Brazilian emeralds in a quartz-pegmatite matrix. Photo credit: Madereugeneandrew / Wikimedia Commons)
Rome wasn’t built in a day, but some of the most beautiful gemstones on earth were, according to new research from Rice University.
Aquamarine, emerald, garnet, zircon, and topaz are just some of the crystalline minerals found primarily in pegmatites, vein-like formations that usually contain both large crystals and hard-to-find elements such as tantalum and niobium. Another common find is lithium, an important component of electric car batteries.
“This is a step in understanding how the earth concentrates lithium in specific locations and minerals,” said Patrick Phelps, co-author of a study published online Communication with nature. “If we can understand the fundamentals of pegmatite growth rates, it is a step toward understanding the bigger picture of how and where they arise.”
Pegmatites are formed when rising magma cools within the earth, and they have some of the largest crystals on earth. For example, the Etta Mine in South Dakota has logarithmic crystals of lithium-rich spodumene, including one 42 feet long with an estimated weight of 37 tons. The research of Phelps, Rices Cin-Ty Lee, and Southern California geologist Douglas Morton attempts to answer a question that has long preoccupied mineralogists: How can such large crystals be in pegmatites?
Black tourmaline turns into pink tourmaline in a quartz pegmatite at Stewart’s Lithia mine, California. Photo credit: Patrick Phelps / Rice University
“In igneous minerals, crystal size has traditionally been related to cooling time,” said Lee, Rice’s Harry Carothers Wiess Professor of Geology and chairman of the Earth, Environmental and Planetary Sciences division at Rice. “The idea is that large crystals take time to grow.”
Magma that cools down quickly, such as rocks in broken lavas, contains microscopic crystals, for example. But that same magma, if cooled over tens of thousands of years, could contain centimeter-sized crystals, Lee said.
“Pegmatites cool relatively quickly, sometimes in just a few years, and yet they have some of the largest crystals on earth,” he said. “The big question really is, ‘How can that be?'”
When Phelps began research, his most immediate questions were how to formulate a series of measurements that would enable him, Lee, and Morton to answer the big question.
“It was more of a ‘Can we figure out how fast they’re actually growing?’” Phelps said. “Can we use trace elements – elements that don’t belong in quartz crystals – to find out the rate of growth?”
It took me more than three years, an excursion to collect sample crystals from a pegmatite mine in Southern California, hundreds of laboratory measurements to accurately map the chemical composition of the samples, and an in-depth look at some 50-year-old materials science work to create a mathematical model, which can convert the chemical profiles into crystal growth rates.
Rice University graduate student Patrick Phelps used cathodoluminescence microscopy to measure the chemical composition of sample crystals. Photo credit: Linda Welzenbach / Rice University
“We looked at crystals that were half an inch wide and over an inch long,” Phelps said. “We showed that these grew within a few hours, and there was nothing to suggest that the physics would be any different for larger crystals a meter or more in length. Based on what we found, such larger crystals could grow in a matter of days. ”
Pegmatites form where pieces of the earth’s crust are drawn off and recycled in the molten earth mantle. Any water trapped in the crust becomes part of the melt, and as the melt rises and cools, many types of minerals are created. Each forms and precipitates from the melt at a characteristic temperature and pressure. However, the water is retained and makes up an increasingly higher percentage of the cooling melt.
“After all, there is so much water left that it becomes a water-dominated liquid rather than a melt-dominated liquid,” Phelps said. “The leftover elements in this aqueous mixture can now move much faster. Chemical diffusion rates are much faster in liquids and the liquids tend to flow faster. So when a crystal forms, elements can get there faster, which means it can grow faster. ”
Crystals are an ordered arrangement of atoms. They are formed when atoms naturally fall into this arranged pattern due to their chemical properties and energy levels. For example, in the mine where Phelps collected his quartz samples, many crystals had formed in what appeared to be cracks that had opened while the pegmatite was still forming.
“You see these come up and go through the pegmatite layers themselves, almost like veins in veins,” Phelps said. “When those cracks opened, it quickly lowered the pressure. So the liquid poured in because everything expands and the pressure drops dramatically. Suddenly all elements in the melt are confused. They no longer want to be in this physical state and quickly come together in crystals. ”
Cin-Ty Lee. Photo credit: Jeff Fitlow / Rice University
To decipher how quickly the sample crystals grew, Phelps used both cathodoluminescence microscopy and laser ablation with mass spectrometry to measure the exact amount of trace elements incorporated into the crystal matrix at dozen of points during the growth. Through experimental work by materials scientists in the mid-20th century, Phelps was able to decipher the growth rates from these profiles.
“There are three variables,” he said. “There is a likelihood that things will be brought in. That is the partition coefficient. There is how fast the crystal grows, the rate of growth. And then there is the diffusivity, i.e. how quickly elementary nutrients are brought into the crystal. ”
Phelps said the rapid growth rates were quite a surprise.
“Pegmatites are pretty short-lived, so we knew they had to grow relatively quickly,” he said. “But we showed that it was a few orders of magnitude faster than anyone had predicted.
“When I finally got one of those numbers, I remember going into Cin-Ty’s office and saying, ‘Can I do that? I don’t think that’s right. “Phelps recalled. “Because I was still thinking about a millennium timescale in my head. And those numbers meant days or hours.
And Cin-Ty said, ‘Well why not? Why can’t it be right? ‘”Said Phelps. “Because we did math and physics. That part was healthy. Although we didn’t expect it to happen that fast, we couldn’t find any reason why it wasn’t plausible. ”
Reference: “Episodes of Rapid Crystal Growth in Pegmatites” by Patrick R. Phelps, Cin-Ty A. Lee and Douglas M. Morton, October 5, 2020, Communication with nature.
DOI: 10.1038 / s41467-020-18806-w
The research was supported by the National Science Foundation.
Morton, Lee’s lifelong friend and mentor, died on September 16, 2020. He was a professor emeritus of geology at the University of California at Riverside.
