Cockle shells, like most other seashells, are primarily composed of calcium carbonate. However, they also contain a small percentage of other minerals and organic compounds, including:

* Magnesium carbonate: This mineral contributes to the shell's strength and resilience.

* Phosphate: This element plays a role in the shell's growth and development.

* Silica: This mineral adds to the shell's hardness and resistance to abrasion.

* Conchiolin: This organic protein acts as a binder, holding the calcium carbonate crystals together and providing flexibility to the shell.

* Trace elements: Other elements like iron, manganese, and zinc are present in small amounts, contributing to the shell's color and overall structure.

The exact composition of a cockle shell can vary depending on factors such as the species of cockle, its diet, and the environment it lives in.

While calcium carbonate is the dominant component, these other minerals and organic compounds play crucial roles in giving the cockle shell its unique properties.