Which compound is the main mineral component of the shells of many mollusks?

Calcium carbonate is the main mineral component of the shells of many mollusks.

Calcium carbonate has the chemical formula CaCO₃. Mollusks commonly build shells by depositing calcium carbonate crystals within an organic matrix. Depending on the species and conditions, the mineral may occur mainly as aragonite, calcite, or a mixture of both crystal forms.

Calcium carbonate is not limited to shells. It is also found in limestone, chalk, marble, coral skeletons, and many geological deposits. In water containing dissolved carbon dioxide, it can participate in equilibria that help explain why acidic water gradually dissolves carbonate shells and limestone.

A frequent mix-up is naming silicon dioxide, SiO₂, which is common in sand and quartz, rather than calcium carbonate. Calcium carbonate is also used in construction materials, paper, plastics, paints, and some antacid products. Its many uses reflect both its abundance and its ability to react with acids.