Chalk: Difference between revisions
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==Uses== |
==Uses== |
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fit. |
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[[File:Carrieres de Meudon.jpg|thumb|right|Former underground chalk [[quarry]] in [[Meudon]], France]] |
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[[File:Chalk-Sidewalk-Art-0092.jpg|thumb|right|Child drawing with [[sidewalk chalk]]]] |
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Chalk is used to make [[quicklime]] and [[slaked lime]], mainly used as [[lime mortar]] in buildings. In southeast England, [[Denehole]]s are a notable example of ancient chalk pits. Such [[bell pit]]s may also mark the sites of ancient flint mines, where the prime object was to remove [[flint]] [[nodule (geology)|nodules]] for [[stone tool]] manufacture. The surface remains at [[Cissbury]] are one such example, but perhaps the most famous is the extensive complex at [[Grimes Graves]] in [[Norfolk]]. |
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The traditional uses of chalk have in some cases been replaced by other substances, although the word "chalk" is often still applied to the usual replacements. |
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*''[[Chalkboard|Blackboard chalk]]'' is a substance used for drawing on rough surfaces, as it readily crumbles leaving particles that stick loosely to these surfaces. Although traditionally composed of natural chalk, modern blackboard chalk is generally made from the mineral [[gypsum]] ([[calcium sulfate]]), often supplied in sticks of compressed powder about 10 cm (4 in) long. |
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*''[[Sidewalk chalk]]'' is similar to blackboard chalk, except that it is formed into larger sticks and often colored. It is used to draw on sidewalks, streets, and driveways, mostly by children, but also by adult artists. |
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*In ''[[agriculture]]'' chalk is used for raising [[pH]] in soils with high [[acidity]]. The most common forms are CaCO<sub>3</sub> ([[calcium carbonate]]) and CaO ([[calcium oxide]]). |
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*In field sports, including grass tennis courts, powdered chalk was used to mark the boundary lines of the playing field or court. This gives the advantage that, if the ball hits the line, a cloud of chalk or pigment dust can be seen. Now the substance used is mostly [[titanium dioxide]].<ref>http://www.rsc.org/learn-chemistry/content/filerepository/CMP/00/001/020/TeacherResource%20Weightlifting.pdf?v=1356513839795</ref> |
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*In [[gymnastics]], [[rock-climbing]], [[weight-lifting]] and [[tug of war]], chalk — now usually [[magnesium carbonate]] — is applied to the hands and feet to remove perspiration and reduce slipping. |
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*''Tailor's chalk'' is traditionally a hard chalk used to make temporary markings on cloth, mainly by tailors. Nowadays it is usually made from [[talc]] ([[magnesium silicate]]). |
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*''[[Toothpaste]]'' also commonly contains small amounts of chalk, to serve as a mild [[abrasive]]. |
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*''Polishing chalk'' is chalk prepared with a carefully controlled grain size, for very fine polishing of metals.<ref>[http://chestofbooks.com/home-improvement/workshop/Workshop-Companion/Polishing-Powders.html Information on polishing powders], from the 1879 book "The Workshop Companion"</ref> |
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*Chalk is a source of [[quicklime]] by [[thermal decomposition]], or [[slaked lime]] following [[quenching]] with water. |
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*Builder's [[putty]] also mainly contains chalk as a filler in [[linseed oil]]. |
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*[[Woodworking joints]] may be fitted by chalking one of the mating surfaces. A trial fit will leave a chalk mark on the high spots of the corresponding surface. Chalk transferring to cover the complete surface indicates a good fit. |
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*[[Fingerprint powder]] |
*[[Fingerprint powder]] |
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* To ease the symptoms of an upset stomach, chalk in small doses works as an [[antacid]]. |
* To ease the symptoms of an upset stomach, chalk in small doses works as an [[antacid]]. |
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Revision as of 18:01, 25 October 2013
| Sedimentary rock | |
 The Needles, situated off the Isle of Wight, are part of the extensive Southern England Chalk Formation. | |
| Composition | |
|---|---|
| calcite (calcium carbonate) |
Chalk /ˈtʃɔːk/ is a soft, white, porous sedimentary rock, a form of limestone composed of the mineral calcite. Calcite is calcium carbonate or CaCO3. It forms under reasonably deep marine conditions from the gradual accumulation of minute calcite plates (coccoliths) shed from micro-organisms called coccolithophores. It is common to find chert or flint nodules embedded in chalk. Chalk can also refer to other compounds including magnesium silicate and calcium sulfate.
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Chalk has greater resistance to weathering and slumping than the clays with which it is usually associated, thus forming tall steep cliffs where chalk ridges meet the sea. Chalk hills, known as chalk downland, usually form where bands of chalk reach the surface at an angle, so forming a scarp slope. Because chalk is porous it can hold a large volume of ground water, providing a natural reservoir that releases water slowly through dry seasons.
Deposits
The Chalk Group is a European stratigraphic unit deposited during the late Cretaceous Period. It forms the famous White Cliffs of Dover in Kent, England, as well as their counterparts of the Cap Blanc Nez on the other side of the Dover Strait. The Champagne region of France is mostly underlain by chalk deposits, which contain artificial caves used for wine storage. Some of the highest chalk cliffs in the world occur at Møns Klint in Denmark.
Formation
Ninety million years ago the chalk downland of Northern Europe was ooze accumulating at the bottom of a great sea. Protozoans such as foraminifera lived on the marine debris that showered down from the upper layers of the ocean. Their shells were made of calcite extracted from the rich sea-water. As they died a deep layer gradually built up and eventually, through the weight of overlying sediments, became consolidated into rock. Later earth movements related to the formation of the Alps raised these former sea-floor deposits above sea level.
Composition
Chalk is composed mostly of calcium carbonate with minor amounts of silt and clay. It is normally formed underwater, commonly on the sea bed, then consolidated and compressed during diagenesis into the form commonly seen today. During diagenesis silica accumulates to form chert or flint nodules within the carbonate rock.
Uses
fit.
- Fingerprint powder
- To ease the symptoms of an upset stomach, chalk in small doses works as an antacid.
See also
References
External links
