Chuck Bednar for redOrbit.com – @BednarChuck
For nearly a century, scientists have wondered why seashells’ minerals sometimes take two chemically identical but slightly different forms in seawater, but new research published this week in the Proceedings of the National Academy of Science reveals the answer.
The material in question, calcium carbonate, sometimes takes the form of calcite and sometimes takes the form of aragonite, a virtually identical form of the mineral that is more soluble and thus more vulnerable to ocean acidification. Scientists previously identified variations in magnesium concentration in the water as a key factor, but were never able to fully explain why.
In the new study, however, experts from MIT and the Lawrence Berkeley National Laboratory (LBNL) have completed a detailed, atomic-level analysis of the process, and they claim that their findings could ultimately lead to on-demand synthesis of new materials in the laboratory.
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“The big-picture problem is about materials formation,” MIT graduate student Wenhao Sun, one of the authors of the PNAS study, explained Tuesday in a statement. “When solids crystallize in solution, you expect them to make the lowest-energy, [most] stable crystal structure.”
One mineral, two forms
As the study authors explain, many types of materials perform better when they are metastable (stable under ordinary conditions, but subject to transformation into an even more stable state over a period of time). They investigated metastability using calcium carbonate because of the availability of several decades worth of quality experimental data, making it a good case study for why some chemical reactions produce one of several different forms of a compound.
In the case of calcium carbonate, it can take the form of two different minerals. Calcite is the stable form, while aragonite is the metastable form, meaning that as time passes, or it becomes heated, it can ultimately transform into calcite. Sun compared it to diamond and graphite, the material found in pencil lead – both substance have the same basic pure-carbon composition. However, diamond is metastable, and over time it will eventually become graphite.
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Typically, when calcium carbonate crystallizes, it forms calcite. In seawater, however, it forms aragonite. This outcome, the study authors said, impacts many different natural processes, such as the global carbon cycle, the neutralization of atmospheric CO2 into a stable mineral, and the formation of shells and corals, whose aragonite shells are vulnerable to ocean acidification.
While scientists have long known that changes in magnesium concentrations in the surrounding waters impacts which form calcium carbonate takes, they could not explain why. The new study, however, shows that the ratio of calcium to magnesium in the water affects the surface energy of the nucleating crystals, and when the ratio passes specific thresholds, it determines whether calcite or aragonite is formed.
Surface energy: solving mysteries since 2015
“The surface energy is the barrier to nucleation. We were able to calculate the effect of magnesium on the surface energy,” explained Sun. While the surface energy is difficult to measure through experiments, the MIT/Berkeley Lab team was able to figure it out through atomic-level calculations.
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As it turns out, this process served as the mechanism through which magnesium halts the formation of calcium carbonate’s stable phase. If a solution contains no magnesium, stable calcite forms quickly, but as you increase the magnesium concentration, the surface energy of the calcite increases, causing its nucleation rate to plummet drastically.
Eventually, the calcite nucleation process gives way to the metastable aragonite phase. The results calculated by the researchers closely matched the proportions of the two forms seen in experiments when the magnesium ratios were varied, Sun explained.
The research could be adapted to serve as a tool to predict how other compounds will form from a solution. Ultimately, the scientists’ goal is to be able to predict and even control which types of materials form under various chemical solutions, thus making it possible to control the formation of new materials whose different traits can be useful for various technological applications.
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