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Definition: chalk from The Hutchinson Unabridged Encyclopedia with Atlas and Weather Guide

Soft limestone (made from gypsum) used in stick form as a drawing medium; mixed with pigment and a binding agent it is an ingredient used to make crayons. Chalk drawings exist from prehistoric times and chalk was popular among Renaissance artists, for example, Leonardo da Vinci. It is still used in art today, for example, by pavement artists as it can be erased easily.

Summary Article: Chalk
From Rock and Gem

A SOFT, FINE-GRAINED, easily pulverized, white to grayish variety of limestone, chalk is composed of the calcite shells of minute marine organisms. Small amounts of other minerals are commonly present, such as glauconite, apatite, and clay minerals. Silica can also be present in small quantities from sponge spicules, diatom and radiolarian skeletons, and nodules of chert and flint. In some localities, the beds of flint nodules are thick enough to have been mined in ancient times. Extensive chalk deposits were formed during the Cretaceous Period (142 to 65 million years ago), the name being derived from the Latin creta, meaning “chalk”.

  • Properties
  • Rock type Marine, organic, sedimentary
  • Fossils Invertebrates, vertebrates
  • Major minerals Calcite
  • Minor minerals Quartz, glauconite, clays
  • Color White, gray, buff
  • Texture Very fine, angular to rounded

White cliffs

These spectacular chalk cliffs are near Dover in Kent, England.

Fossil in chalk

This well-preserved sea urchin (Tylocidaris) fossil is a common large fossil in chalk.

Industrial uses

Like other limestones, chalk is used for making lime and cement and as a fertilizer. It 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. Modern-day blackboard chalk is a manufactured substance rather than natural chalk. In past times chalk was mined extensively to be burned for making quicklime for mortar. There are buildings lime-mortared 500 years ago that are still standing today.

Wellington boots

These loose-fitting, waterproof rubber boots are based upon a design worn by the first Duke of Wellington (1769–1852).

© 2008 Dorling Kindersley Limited

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