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mineral of calcium carbonatecalcium carbonate,
CaCO3, white chemical compound that is the most common nonsiliceous mineral. It occurs in two crystal forms: calcite, which is hexagonal, and aragonite, which is rhombohedral.
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, 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 foraminiferansforaminiferan
, common name for members of the class Foraminifera, large, shelled ameboid protozoans belonging to the phylum Sarcodina. Most foraminiferan shells are calcareous, but some are siliceous, and others are built of sand grains.
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. 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 periodCretaceous period
, third and last period of the Mesozoic era of geologic time (see Geologic Timescale, table), lasting from approximately 144 to 65 million years ago. The Cretaceous was marked, in both North America and Europe, by extensive submergences of the continents.
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. 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.



a weakly cemented, fine-grained variety of carbonate rock that has the property of rubbing off when pressed against a surface. It consists primarily of calcium carbonate that is of natural origin or is synthetically obtained. Natural chalk is composed primarily of calcite skeletal particles of microorganisms: the calcareous algae Coccolithophoridae (70–90 percent) and the rhizopods Foraminifera (1–20 percent). Chalk occasionally contains mollusk shells and the skeletons of pearlworts, sea urchins, sea lilies, siliceous sponges, and corals.

The chemical composition of chalk is 50–55 percent CaO, 0.2–0.3 percent MgO, 0.5–6.0 percent SiO2, 0.2–4.0 percent Al2O3, 0.02–0.7 percent Fe2O3 + FeO, and 40–43 percent CO2. The mineral composition is 90–99 percent calcite, 1–8 percent clay minerals (montmorillonite, hydromica, and kaolinite), 0.01–0.1 percent pyrite, 0.1–0.5 percent glauconite, 0.2–6 percent quartz, 0.01–7.0 percent opal, 0.01–0.50 percent zeolite-heulandite, and 0.01 percent barite. More than 90 percent of the particles in chalk are usually less than 0.01 mm in size. The density of chalk is 2.70–2.72 g/cm3. The volumetric mass of the skeleton is 1.42–1.56 g/cm3. Porosity is 45–50 percent. Natural moisture is 30–33 percent. Wet chalk has a compressive strength of 1–2 meganewtons per sq m (10–20 kilograms-force per sq cm); the corresponding figure for dry chalk is 4–5 meganewtons per sq m (40–50 kilograms-force per sq cm). Concretions of flint, pyrite, and phosphorite are sometimes scattered throughout the chalk. Chalk is a semihardened sea ooze deposited at depths of 30–500 m and more. It is common in nature and is primarily confined to Upper Cretaceous and Lower Paleogene beds.

The largest zone of chalk deposits stretches from the Emba River in Western Kazakhstan to Great Britain. In places the beds are hundreds of meters thick—for example, 600 m in the Kharkov region.

Depending on the method of production and area of primary use, chalk in the USSR is subdivided into types, brands, and grades established by the All-Union State Standards (1972). Chalk is used in agriculture for liming soils and for animal feed supplements. In industry chalk is used to produce cement and lime; as a filler for rubber, plastics, and paints and varnishes; in obtaining soda and glass; in sugar refining; and in the production of chalk for school. Precipitated chalk is used in medicine (as a therapeutic preparation) and in the toiletries industry (as a constituent of tooth powders). In the plastic arts chalk is used as a base for levkas and other grounds and as a component in making paints (including pastels). White chalk and black chalk are used for drawing.

In the USSR chalk deposits are concentrated in Briansk, Belgorod, Ul’ianovsk, and Saratov oblasts of the RSFSR and in the Ukrainian SSR, the Byelorussian SSR, and the Kazakh SSR; the major deposits abroad are in France (the Paris Basin), Great Britain, and Denmark.



Artificially prepared pure calcium carbonate; used as the basis for pastels. Also known as whiting.
A variety of limestone formed from pelagic organisms; it is very fine-grained, porous, and friable; white or very light-colored, it consists almost entirely of calcite.


A soft limestone, usually white, gray, or buff in color, composed chiefly of the calcareous remains of marine organisms.


1. a soft fine-grained white sedimentary rock consisting of nearly pure calcium carbonate, containing minute fossil fragments of marine organisms, usually without a cementing material
2. Billiards Snooker a small cube of prepared chalk or similar substance for rubbing the tip of a cue
References in classic literature ?
It is a thousand pities that we have not a reproduction of those which were done in chalk upon the window-sill.
They had been drawn in chalk upon the black wooden door of the tool-house, which stands beside the lawn in full view of the front windows.
Open the note, take the chalk, and do as I command you.
With flashing eyes she tore open the letter, snatched up the chalk, and dashed at the blackboard.
When Cecily had finished and had thrown down the chalk with bitter vehemence, he turned around with a very red face.
For instance, the several species of the Chthamalinae (a sub-family of sessile cirripedes) coat the rocks all over the world in infinite numbers: they are all strictly littoral, with the exception of a single Mediterranean species, which inhabits deep water and has been found fossil in Sicily, whereas not one other species has hitherto been found in any tertiary formation: yet it is now known that the genus Chthamalus existed during the chalk period.
There it stands right up above all the rest, nine hundred feet above the sea, and the boldest, bravest shape for a chalk hill that you ever saw.
After which crowning mercy, the pious king, that there might never be wanting a sign and a memorial to the country-side, carved out on the northern side of the chalk hill, under the camp, where it is almost precipitous, the great Saxon White Horse, which he who will may see from the railway, and which gives its name to the Vale, over which it has looked these thousand years and more.
she said, and laying down the chalk, she made a movement as though to get up.
He quickly rubbed out what he had written, gave her the chalk, and stood up.
Before sitting down to it, he opens his corner- cupboard door; takes his bit of chalk from its shelf; adds one thick line to the score, extending from the top of the cupboard door to the bottom; and then falls to with an appetite.
Let them come and pull the roof about our ears; let them burn us out of house and home; I'd neither have the protection of their leader, nor chalk their howl upon my door, though, for not doing it, they shot me on my own threshold.