Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate. The Mohs scale of mineral hardness, based on scratch hardness comparison, defines value 3 as "calcite". Other polymorphs of calcium carbonate are the minerals vaterite. Aragonite will change to calcite over timescales of days or less at temperatures exceeding 300 °C, vaterite is less stable. Calcite is derived from the German Calcit, a term coined in the 19th century from the Latin word for lime, calx with the suffix -ite used to name minerals, it is thus etymologically related to chalk. When applied by archaeologists and stone trade professionals, the term alabaster is used not just as in geology and mineralogy, where it is reserved for a variety of gypsum. In publications, two different sets of Miller indices are used to describe directions in calcite crystals - the hexagonal system with three indices h, k, l and the rhombohedral system with four indices h, k, l, i. To add to the complications, there are two definitions of unit cell for calcite.
One, an older "morphological" unit cell, was inferred by measuring angles between faces of crystals and looking for the smallest numbers that fit. A "structural" unit cell was determined using X-ray crystallography; the morphological unit cell has approximate dimensions a = 10 Å and c = 8.5 Å, while for the structural unit cell they are a = 5 Å and c = 17 Å. For the same orientation, c must be multiplied by 4 to convert from morphological to structural units; as an example, the cleavage is given as "perfect on " in morphological coordinates and "perfect on " in structural units. Twinning and crystal forms are always given in morphological units. Over 800 forms of calcite crystals have been identified. Most common are scalenohedra, with faces in the hexagonal directions or directions. Habits include acute to tabular forms, prisms, or various scalenohedra. Calcite exhibits several twinning types adding to the variety of observed forms, it may occur as fibrous, lamellar, or compact. A fibrous, efflorescent form is known as lublinite.
Cleavage is in three directions parallel to the rhombohedron form. Its fracture is difficult to obtain. Scalenohedral faces are chiral and come in pairs with mirror-image symmetry. Rhombohedral faces are achiral, it has a defining Mohs hardness of 3, a specific gravity of 2.71, its luster is vitreous in crystallized varieties. Color is white or none, though shades of gray, orange, green, violet, brown, or black can occur when the mineral is charged with impurities. Calcite is transparent to opaque and may show phosphorescence or fluorescence. A transparent variety called. Acute scalenohedral crystals are sometimes referred to as "dogtooth spar" while the rhombohedral form is sometimes referred to as "nailhead spar". Single calcite crystals display; this strong birefringence causes objects viewed through a clear piece of calcite to appear doubled. The birefringent effect was first described by the Danish scientist Rasmus Bartholin in 1669. At a wavelength of ≈590 nm calcite has ordinary and extraordinary refractive indices of 1.658 and 1.486, respectively.
Between 190 and 1700 nm, the ordinary refractive index varies between 1.9 and 1.5, while the extraordinary refractive index varies between 1.6 and 1.4. Calcite, like most carbonates, will dissolve with most forms of acid. Calcite can be either dissolved by groundwater or precipitated by groundwater, depending on several factors including the water temperature, pH, dissolved ion concentrations. Although calcite is insoluble in cold water, acidity can cause dissolution of calcite and release of carbon dioxide gas. Ambient carbon dioxide, due to its acidity, has a slight solubilizing effect on calcite. Calcite exhibits an unusual characteristic called retrograde solubility in which it becomes less soluble in water as the temperature increases; when conditions are right for precipitation, calcite forms mineral coatings that cement the existing rock grains together or it can fill fractures. When conditions are right for dissolution, the removal of calcite can increase the porosity and permeability of the rock, if it continues for a long period of time may result in the formation of caves.
On a landscape scale, continued dissolution of calcium carbonate-rich rocks can lead to the expansion and eventual collapse of cave systems, resulting in various forms of karst topography. Ancient Egyptians carved many items out of calcite, relating it to their goddess Bast, whose name contributed to the term alabaster because of the close association. Many other cultures have used the material for similar carved applications. High-grade optical calcite was used in World War II for gun sights in bomb sights and anti-aircraft weaponry. Experiments have been conducted to use calcite for a cloak of invisibility. Microbiologically precipitated calcite has a wide range of applications, such as soil remediation, soil stabilization and concrete repair. Calcite, obtained from an 80 kg sample of Carrara marble, is used as the IAEA-603 isotopic standard in mass spectrometry for the calibration of δ18O and δ13C. Calcite is a common constituent
Silt is granular material of a size between sand and clay, whose mineral origin is quartz and feldspar. Silt may occur as a soil or as sediment mixed in suspension with water and soil in a body of water such as a river, it may exist as soil deposited at the bottom of a water body, like mudflows from landslides. Silt has a moderate specific area with a non-sticky, plastic feel. Silt has a floury feel when dry, a slippery feel when wet. Silt can be visually observed with a hand lens, it can be felt by the tongue as granular when placed on the front teeth. Silt is created by a variety of physical processes capable of splitting the sand-sized quartz crystals of primary rocks by exploiting deficiencies in their lattice; these involve chemical weathering of rock and regolith, a number of physical weathering processes such as frost shattering and haloclasty. The main process is abrasion through transport, including fluvial comminution, aeolian attrition and glacial grinding, it is in semi-arid environments.
