Rome wasn’t built in a day, but some of Earth’s finest gemstones were, according to new research from Rice University.
Aquamarine, emerald, garnet, zircon and topaz are but a few of the crystalline minerals found mostly in pegmatites, veinlike formations that commonly contain both large crystals and hard-to-find elements like tantalum and niobium. Another common find is lithium, a vital component of electric car batteries.
“This is one step towards understanding how Earth concentrates lithium in certain places and minerals,” said Rice graduate student Patrick Phelps, co-author of a study published online in Nature Communications. “If we can understand the basics of pegmatite growth rates, it’s one step in the direction of understanding the whole picture of how and where they form.”
Pegmatites are formed when rising magma cools inside Earth, and they feature some of Earth’s largest crystals. South Dakota’s Etta mine, for example, features log-sized crystals of lithium-rich spodumene, including one 42 feet in length in weighing an estimated 37 tons. The research by Phelps, Rice’s Cin-Ty Lee and Southern California geologist Douglas Morton attempts to answer a question that has long vexed mineralogists: How can such large crystals be in pegmatites?
“In magmatic minerals, crystal size is traditionally linked to cooling time,” said Lee, Rice’s Harry Carothers Wiess Professor of Geology and chair of the Department of Earth, Environmental and Planetary Sciences at Rice. “The idea is that large crystals take time to grow.”
Magma that cools rapidly, like rock in erupted lavas, contains microscopic crystals, for example. But the same magma, if cooled over tens of thousands of years, might feature centimeter-sized crystals, Lee said.
“Pegmatites cool relatively quickly, sometimes in just a few years, and yet they feature some of the largest crystals on Earth,” he said. “The big question is really, ‘How can that be?’”
When Phelps began the research, his most immediate questions were about how to formulate a set of measurements that would allow him, Lee and Morton to answer the big question.
“It was more a question of, ‘Can we figure out how fast they actually grow?’” Phelps said. “Can we use trace elements — elements that don’t belong in quartz crystals — to figure out the growth rate?”
It took more than three years, a field trip to gather sample crystals from a pegmatite mine in Southern California, hundreds of lab measurements to precisely map the chemical composition of the samples and a deep dive into some 50-year-old materials science papers to create a mathematical model that could transform the 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 form where pieces of Earth’s crust are drawn down and recycled in the planet’s molten mantle. Any water that’s trapped in the crust becomes part of the melt, and as the melt rises and cools, it gives rise to many kinds of minerals. Each forms and precipitates out of the melt at a characteristic temperature and pressure. But the water remains, making up a progressively higher percentage of the cooling melt.
“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.”
Crystals are ordered arrangement of atoms. They form when atoms naturally fall into that arranged pattern based on 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 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.”
To decipher how quickly the sample crystals grew, Phelps used both cathodoluminescence microscopy and laser ablation with mass spectrometry to measure the precise amount of trace elements that had been incorporated into the crystal matrix at dozens of points during growth. From experimental work done 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’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.”
Phelps said the fast growth rates were quite a surprise.
“Pegmatites are pretty short-lived, so we knew they had to grow relatively fast,” he said. “But we were showing it was a few orders of magnitude faster than anyone had predicted.
“When I finally got one of these numbers, I remember going into Cin-Ty’s office, and saying, ‘Is this feasible? I don’t think this is right.’” Phelps recalled. “Because in my head, I was still kind of thinking about a thousand-year time scale. And these numbers were meaning days or hours.
“And Cin-Ty said, ‘Well, why not? Why can’t it be right?’” Phelps said. “Because 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.”
Reference: “Episodes of fast crystal growth in pegmatites” by Patrick R. Phelps, Cin-Ty A. Lee and Douglas M. Morton, 5 October 2020, Nature Communications.
The research was supported by the National Science Foundation.
Morton, Lee’s lifelong friend and mentor, died on September 16, 2020. He was an adjunct professor emeritus of geology at the University of California, Riverside.
“… there is nothing to suggest the physics would be different in larger crystals that measure a meter or more in length.” Well, for one thing there is a scaling problem. The electrostatic force that attracts ions to the growing crystal faces weakens with the square of the distance from the crystal. Therefore, the magnitude of the attractive force in a 25mm crystal scavenging ions from its immediate vicinity will be 10-thousands times stronger than for a 2500mm crystal that has depleted its immediate surroundings and has to attract ions at much greater distances. To overcome the weaker attractive force, it would be necessary for the pneumatolytic fluid to have very high rates of circulation to bring ions within a distance that they will be affected and incorporated into the crystal lattice. That suggests that to maintain a constant growth rate, the fluid would need to have very high rates of circulation, which mitigates against clean crystals without inclusions trapped by the rapid rate of deposition. These aren’t the first researchers to suggest very high rates of crystal growth in pegmatites. However, I remain unconvinced.
