How limestone rock are form?

Limestone is a sedimentary rock made up of calcite (CaCO3) as its main mineral. Some limestones were formed by chemical deposition and others by the accumulation of shells from minute sea creatures. Many invertebrate animals (animals with no backbones) take calcite from sea water to construct their shells. When they die the shells fall to the sea bed. Areas where there is little deposition of mud or sand will be ideal for the formation of limestone. One type of limestone which is very pure is called chalk, but most other limestones contain variable amounts of mud or sand or other material.