I’ve been on a little internet ramble. The topic is aragonite, a form of calcium carbonate. Here are some pictures of inorganic crystaline aragonite, though mostly this post is about biogenic aragonite in sea shells.

I began with the following bits of knowledge:

Aragonite is one of two*^/** polymorphs. In terms of minerals, calcium carbonate has two polymorphs: calcite and aragonite. That is, both are calcium carbonate, but they have different crystal structures, and are thus considered different minerals.
It is metastable. I also knew that aragonite is metastable (that is, stable under very limited conditions, and easily nudged into instability).
It occurs in sea shells. I recently learned that aragonite occurs in sea shells and corals, and was curious why an ‘unstable’ mineral would be useful in a biological setting. Is it common in shells, or is it rare? Does it have advantages over calcite? Or is it just easier to synthesize?

(* Later: There is another! ; – ) I checked, and there is a third polymorph of calcium carbonate called “vaterite.” It too is unstable, more so than aragonite, to the extent that it is rarely found in nature. So I’ve ignored it.)

(** Later still: There are eight polymorphs of calcium carbonate. Two are hydrated forms; the others include Mg and Pb, and I’m not sure are properly called polymorphs. But the paper — “An Overview of Biomineralization Processes and the Problem of the Vital Effect,” Weiner and Dove — is fascinating, and lists over 50 biogenic minerals.)

Shells, in general. Here’s what I learned. Sea shells are often composed of both types of calcium carbonate: calcite on the outside, and aragonite on the inside. Calcite is useful because it is more durable than aragonite and withstands erosion well (think of currents pulling sand grains across shells); is better at resisting drilling/penetration by predators; and is also more resistant to acidic conditions. Aragonite is useful because it is better at withstanding impacts. This is because aragonite forms long needle-like crystals that the organism can lay down in various patterns (think brickwork, or a mesh). I would guess that these ordered alignments of accicular aragonite crystals are also responsible for the luster and iridescence of nacre. In shells, organic material is used to pad and align the both calcite and aragonite crystals, adding even more durability to the shell.

Heart Cockle Shells. The starting point for this ramble was a remark by David O’Hara about shells that use aragonite to admit light for symbiotic algae. I’ve found the research that he was presumably referring to: “Heart cockles have windows in their shells to let in light for symbiotic algae.“

To summarize, Heart Cockle shells, like many mollusks, have a symbiotic relationship with algae that dwell within their shells. However, whereas most mollusks open, and thus admit light that their algae use for photosynthesis, Heart Cockles do not open (seems odd to me but that’s what the paper says!), and instead uses ‘windows’ composed of aligned* crystals of aragonite to admit light. (*The paper is a little unclear here, but I’d think they’d have to be aligned.) Furthermore, “beneath the windows, the aragonite forms into bundled fiber optic cables that act as condensing lenses to focus light. Testing showed the structures allowed twice as much light to pass through as would be the case if they were just simple windows.” Who knew you could do so much with long, needle-like crystals!

Questions. I don’t (yet) know about how shells selectively generate calcite or aragonite. Weiner and Dove, cited earlier, say: “One of the major challenges
in the field of biomineralization is to understand the mechanism(s) by which biological systems determine which polymorph will precipitate. This is genetically controlled and is almost always achieved with 100% fidelity.” 100% !!!
One possibility (this is me thinking out loud, not a paraphrase from the paper) is that they have enzymes that selectively generate one or that other. Another possibility is that they have mechanisms for controlling the concentration of magnesium ions – it appears that a concentration of Mg+ inhibits the crystallization of calcite, but does not impede that of aragonite (more on this below in the ‘(inorganic) geology ‘section).

I have some tentative answers from GPT-AI; I don’t like to trust it unless I know something about the domain, which is not the case here. So we can think of these as hypotheses:

“The mantle epithelial cells secrete an organic matrix composed of proteins, glycoproteins, and polysaccharides [that] act as a scaffold, influencing nucleation sites for crystal growth. Certain proteins are known to bind specifically to calcite surfaces, while others favor the formation of aragonite. […] For example: Acidic proteins with aspartic acid-rich domains often interact strongly with Ca²⁺, influencing crystal shape and orientation. Chitin and silk-like proteins in the shell layers provide a framework that can stabilize specific crystal polymorphs.
[…]
The mantle cells create localized microenvironments beneath the shell surface. By regulating ion concentrations and pH, they maintain conditions that are supersaturated with respect to certain crystal forms of CaCO₃. This biochemical “fine-tuning” ensures that once nucleation begins, the chosen crystal polymorph will continue to grow preferentially.

In (inorganic) geology, Aragonite is primarily found in ‘young’ limestones. As limestones age, and especially if they undergo significant heat and pressure, for instance when metamorphosing into marble, the aragonite changes to calcite (over geologic time, meaning millions of years). However, aragonite *can* form under ordinary earth-surface conditions. It is often found in caves. This was a bit of a mystery for a while, but it turns out that aragonite forms when there is a significant amount of magnesium present. Magnesium ions act to prevent the formation of calcite crystals (the magnesium ions apparently stick to a key place in an about-to-nucleate calcite seed crystal), and so a solution of with dissolved calcium carbonate becomes more and more supersaturated, until it reaches a point where aragonite will precipitate. I found this information on a caving site (https://caves.org/virtualcave/aragonite/) that described structures found in caves – it is quite a nice site because it pays a lot of attention to the factors that affect the rheology and morphology of minerals.)

Later:I discovered that it is that Mg++ Ions are NOT getting in the way of calcite crystal formation per se: “The MIT team’s analysis shows that the ratio of calcium to magnesium in the water affects the surface energy of the nucleating crystals; when that ratio passes a specific value, it tips the balance from forming calcite to forming aragonite.” (https://news.mit.edu/2015/why-seashell-mineral-forms-differently-in-seawater-0302 ) All that said, I don’t understand the mechanism by which the Mg++ to lowers the surface energy. However, I am now reading Kenneth Libbrecht’s book on Snow Crystals, which appears to go deeply into these issues, so stay tuned.

Also see:

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