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Supersaturates solutions can be unstable and decay beautifully once a seed crystal is introduced.

Our oceans near the surface are supersaturated with respect to calcium carbonate. This map shows the solubility product of calcium and carbonate ions, scaled such that a value of one indicates saturation with respect to aragonite. The thermodynamically stable form of calcium carbonate is actually calcite which means the supersaturation level is even higher. Falling levels of supersaturation are a climate change concern for the welfare of calcium-shelled organisms.

What prevents the seed crystals (of which there are plenty) from removing this super-saturation?

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    $\begingroup$ Your question is addressed in detail in a 1984 paper titled "Apparent calcite supersaturation at the ocean surface: Why the present solubility product of pure calcite in seawater does not predict the correct solubility of the salt in nature?" by Cooke, Robert C., and Paul E. Kepkay. Check Google Scholar for this title. $\endgroup$
    – AChem
    Oct 27, 2021 at 6:18
  • $\begingroup$ If there was oversaturation, sea creatures forming aragonite shells would not have recently observed troubles to form them. $\endgroup$
    – Poutnik
    Oct 27, 2021 at 6:49
  • $\begingroup$ Hint: water evaporates at the surface. What happens if crystals do form in deeper layers? $\endgroup$ Oct 28, 2021 at 1:19

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