How are shells formed or created?
What you likely are referring to are seashells you might buy at a store, for ornamental purposes. If so, then they are formed by secretion of calcium carbonate by a structure in the mollusks known as the mantle. The mantle is sort of a membrane covering part of the mollusk, and at the open end of the shell (with mollusk inside) it combines oxygen, calcium, and carbon dioxide to produce one of two minerals, either calcite or aragonite. Both of them are CaCO3, but they have different crystal structures. It is only the outer part of the mantle that secretes the minerals; previously secreted shell remains almost the way it was earlier, although there is some overlap of new and old. Other organisms, such as brachiopods and echinoderms secrete shells as well. Some brachiopods secrete shells made of an organic material known as Chitin, which is similar to fingernail material. Interestingly, some single-celled organisms secrete tiny shells, and those organisms (diatoms, foraminiferans, and so
