Home Sedimantery Rocks Non-Clastic Sedimentary Rock Chalk

Chalk

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Chalk is a non-clastic carbonate sedimentary rock that is form of limestone compesed of the mineral calcite. It is soft, fine-grained and easily pulverized.  Color is white-to-grayish variety of limestone rock. It is composed of the shells of such minute marine organisms as foraminifera, coccoliths, and rhabdoliths. The purest varieties contain up to 99 percent calcium carbonate in the form of the mineral calcite. The sponge spicules, diatom and radiolarian tests (shells), detrital grains of quartz, and chert nodules (flint) found in chalk contribute small amounts of silica to its composition. Small proportions of clay minerals, glauconite, and calcium phosphate also are present.

It 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 well jointed it can hold a large volume of ground water, providing a natural reservoir that releases water slowly through dry seasons.

Origin: Biochemical

Texture: Nonclastic; Fine-grained

Composition: Calcite

Color: White

Miscellaneous: Massive, Earthy; Reacts with HCl; Hardness < Glass

Depositional Environment: Deep Marine

Color: white-to-grayish

Chalk Classification

Natural chalk

Natural chalk occurs in deep sea conditions and consists of gradual accumulation of very small calcite plates. These plates are poured from microorganisms; microorganisms are called kokkolithofors. Natural chalk is highly resistant to erosion due to its porous structure. It is very often associated with clay, but is less resistant to clay erosion and weather conditions. This is more resistant and limestone when the clay is worn, mostly where the chalk ridges meet the sea, steep rocks and shelves. The best example of this is the white cliffs of Dover. It is porous and therefore can hold a lot of water. This is a great advantage for areas that suffer from drought and that have large amounts of limestone and limestone, because these rock types can provide a natural reservoir that releases water slowly.

Manmade chalk

The natural chalk used in sport has been replaced with a man-made chalk in most cases. Artificial chalk is called magnesium carbonate. For this, the empirical formula is MgCO3. Similar to natural chalk, magnesium carbonate has the same properties and properties; white and soft solid. Magnesium carbonate can be mapped or produced under a carbon dioxide atmosphere by a series of chemical processes involving the release of the mixture of magnesium and carbonate ions or the release of the magnesium hydroxide slurry under pressure. Magnesium carbonate has many other uses other than sports. Some have already been mentioned, ie toothpaste. Magnesium carbonate may also be present in laying, non-flammable, fire-extinguishing compositions, cosmetic products and as a powdered powder. The versatility of the substance also means that it is used as a drying agent, as a laxative for loosening the intestines and as a reinforcing agent for neoprene rubber.

Chalk Chemical Composition

Chemical composition of chalk is calcium carbonate and with minor about silt and clay minerals.  It is formed in the sea by sub-microscopic plankton, which fall to the sea floor and are then consolidated and compressed during diagenesis into rock.

Chalk Formation

Ninety million years ago what is now the chalk downland of Northern Europe was ooze accumulating at the bottom of a great sea.It was one of the earliest rocks made up of microscopic particles to be studied under the microscope, when it was found to be composed almost entirely of coccoliths. Their shells were made of calcite extracted from the rich seawater. As they died, a substantial layer gradually built up over millions of years and, through the weight of overlying sediments, eventually became consolidated into rock. Later earth movements related to the formation of the Alps raised these former sea-floor deposits above sea level.

Where is It Located

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 Jasmund National Park in Germany and at Møns Klint in Denmark – both once formed a single island.

Chalk Characteristics and Properties

Chalk, in both its natural and man-made form, is white in colour and is considered to be a fairly soft solid. Naturally, It comes from the ground where it is found as a porous (can hold water) sedimentary rock. It is a form of limestone and is composed of the mineral calcite. It has been quarried for centuries and in ancient times was quarried for building materials and has been used on fields. Most people associate chalk as being white, however red chalk also occurs naturally. It comes from very pure limestone which is normally found in the ground in layers which can be 300 to 400 m thick. Within these layers of limestone there are often layers of other sediments such as flint or chert.

Chalk Uses

  • Blackboard chalk is a substance used for drawing on rough surfaces because the particles adhering to these surfaces are easily crumbled. Although traditionally composed of natural chalks, modern wood chalk is generally made of mineral gypsum (calcium sulfate), which is usually supplied with compressed powdered rods about 10 cm in length.
  • The pavement chalk is similar to the blackboard rock, but it is the same as it turns into larger bars and is usually colored. It is also used to draw sidewalks, streets and car roads, mostly by children, but also by adult artists.
  • In agriculture, It is used to raise pH in soils with high acidity. The most common forms are CaCO3 (calcium carbonate) and CaO (calcium oxide).
  • In field sports, including grass tennis courts, powder It is used to mark the playing field or field boundaries. This provides the advantage that a chalk or pigment powder cloud can be seen when the ball hits the line. The material used today is mostly titanium dioxide.
  • In gymnastics, rock climbing, weight lifting and combat traction, chalk – now generally magnesium carbonate – apply to the hands to remove perspiration and reduce slippage.
  • Tailor chalk is traditionally used, especially by tailors, to make a hard it used to make temporary markings on the fabric. Today, it is usually made of talc (magnesium silicate).
  • Toothpaste also usually contains a small amount of chalk to function as a mild abrasive.
  • The polishing chalk is a carefully controlled grain-sized chalk for very fine polishing of metals.
  • It is a source of slaked lime by thermal decomposition or a source of slaked lime after quenching with water.
  • Builders putty also contains chalk as a filler, which is essentially flaxseed oil.
  • Woodworking joints can be placed by chalking of one of the bonding surfaces. The trial fit will leave a chalk mark on the high points of the corresponding surface. The chalk transfer to cover the entire surface indicates a good fit.

References

  • Bonewitz, R. (2012). Rocks and minerals. 2nd ed. London: DK Publishing.
  • Wikipedia contributors. (2019, April 12). Chalk. In Wikipedia, The Free Encyclopedia. Retrieved 20:32, May 9, 2019, from https://en.wikipedia.org/w/index.php?title=Chalk&oldid=892176455

Cite this article as: Geology Science. (2019). Chalk. [online] Available at: http://geologyscience.com/rocks/sedimantery-rocks/chalk/ [9th December 2019 ]
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