chalk

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chalk,

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.

Chalk

 

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.

G. I. BUSHINSKII

chalk

[chȯk]
(materials)
Artificially prepared pure calcium carbonate; used as the basis for pastels. Also known as whiting.
(petrology)
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.

chalk

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

chalk

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