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Atomic-level calculations explain metastable aragonite formation

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Two forms of CaCO3 have identical chemical composition, but different properties and appearances. The flat, clear crystal is calcite; the dark, multifaceted one is aragonite.

Scientific Achievement
Explained how magnesium (Mg) concentrations in seawater determine when calcium carbonate (CaCO3) precipitates as aragonite instead of calcite.

Significance and Impact
The new explanation of CaCO3 precipitation is a step toward enabling the prediction of synthesis conditions for desired technological materials in a laboratory setting.

Research Details

  • When solids crystallize in solution, one expects them to form the lowest-energy, stable structure
  • While CaCO3 forms calcite in pure water, it precipitates as metastable phase aragonite in seawater
  • New atomic-level calculations showed the effect of aqueous Mg on surface energy, which is the barrier to crystal nucleation
  • Analysis confirmed that at a certain Mg:Ca ratio in seawater, the nucleation barrier of calcite exceeds that of aragonite, tipping the balance from forming calcite to forming aragonite
  • The study provides a tool to predict how solution chemistries govern which compounds may form

W Sun, S Jayaraman, W Chen, K Persson, and G Ceder, PNAS 2015, doi:10.1073/pnas.1423898112