A soft, porous white mineral composed of fine-grain limestone. Also called whiting, pure varieties of chalk contain up to 99 percent calcium carbonate as the mineral calcite. Chalk was formed in the Cretaceous period and occurs naturally in thick beds in many parts of the world, such as the chalk cliffs along the English Channel. Chalk beds are collections of the shells of such tiny marine organisms as foraminifera, coccoliths, diatoms, and rhabdoliths. Ground chalk has been used as a pigment since ancient times. When mixed with glue, it was the most common ground for northern European paintings from medieval times well into the 18th century. Chalk was also commonly used for making lime, portland cement, putty, and polishing powders. Industrially, it is still used as a filler, extender, or pigment in a wide variety of materials, including ceramics, putty, cosmetics, crayons, plastics, rubber, paper, paints, and linoleum. Chalk is stable and inert, but has poor covering power in oil. It helps control gloss, add texture, absorb oil, and provide plasticity. Chalk is made synthetically by precipitating fine particles of calcium carbonate.
Synonyms and Related Terms
calcium carbonate; Pigment White 18; CI 77220; whiting; Kreide (Deut.); craie (Fr.); calcare (It.); creta (Esp., It.); calcita (Esp.); tiza (Esp.); kimolia (Gr.); krijt (Ned.); kreda (Pol.); kalk (Sven.); cré (Port.); English white; Paris white; gilder's whiting; Champagne chalk; calcite; limestone; marble white;
Reacts with acids to evolve carbon dioxide.
Microscopically, coccoliths may be seen at about 500x in natural chalk. Precipitated chalk has fine, uniform particles.
High birefringence with strong interference colors
|Density||1.8 - 2.7|
|Refractive Index||e =1.486, w =1.64 -1.66|
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- Teri Hensick, contributed information, 1998
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- Website address 1 Comment: http://webexhibits.org/pigments/indiv/overview/chalk.html e =1.486, w =1.64 -1.66
- CRC Handbook of Chemistry and Physics, Robert Weast (ed.), CRC Press, Boca Raton, Florida, v. 61, 1980 Comment: density=1.9-2.8