Chalk is a soft, white,porous,sedimentarycarbonate rock. It is a form oflimestone composed of the mineralcalcite and originally formed deep under the sea by the compression of microscopicplankton that had settled to thesea floor. Chalk is common throughoutWestern Europe, where deposits underlie parts of France, and steep cliffs are often seen where they meet the sea in places such as theDover cliffs on theKent coast of theEnglish Channel.
Chalk is mined for use in industry, such as forquicklime,bricks and builder'sputty, and inagriculture, for raisingpH in soils with highacidity. It is also used for "blackboard chalk" for writing and drawing on various types of surfaces, although these can also be manufactured from other carbonate-based minerals, orgypsum.
"Nitzana Chalk curves" situated at WesternNegev,Israel, are chalk deposits formed in theMesozoic era'sTethys OceanOpen chalk pit, Seale, Surrey, UK
Chalk is a fine-textured, earthy type oflimestone distinguished by its light colour, softness, and high porosity.[1][2] It is composed mostly of tiny fragments of thecalcite shells or skeletons ofplankton, such asforaminifera orcoccolithophores.[1] These fragments mostly take the form of calcite plates ranging from 0.5 to 4 microns in size, though about 10% to 25% of a typical chalk is composed of fragments that are 10 to 100 microns in size. The larger fragments include intact plankton skeletons and skeletal fragments of larger organisms, such asmolluscs,echinoderms, orbryozoans.[3][4][5]
Chalk is typically almost pure calcite,CaCO3, with just 2% to 4% of other minerals. These are usuallyquartz andclay minerals, though collophane (cryptocrystallineapatite, aphosphate mineral) is also sometimes present, as nodules or as small pellets interpreted as fecal pellets. In some chalk beds, the calcite has been converted todolomite,CaMg(CO3)2, and in a few cases the dolomitized chalk has been dedolomitized back to calcite.[3]
Chalk is highly porous, with typical values of porosity ranging from 35 to 47 per cent.[3] While it is similar in appearance to bothgypsum anddiatomite, chalk is identifiable by its hardness,fossil content, and its reaction toacid (it produceseffervescence on contact).[5]
In Western Europe, chalk was formed in the LateCretaceous Epoch and the earlyPalaeocene Epoch (between 100 and 61 million years ago).[6][7] It was deposited on extensivecontinental shelves at depths between 100 and 600 metres (330 and 1,970 ft), during a time of nonseasonal (likely arid) climate that reduced the amount of erosion from nearby exposed rock. The lack of nearby erosion explains the high purity of chalk. The coccolithophores, foraminifera, and other microscopic organisms from which the chalk came mostly form low-magnesium calcite skeletons, so the sediments were already in the form of highly stable low-magnesium calcite when deposited. This is in contrast with most other limestones, which formed from high-magnesium calcite or aragonite that rapidly converted to the more stable low-magnesium calcite after deposition, resulting in the earlycementation of such limestones. In chalk, the absence of calcium carbonate conversion process prevented early cementation, and it accounts for chalk's high porosity.[3] Additionally, chalk is the only form of limestone that commonly shows signs of compaction.[8]
Flint (a type ofchert) is very common as bands parallel to thebedding or asnodules inseams, or linings tofractures, embedded in chalk. It is probably derived fromsponge spicules[4] or othersiliceous organisms as water is expelled upwards during compaction. Flint is often deposited around largerfossils such asEchinoidea which may besilicified (i.e. replaced molecule by molecule by flint).[9]
Chalk is so common in Cretaceous marine beds that theCretaceousPeriod was named for these deposits. The name Cretaceous was derived fromLatincreta, meaningchalk.[10] Some deposits of chalk were formed after the Cretaceous.[11]
Chalk in different colorsChild drawing withsidewalk chalk
Most people first encounter chalk in school where it refers toblackboard chalk, which was originally made of mineral chalk, since it readily crumbles and leaves particles that stick loosely to rough surfaces, allowing it to make writing that can be readily erased. Blackboard chalk manufacturers now may use mineral chalk, othermineral sources of calcium carbonate, or the mineralgypsum (calcium sulfate). While gypsum-based blackboard chalk is the lowest cost to produce, and thus widely used in thedeveloping world, use of carbonate-based chalk produces larger particles and thus less dust, and it is marketed as "dustless chalk".[20][5]
Coloured chalks, pastel chalks, andsidewalk chalk (shaped into larger sticks and often coloured), used to draw onsidewalks, streets, anddriveways, are primarily made of gypsum rather than calcium carbonate chalk.[21]
ClimberJan Hojer blows surplus chalk from his hand.
