Gypsum, a common and economically important mineral that occurs on Earth’s surface, has long been thought by scientists to grow simply from the right ions meeting up in a liquid solution. But experiments by a team of European researchers show that gypsum, which is the main ingredient in plaster wallboard used in construction throughout the world, forms by means of a much more complex process than that.
Gypsum, it seems, gets made not in one step but in a series of four distinct stages, starting with the self-assembly of submicroscopic cylindrical particles the experimenters call “nanobricks.” By manipulating this four-stage process, the experimenters say, manufacturers may one day make the annual production of billions of kilograms of plaster of paris far more energy efficient than current methods permit.
The new work may have otherworldly implications as well, the researchers said: It might yield new insights into the evolution of the surface of Mars.
“I think the most important highlight…is that gypsum grows from nanobricks rather than from simple ions dissolved in solution,” said Tomasz Stawski, a geochemist with the University of Leeds in the United Kingdom as well as the German Research Centre for Geosciences in Germany and lead author of the recent Nature Communications study. “It seems that [the nanobricks] form immediately in solution, and then the entire process is driven by the interaction of those nanobricks…with each other,” he said.
“Stem Cell” Building Blocks
Stawski and his colleagues achieved their novel insights into gypsum formation by using X-ray beams produced by the Diamond Light Source synchrotron in Harwell, U.K., to observe the process as it unfolded. Because gypsum is a type of calcium sulfate, the researchers first mixed in a glass reactor an aqueous solution rich in calcium chloride and another rich in sodium sulfate to bring together the ingredients necessary to make the basic compound. Then circulating the mixed solutions through a much smaller chamber for observation, the team fired X-ray beams “head-on through the sample,” Stawski said, and studied the ways those beams scattered off of building blocks of gypsum as they formed in the liquid.
“Think about the sunset,” Stawski said. “When you start seeing red colors in the sky, it is due to the scattering of sunlight at very small angles when it hits dust particles in the air.” Similarly, he explained, when X-rays strike newly formed structures in the solution, the beams deflect in specific directions based on the unique arrangement of the atoms composing those structures. The scattering patterns, interpreted with the help of a mathematical model, allowed the scientists “to extrapolate how large those [precursors of gypsum] are, or what their shape is,” Stawski said. “Everything has a structure, and scattering methods are fantastic [for] accessing this hidden information.”
The four stages of the pathway unfolded as follows, according to Stawski. First, nanobricks about 3 nanometers in length and 1 nanometer in diameter formed. Then the bricks clustered into loose groups, or “domains”—a very important development, Stawski said, “because the chances that the bricks will meet each other increases,” leading to mineral precipitation.
In the third stage, “there is a triggering moment during which all the nanobricks come together, and this is what we see as precipitation, but at this stage there is still no gypsum.” Finally comes the fourth stage: Within the structures that formed in stage 3, “the nanobricks begin to reassemble, maybe to grow in size—we’re not exactly sure—and only in this stage does gypsum form,” Stawski noted.
“What this group has done is figure out how to use light sources in order to probe all of these early events and get information that is very difficult to get any other way,” said Jim De Yoreo, a materials scientist at Pacific Northwest National Laboratory in Richland, Wash., and the University of Washington in Seattle, who was not involved in the new work. “They have done what I think is an incredible job in using a modern tool to track down and scoop out this pathway.”
De Yoreo compared the nanobricks loosely to stem cells that can ultimately turn into a variety of different tissues in the body. Like stem cells, the nanobricks will not necessarily go on to form only gypsum: They can also lead to other calcium sulfate minerals like bassanite and also anhydrite, depending on the conditions under which mineral formation takes place. Which mineral you get depends on how much water permeates the mineral structure. Whereas a minuscule single crystal of gypsum harbors two water molecules, two bassanite crystals share just one water molecule. Anhydrite crystals contain no water molecules.
Depending on the “physiochemical conditions—such as very high salinity, higher temperatures, or rapid quenching—the reaction reported in our paper can lead to bassanite or anhydrite instead of gypsum,” agreed Stawski. Prior to these new experiments, it was thought that dehydrating gypsum, or causing it to lose its water molecules, was the only way of forming a phase like bassanite.
Industrial Applications and Water on Mars
Knowing what’s really going on in gypsum formation could eventually help curb the voracious energy appetite of the construction industry, the research team reports in its 1 April paper. Each year humanity produces about 100 billion kilograms of plaster of paris by heating gypsum to about 150°C to remove the water molecules, the experimenters note. “It is superinefficient—it’s a very energy intensive process, and you need to remove the water to get bassanite,” which, Stawski said, is dehydrated gypsum—plaster of paris’s primary ingredient.
By manipulating the four-stage formation pathway, it might be possible to produce plaster of paris without using so much energy, he explained: “If you take gypsum and put it into water and dissolve it, [you can] get the nanobricks again,” he said. Then, instead of doing all that heating, “we need simply force the nanobricks to rearrange themselves into bassanite rather than gypsum, by controlling the chemistry.”
An extraterrestrial consequence of the new findings might even await. That’s because understanding calcium sulfate minerals’ formation pathway might help us better understand the evolution of the surface of a sister planet—Mars—the scientists propose. Whereas bassanite can occasionally be found on Earth, “it’s not very common because the moment it gets any moisture it converts into gypsum,” said Stawski. “But it’s very abundant on Mars, [and] the question is: How does it form there?” he asks.
“If it formed by the dehydration of gypsum, that will tell us something about the [early] conditions on Mars,” where, if gypsum had once been present, then there may also have been more water there than exists today. Alternatively, if rearranging nanobricks can also form bassanite, then Martian bassanite may not have formed simply by drying out, added Stawski.
For all the light that Stawski and his colleagues have now shed on gypsum’s formation, a lot about the four-stage pathway still remains to be discovered, said De Yoreo, who calls the four-stage pathway “a solid concept seen through very foggy glasses.”
“It’s an advance in that we can say, ‘yes, there are these objects, and we can track the time scale over which this happens,’” he explained. However, exactly how does each stage transform into the next? he asked. That’s still a mystery.
—Lucas Joel, Freelance Writer; e-mail: email@example.com
Citation: Joel, L. (2016), Gypsum forms in an unexpected way, Eos, 97, doi:10.1029/2016EO050975. Published on 21 April 2016.