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chalk, mineral of calcium carbonate, similar in composition to limestone, but softer. It is characteristically a marine formation and sometimes occurs in great thickness; the chief constituents of these chalk deposits are the shells of minute animals called foraminiferans. Chalk has been laid down in all periods of geologic time, but most of the best-known deposits, e.g., the cliffs of the English Channel, date from the Cretaceous period. Chalk is used in the manufacture of putty, plaster, cement, quicklime, mortar, and rubber goods and also for blackboard chalk. Harder forms are used as building stones. Poor soils containing an excessive proportion of clay are frequently improved and sweetened by mixing chalk into them.

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chalk

Soft, fine-grained, easily pulverized, white-to-grayish variety of limestone, composed of the shells of minute marine organisms. The purest varieties contain up to 99% calcium carbonate in the form of the mineral calcite. Extensive deposits occur in western Europe south of Sweden and in England, notably in the chalk cliffs of Dover along the English Channel. Other extensive deposits occur in the U.S. from South Dakota to Texas and eastward to Alabama. Chalk is used for making lime and portland cement and as a soil additive. Finely ground and purified chalk is known as whiting and is used as a filler, extender, or pigment in a wide variety of materials, including ceramics, putty, cosmetics, crayons, plastics, rubber, paper, paints, and linoleum. The chalk commonly used in classrooms is a manufactured substance rather than natural chalk.