What is the main structural mineral in the shells of most mollusks?

Calcium carbonate is the main structural mineral in the shells of most mollusks.

Mollusks typically build shells from calcium carbonate crystals embedded in an organic protein matrix. The mineral commonly occurs as aragonite, calcite or layers of both. A mantle, the tissue surrounding the animal’s body, secretes the shell as the mollusk grows.

Shell architecture differs widely among clams, snails, chitons and cephalopods. Many bivalves use layered structures for strength, while nautiluses form internal chambers with aragonite walls. Some shells also contain pigments and microscopic arrangements that create iridescent colors, as seen in mother-of-pearl.

Ocean chemistry affects this process. When seawater absorbs excess carbon dioxide, its pH falls and carbonate ions become less available. This ocean acidification can make shell formation more difficult for some organisms, especially larvae and species that build aragonite shells. Calcium carbonate is also present in coral skeletons, but corals are not mollusks and should not be confused with shell-producing animals.