Calcium carbonate
Biological sources

Eggshells, snail shells and most seashells are predominantly calcium carbonate and can be used as industrial sources of that chemical. Oyster shells have enjoyed recent recognition as a source of dietary calcium, but are also a practical industrial source. Dark green vegetables such as broccoli and kale contain dietarily significant amounts of calcium carbonate, but they are not practical as an industrial source.

Annelids in the family Lumbricidae, earthworms, possess a regionalization of the digestive track called calciferous glands, Kalkdrüsen, or glandes de Morren, that processes calcium and CO into calcium carbonate, which is later excreted into the dirt. The function of these glands is unknown but is believed to serve as a CO regulation mechanism within the animals' tissues. This process is ecologically significant, stabilizing the pH of acid soils.
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