Calcium Supersaturation

[Bob180]

One thing that really puzzles me is how our system can be so far from equilibrium. I understand the argument that Mg can kinetically hinder the growth of CaCO3 precipitate size. But, what I would of expected then is that one would then end up with very many micro-precipitates. One thought that I had was that in fact our systems are close to equilibrium, but our test kits are counting suspended CaCO3 micro-precipitates as free Ca++.
Along the same lines I wonder how products like Argamight can possibly work. Adding fine particulates to an already supersaturated system should act as nucleation sites and lower the calcium concentrations rather than raise it??

Bob

 

[Chris Jury1]

Good question. Ion-pairing between Mg and carbonate and bicarbonate as well as poisioning of the growing crystal surface seems to explain most of the maintenance of the supersaturation. Organic ligands of calcium also likely play a significant part in maintaing supersturation in the shallow ocean. It does seem couniterintuitive, but nonetheless, it works. Take a beaker of sea water that is supersaturated with respect to aragonite by 300 - 400% (or 500 - 600% with respect to calcite) and you can wait from now until forever for much of that to precipitate out. In sea water it just doesn't precipitate appreciably, despite the significant supersaturation.

Aragamight and like products aren't very soluble in freshwater or sea water (they're mostly aragonite, so the same as aragonite) hence they really don't "work" per se.

Chris