Interesting. But how much of a scaling problem is it to go from cm to meters? The described time scaling suggest 1-3 order of magnitude difference in formation times.
From the paper, a teaser: “Pegmatites appear to form on much shorter timescales than typical plutonic rocks, but paradoxically, they have much larger crystals, which alone requires that crystal growth rates must be much higher in pegmatitic systems compared to typical plutonic systems.”
The article claims a simple model: “”There are three variables,” he 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.””
From the abstract: “Kinetic crystal growth theory is used to show that crystals accelerated from an initial growth rate of 10^−6–10^−7 m s^−1 to 10^−5–10−4 m s^^−1 (10-100 mm day^−1 to 1–10 m day^−1), indicating meter sized crystals could have formed within days, if these rates are sustained throughout pegmatite formation. The rapid growth rates require that quartz crystals grew from thin (micron scale) chemical boundary layers at the fluid-crystal interfaces. A strong advective component is required to sustain such thin boundary layers. Turbulent conditions (high Reynolds number) in these miarolitic cavities are shown to exist during crystallization, suggesting that volatile exsolution, crystallization, and cavity generation occur together.”
Browsing the paper they use trace element analysis applied to their kinetic model to match and estimate growth rates from measured trace element profiles.
“For context, these crystal growth rates are compared to other geological rates in Fig. 9. Growth rates of pegmatitic quartz in this study are clearly fast compared to growth rates inferred for metamorphic garnets45,46 and quartz in granitic plutons47. Quartz phenocrysts in volcanic rocks are thought to have also grown fast48,49, but pegmatitic quartz growth rates are still at least as fast as the fastest inferred growth rates for volcanic quartz. Finally, crystal growth rates are compared with other geologic rates, such as plate motions50 and earthquake-related deformation, such as slow slip and aseismic creep51 (and references therein). This comparison allows us to contemplate whether crystal growth could play a significant role in other systems. We note that crystal growth timescales in pegmatitic systems can approach that of slow slip and aseismic creep, begging the question of whether crystal growth may be important during faulting or fault healing.”
Here their figure 9 comparison of growth rates is illustrative I think, since they find tentative overlap with volcanic quartz.
Skepticism is warranted. But I’m just here mostly because I’m interested in crust formation, so YMMV on my (essentially the paper) analysis.
I’ve walked through lava tubes and wondered about the lava velocity that created these huge tubes. I’ve seen eruptions that pushed up gas and flames igniting praires. My small brain wandered around these kinds of things thinking soil porosity and thus air and water infusion had a lot to do with so many perturbations of mineral, crystal and metal formation. Anyone ever look into porosity effects? Anyone that ever hiked down into extinct volcanoes, like Sunset Crater Volcano National Monument (where lunar astronauts practiced walking on the moon) or trecked around Roosevelt Dam in Grand Coulee – as I have – trudging through schist at times knee deep, left me thinking about how the porosity of these areas enabled lots of permutations through subsequent volcanic eruptions, glacier movement, big temperature swings and massive flooding. What a wicked issue to ponder.
Dr Douglas Morton kept calling me to get back down in the Stewart Lithia Mine and get more and more of the quartz crystals from the miaroles in the pegmatite, especially if they displayed odd morphology. Well, I think that you have definitely put those to good use and productive study! A wonderful paper! Not only have you made Doug and Cin-Ty proud, but me too. If you need a field trip or more study minerals or anything from the Mine, just ask, i will be happy to be there for you. Keep up the good work.
Having made an extensive study of lab grown mineral crystals, I have speculated that such crystals might grow in nature at a much faster rate than was previously assumed. If you look at a commercial quartz growing setup, you have high pressure, temperature, and saturation conditions resulting in very large crystals in a matter of months. Because the conditions favorable to crystal growth are likely quite fleeting, it is logical to assume that crystal growth must occur over a relatively short time span. When producing large crystals from commercial autoclaves, we are looking at maintaining the required combinations of heat and pressure over a period of months. It is hard to imagine such conditions occurring naturally lasting for more than a few months. Very high pressures within the pegmatite might be far far higher than what can be achieved in today’s crystal growing labs. It is NOT hard to imagine a crack developing spontaneously in the strata containing extremly high pressure supersaturated pegmatite fluids, If the pressure were to suddenly be released through fracturing of the surrounding rock, the resulting extreme supersaturation of hydrothermal fluids, now suddenly without the pressure necessary to maintain them at equallibrium, would be akin to the kind of rapid growth seen when super-chilled water spontaneously nucleates, resulting in very rapid crystal formation. The higher the temperature and pressure, the more critically supersaturated the hot hydrothermal fluids will be when/if that pressure is suddenly released.