The most common chemical formulas for chalk are CaCO3 (calcium carbonate) and CaO (calcium oxide). Chalk is a soft, porous sedimentary rock that is a form of limestone and is mainly composed of calcite (calcium carbonate).
Chalk forms under deep marine conditions, when micro-organisms called coccolithophores shed calcite plates and these tiny plates accumulate. Chalk has high resistance to weathering and can form tall cliffs where chalk ridges connect with the sea.
Chalk is used in agriculture to raise the pH in highly acidic soils. In small doses chalk can be used as an antacid. Toothpaste often contains small amounts of chalk because of its usefulness as a mild abrasive.