Magnesium carbonate chalk is commonly used as a drying agent to obtain better grip by gymnasts and rock climbers.
Chalk and other forms of limestone may be used for their properties as abase.[23] Chalk is a source ofquicklime bythermal decomposition, orslaked lime followingquenching of quicklime with water.[24] Inagriculture, chalk is used for raisingpH in soils with highacidity.[25] Small doses of chalk can also be used as anantacid.[26] Additionally, the small particles of chalk make it a substance ideal for cleaning and polishing. For example, toothpaste commonly contains small amounts of chalk, which serves as a mildabrasive.[27] Polishing chalk is chalk prepared with a carefully controlled grain size, for very fine polishing of metals.[28]
French chalk (also known as tailor's chalk) is traditionally a hard chalk used to make temporary markings on cloth, mainly bytailors. It is now usually made oftalc (magnesium silicate).[29]
In southeast England,deneholes are a notable example of ancient chalk pits. Suchbell pits may also mark the sites of ancientflint mines, where the prime object was to remove flintnodules forstone tool manufacture. The surface remains atCissbury are one such example, but perhaps the most famous is the extensive complex atGrimes Graves inNorfolk.[31]
Chalk was traditionally used in recreation. In field sports, such as tennis played on grass, powdered chalk was used to mark the boundary lines of the playing field or court. If a ball hits the line, a cloud of chalk or pigment dust will be visible. In recent years, powdered chalk has been replaced withtitanium dioxide.[32] In gymnastics, rock-climbing, weightlifting andtug of war, chalk — now usuallymagnesium carbonate — is applied to the hands and feet to remove perspiration and reduce slipping.[33]
Chalk may also be used as a house construction material instead ofbrick orwattle and daub: quarried chalk was cut into blocks and used asashlar, or loose chalk was rammed into blocks and laid in mortar.[34][35] There are still houses standing which have been constructed using chalk as the main building material. Most are pre-Victorian though a few are more recent.[36]
A mixture of chalk andmercury can be used asfingerprint powder. However, because of the toxicity of the mercury, the use of such mixtures for fingerprinting was abandoned in 1967.[37]
^Prothero, Donald R. (9 July 2018). "The Story of the Earth in 25 Rocks: Tales of Important Geological Puzzles and the People Who Solved Them".doi:10.7312/prot18260-021.{{cite journal}}:Cite journal requires|journal= (help)
^Tewksbury, B. J.; Hogan, J. P.; Kattenhorn, S. A.; Mehrtens, C. J.; Tarabees, E. A. (1 June 2014). "Polygonal faults in chalk: Insights from extensive exposures of the Khoman Formation, Western Desert, Egypt".Geology.42 (6):479–482.Bibcode:2014Geo....42..479T.doi:10.1130/G35362.1.
^Thakker, M., Shukla, P. and Shah, D.O., 2015. Surface and colloidal properties of chalks: A novel approach using surfactants to convert normal chalks into dustless chalks. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 480, pp. 236–244.doi:10.1016/j.colsurfa.2015.01.054
^Clayman, Charles B. (5 December 1980). "The Carbonate Affair: Chalk One Up".JAMA: The Journal of the American Medical Association.244 (22): 2554.doi:10.1001/jama.1980.03310220052030.PMID7431595.
^Baxter, P. M.; Davis, W. B.; Jackson, J. (January 1981). "Toothpaste abrasive requirements to control naturally stained pellicle.: The relation of cleaning power to toothpaste abrasivity".Journal of Oral Rehabilitation.8 (1):19–26.doi:10.1111/j.1365-2842.1981.tb00471.x.PMID6935391.
^Kumar, Sublania Harish; J., Singh K.; K., Somani A. (2016).Estimatation of talc properties after milling. International Conference on Condensed Matter and Applied Physics (Icc 2015). AIP Conference Proceedings. Vol. 1728. p. 020139.Bibcode:2016AIPC.1728b0139K.doi:10.1063/1.4946190.
^Sieveking, G. De G.; Bush, P.; Ferguson, J.; Craddock, P. T.; Hughes, M. J.; Cowell, M. R. (August 1972). "Prehistoric flint mines and their identification as sources of raw material".Archaeometry.14 (2):151–176.doi:10.1111/j.1475-4754.1972.tb00061.x.
^"Archived copy"(PDF).Archived(PDF) from the original on 29 October 2013. Retrieved24 October 2013.{{cite web}}: CS1 maint: archived copy as title (link)
"Landscapes".White Rocks. Archived fromthe original on 7 June 2009. Retrieved1 December 2005. The "White Rocks" is the name given to cliffs to the east of Portrush in County Antrim, Northern Ireland.
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