Calcite polymorphs

[KirstyH]

Good afternoon PHREEQC users,

Would anyone be able to advise: If I use EQUILIBRIUM PHASES to predict how much calcite will precipitate out of my super-saturated solution, does that also include polymorphs of calcite, such as metastable vaterite? I see that aragonite is separately listed in EQUILIBRIUM PHASES, but vaterite isn't- so perhaps it is grouped with calcite?

Just wondering because there is a lot more calcium carbonate precipitating in my experiments than what I am predicting in PHREEQC (I haven't yet confirmed if my precipitate is calcite or other), and thinking it may be due to the presence of another polymorph.

Many thanks!
Kirsty

[dlparkhurst]

Each polymorph has a different (temperature-dependent) equilibrium constant. See Plummer, L. Niel, and Eurybiades Busenberg. "The solubilities of calcite, aragonite and vaterite in CO2-H2O solutions between 0 and 90 C, and an evaluation of the aqueous model for the system CaCO3-CO2-H2O." Geochimica et cosmochimica acta 46.6 (1982): 1011-1040.

For the temperature range of the paper, calcite is the most stable (least soluble, lowest log K) phase. So, even if a different polymorph forms because of kinetics or inhibition, calcite will still be calculated to be supersaturated. In other words, calcite should precipitate to a greater extent than any other polymorph and, thermodynamically, other polymorphs should convert to calcite.

Don't know why you have more precipitate, but degassing of CO2 will cause calcite precipitation

[KirstyH]

Thank you for your response Dr Parkhurst, I'll check out the paper you suggested!

Best Wishes,
Kirsty