Calcium carbonate

Calcium carbonate is made of one calcium atom, one carbon atom and three oxygen atoms. It's one of the most common minerals on Earth. Limestone, chalk and marble are mostly calcium carbonate, and so are seashells, coral, eggshells and pearls.

Most of it was built by living things. Sea creatures take calcium and dissolved carbon dioxide from seawater and lay them down as shells and skeletons. When they die, their remains pile up on the seafloor and, over millions of years, are pressed into limestone. The white cliffs of Dover are chalk made from the shells of microscopic algae. Industry simply quarries the rock and crushes it. A purer form, called precipitated calcium carbonate, is made by bubbling carbon dioxide through a mix of lime and water.

In a blast furnace, limestone is the flux. The heat breaks it into lime, which combines with sand and clay in the iron ore to form slag that can be drained off, leaving cleaner pig iron. Heated in a kiln to around 900 °C, it gives off carbon dioxide and becomes calcium oxide, which goes into glass. Dripping cave water deposits it too, slowly building stalactites.