Chalk is a soft, white, porous, sedimentarycarbonate rock. It is a form of limestone composed of the mineral calcite and originally formed deep under the sea by the compression of microscopic plankton that had settled to the sea floor. Chalk is common throughout Western Europe, where deposits underlie parts of France, and steep cliffs are often seen where they meet the sea in places such as the Dover cliffs on the Kent coast of the English Channel.
Chalk is mined for use in industry, such as for quicklime, bricks and builder's putty, and in agriculture, for raising pH in soils with high acidity. 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, or gypsum.
Chalk is a fine-textured, earthy type of limestone distinguished by its light colour, softness, and high porosity.[1][2] It is composed mostly of tiny fragments of the calcite shells or skeletons of plankton, such as foraminiferaorcoccolithophores.[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 as molluscs, echinoderms, or bryozoans.[3][4][5]
Chalk is typically almost pure calcite, CaCO3, with just 2% to 4% of other minerals. These are usually quartz and clay minerals, though collophane (cryptocrystalline apatite, a phosphate mineral) is also sometimes present, as nodules or as small pellets interpreted as fecal pellets. In some chalk beds, the calcite has been converted to dolomite, 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 both gypsum and diatomite, chalk is identifiable by its hardness, fossil content, and its reaction to acid (it produces effervescence on contact).[5]
In Western Europe, chalk was formed in the Late Cretaceous Epoch and the early Palaeocene Epoch (between 100 and 61 million years ago).[6][7] It was deposited on extensive continental 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 early cementation of such limestones. In chalk, absence of this calcium carbonate conversion process prevented early cementation, which partially accounts for chalk's high porosity.[3] Chalk is also the only form of limestone that commonly shows signs of compaction.[8]
Flint (a type of chert) is very common as bands parallel to the bedding or as nodulesinseams, or linings to fractures, embedded in chalk. It is probably derived from sponge spicules[4] or other siliceous organisms as water is expelled upwards during compaction. Flint is often deposited around larger fossils such as Echinoidea which may be silicified (i.e. replaced molecule by molecule by flint).[9]
Chalk is so common in Cretaceous marine beds that the CretaceousPeriod was named for these deposits. The name Cretaceous was derived from Latincreta, meaning chalk.[10] Some deposits of chalk were formed after the Cretaceous.[11]
Most people first encounter chalk in school where it refers to blackboard 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, other mineral sources of calcium carbonate, or the mineral gypsum (calcium sulfate). While gypsum-based blackboard chalk is the lowest cost to produce, and thus widely used in the developing 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, and sidewalk chalk (shaped into larger sticks and often coloured), used to draw on sidewalks, streets, and driveways, are primarily made of gypsum rather than calcium carbonate chalk.[21]
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 a base.[23] Chalk is a source of quicklimebythermal decomposition, or slaked lime following quenching of quicklime with water.[24]Inagriculture, chalk is used for raising pH in soils with high acidity.[25] Small doses of chalk can also be used as an antacid.[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 mild abrasive.[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 by tailors. It is now usually made of talc (magnesium silicate).[29]
In southeast England, deneholes are a notable example of ancient chalk pits. Such bell pits may also mark the sites of ancient flint mines, where the prime object was to remove flint 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 GravesinNorfolk.[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 with titanium dioxide.[32] In gymnastics, rock-climbing, weightlifting and tug of war, chalk — now usually magnesium 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 of brickorwattle and daub: quarried chalk was cut into blocks and used as ashlar, 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 and mercury can be used as fingerprint 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. Retrieved 24 October 2013.{{cite web}}: CS1 maint: archived copy as title (link)
"Landscapes". White Rocks. Archived from the original on 7 June 2009. Retrieved 1 December 2005. The "White Rocks" is the name given to cliffs to the east of Portrush in County Antrim, Northern Ireland.