What is the main mineral component of most mollusc shells?

Calcium carbonate is the main mineral component of most mollusc shells.

Molluscs such as clams, oysters, mussels, snails, and many cephalopods build shells or shell structures using calcium carbonate. The mineral is deposited by the mantle, a tissue layer that secretes an organic matrix and controls shell growth. Molluscs commonly use the mineral in the forms aragonite, calcite, or both.

A shell is not a lifeless stone. It grows by adding material at its edges and can contain layered structures that improve strength. The organic proteins and polysaccharides between mineral crystals help regulate formation and reduce brittleness.

Ocean acidification matters because seawater with lower pH contains fewer carbonate ions available for shell building. Acidification does not instantly dissolve every shell, but it can make calcification more difficult, especially for young or already stressed animals. Crustaceans and corals also use calcium carbonate, but they are not molluscs.