Looking for a way to imitate shell formation for art project
I've been reading up on the lime cycle. Slaked lime, or calcium hydroxide as I guess it is known round these parts, will absorb atmospheric CO2 and turn into calcium carbonate. From what I've seen, it's a rather boring (though super useful!) chalky, plaster-like stuff, used in mortar and plaster. Is there anyway to persuade it to crystallize in a mold? One of the crystalline form of CaCO3 is argonite, and it is very beautiful. I'm curious to know if it is possible to form argonite, or one of the other two crystalline forms of CaCO3 into a mold. Any thoughts? Short of solutions, where else might I go to learn about the possibilities? My level of chemistry knowledge is slight (though I did get an 93 in my first year uni chemisty class !) Any help enormously appreciated!!
Posted Jul 26, 2021, 12:12 AM UTC
You will have to look into the phase diagram for calcium carbonate. It will tell you the circumstances under which each phase of the mineral will form. You can even look for geology or biology papers via google scholar. Search for “crystallization of aragonite” and you may be able to find a paper with a descriptive synthesis. follow their methods.
If you are looking to imitate the look of a shell, it may be very difficult to do. The parallel lines in a shell are there because molluscs secrete aragonite very slowly from their mantle, and the rate differs bepending on the water temperature and quality.
Regarding using a mold, by my understanding of crystallization (I work in inorganic materials chemistry), you would expect the aragonite to crystallize at several nucleation sites around your dish rather than filling the mold with a single crystal, and the solution you crystallize from will need to be much bigger than your mold. You should try this, but depending on the hardness of aragonite, you might be more easily able to chisel it into a shell shape, a more sculptural approach.
On a side note, there is a kind of fossilization called recrystallization you may be ineterested in. Shells of ancient molluscs sometimes encounter oceans of different acidity than when they lived, which can dissolve the aragonite of their shells, but the different pH of the current environment sets it somewhere else on the phase digram, meaning it recrystallizes as calcite, another form on calcium carbonate.
I looked into it for you a little bit. Aragonite in particulr is a high pressure phase, meaning that It can only be formed inorganically in specialized labs and deep in the crust. It is metastable, meaning it won’t turn into calcite unless disturbed (by acid for example). You are better off trying to crystallize calcite or doing a subtractive method on a purchased sample of aragonite.
Many thanks for this write-up, MQ, much for me to follow up on. It does seem like odds are slim that I'll be able to achieve what I had in mind, but you have given me much to think about.