chalk (rock)

Soft, fine-grained, whitish sedimentary rock composed of calcium carbonate, CaCO₃, extensively quarried for use in cement, lime, and mortar, and in the manufacture of cosmetics and toothpaste. Blackboard chalk in fact consists of gypsum (calcium sulphate, CaSO₄.2H₂O).

Chalk was once thought to derive from the remains of microscopic animals or foraminifera. In 1953, however, it was seen under the electron microscope to be composed chiefly of coccolithophores, unicellular lime-secreting algae, and hence primarily of plant origin. It is formed from deposits of deep-sea sediments called oozes.