Dear Mathieu,

If you "simply" want to precipitate a specific CaCO3 polymorph then this paper claims to contain a reliable method.

H. Nebel and Matthias Epple. Continuous Preparation of Calcite, Aragonite and Vaterite, and of Magnesium Substituted Amorphous Calcium Carbonate (Mg-ACC). DOI: 10.1002/zaac.200800134

If you look in the materials science or crystal design literature then you can find all sorts of recipes to produce specific forms of CaCO3.

That said, I've never actually used any of these methods so I can't speak from practical experience.

If you want to "quantitatively" precipitate DIC in a solution as pure calcite then I suspect that this is much more difficult/impossible. The original method for measuring the carbon isotope composition of DIC was based on quantitative precipitation by addition of a solution containing excess SrCl2 or BaCl2 adjusted to a suitably alkaline pH (>10). Of course the method didn't need to worry too much about the form of solid carbonate precipitated, just that all DIC had been removed from solution. In any respect this procedure would only produce either strontianite or witherite. I  suspect that if you tried something similar with CaCl2 then the outcome would be highly unpredictable and lead to precipitation of a mixture of CaCO3 polymorphs as suggested by William.

Regards,

Steve Crowley

-----Original Message-----
From: Stable Isotope Geochemistry <[log in to unmask]> On Behalf Of Mathieu Daëron
Sent: 16 June 2021 14:24
To: [log in to unmask]
Subject: Quantitative precipitation of DIC to calcite rather than witherite

Dear isogeochemists,

Uchikawa & Zeebe (2012) write, regarding quantitative precipitation of DIC to witherite (BaCO3):

"During the method development, our test samples precipitated as CaCO3 [...] resulted in co-precipitation of calcite and variable amounts of vaterite (roughly from 30 to 50% of total CaCO3)."

Does anybody have ideas about potential strategies to "push the system" towards exclusive calcite precipitation? I suppose this is actually a challenging problem, otherwise some clever people would have already solved it, but I figure it might be worth asking here.

Cheers to all,

- Mathieu

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Mathieu Daëron
Centre National de la Recherche Scientifique Laboratoire des Sciences du Climat et de l’Environnement
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