth rate?”

After surveying dozens of scientific papers on the chemistry of crystal formation and closely examining a variety of crystals from a pegmatite mine in Southern California, researchers developed a formula for translating chemical profiles into crystal growth rates.

“We examined crystals that were half an inch wide and over an inch long,” Phelps said. “We showed those grew in a matter of hours, and there is nothing to suggest the physics would be different in larger crystals that measure a meter or more in length. Based on what we found, larger crystals like that could grow in a matter of days.”

Pegmatites are formed when pieces of Earth’s crust are pulled down and recycled inside Earth’s mantle. Water trapped in the crust is dissolved into the melt. As the pegmatite rises and cools, a diversity of minerals are formed. With the precipitation of each new mineral, the remaining melt becomes increasingly water-logged.

“Eventually, you get so much water left over that it becomes more of a water-dominated fluid than a melt-dominated fluid,” Phelps said. “The leftover elements in this watery mixture can now move around a lot faster. Chemical diffusion rates are much faster in fluids and the fluids tend to flow more quickly. So when a crystal starts forming, elements can get to it faster, which means it can grow faster.”

While collecting pegmatite crystals, Phelps noticed many of the quartz crystals had formed in cracks that had opened up in the still-forming pegmatite.

“You see these pop up and go through the layers of pegmatite itself, almost like veins within veins,” Phelps said. “When those cracks opened, that lowered the pressure quickly. So the fluid rushed in, because everything’s expanding, and the pressure dropped dramatically. All of a sudden, all the elements in the melt are now confused. They don’t want to be in that physical state anymore, and they rapidly start coming together in crystals.”

Researchers used a combination of advanced imaging techniques — cathodoluminescence microscopy and laser ablation with mass spectrometry — to measure the concentration of trace elements in the quartz crystals.

Using discoveries made by material scientists several decades ago, Phelps and his research partners were able to determine the growth rates.

“There are three variables,” Phelps said. “There’s the likelihood of things getting brought in. That’s the partition coefficient. There’s how fast the crystal is growing, the growth rate. And then there’s the diffusivity, so how quickly elemental nutrients are brought to the crystal.”

Their analysis showed the quartz pegmatite crystals had grown at rates several orders of magnitude faster than anyone had previously predicted.

The research team had a hard time accepting the numbers, though. Their formulas showed the crystals were growing in a matter of hours. But after double and triple checking their work, Phelps and his colleagues accepted their discovery.

“We’d done the math and the physics. That part was sound. While we didn’t expect it to be that fast, we couldn’t come up with a reason why it wasn’t plausible.”

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