Silt is sometimes known as "rock flour" or "stone dust" when produced by glacial action. Mineralogically, silt is composed of quartz and feldspar. Sedimentary rock composed of silt is known as siltstone. Liquefaction created by a strong earthquake is silt suspended in water, hydrodynamically forced up from below ground level. In the Udden–Wentworth scale, silt particles range between 0.0039 and 0.0625 mm, larger than clay but smaller than sand particles. ISO 14688 grades silts between 0.063 mm. In actuality, silt is chemically distinct from clay, unlike clay, grains of silt are the same size in all dimensions. Clays are formed from thin plate-shaped particles held together by electrostatic forces, so present a cohesion. Pure silts are not cohesive. According to the U. S. Department of Agriculture Soil Texture Classification system, the sand–silt distinction is made at the 0.05 mm particle size. The USDA system has been adopted by the Agriculture Organization. In the Unified Soil Classification System and the AASHTO Soil Classification system, the sand–silt distinction is made at the 0.075 mm particle size.
Silts and clays are distinguished mechanically by their plasticity. Silt is transported in water or other liquid and is fine enough to be carried long distances by air in the form of dust. Thick deposits of silty material resulting from deposition by aeolian processes are called loess. Silt and clay contribute to turbidity in water. Silt is transported by water currents in the ocean; when silt appears as a pollutant in water the phenomenon is known as siltation. Silt, deposited by annual floods along the Nile River, created the rich, fertile soil that sustained the Ancient Egyptian civilization. Silt deposited by the Mississippi River throughout the 20th century has decreased due to a system of levees, contributing to the disappearance of protective wetlands and barrier islands in the delta region surrounding New Orleans. In southeast Bangladesh, in the Noakhali district, cross dams were built in the 1960s whereby silt started forming new land called "chars"; the district of Noakhali has gained more than 73 square kilometres of land in the past 50 years.
With Dutch funding, the Bangladeshi government began to help develop older chars in the late 1970s, the effort has since become a multi-agency operation building roads, embankments, cyclone shelters and ponds, as well as distributing land to settlers. By fall 2010, the program will have allotted some 100 square kilometres to 21,000 families. A main source of silt in urban rivers is disturbance of soil by construction activity. A main source in rural rivers is erosion from plowing of farm fields, clearcutting or slash and burn treatment of forests; the fertile black silt of the Nile river's banks is a symbol of rebirth, associated with the Egyptian god Anubis. Erosion control Nonpoint source pollution Sediment control Silt fence Siltation
The Centennial International Exhibition of 1876, the first official World's Fair in the United States, was held in Philadelphia, from May 10 to November 10, 1876, to celebrate the 100th anniversary of the signing of the Declaration of Independence in Philadelphia. Named the International Exhibition of Arts and Products of the Soil and Mine, it was held in Fairmount Park along the Schuylkill River on fairgrounds designed by Herman J. Schwarzmann. Nearly 10 million visitors attended thirty-seven countries participated in it; the Great Central Fair, Pennsylvania held in 1864, one of the many sanitary fairs held during the Civil War, anticipated the combination of public and commercial efforts that were necessary for the Centennial. The Great Central Fair, held on Logan Square, had a similar gothic appearance, the waving flags, the huge central hall, the "curiosities" and relics and industrial exhibits, a visit from the President and his family, provided a creative and communal means for ordinary citizens to promote the welfare of Union soldiers and dedicate themselves to the survival of the nation.
They made Philadelphia a vital center in the Union war effort. The idea of the Centennial Exposition is credited to John L. Campbell, a professor of mathematics, natural philosophy and astronomy at Wabash College, Indiana. In December 1866, Campbell suggested to Philadelphia Mayor Morton McMichael that the United States Centennial be celebrated with an exposition in Philadelphia. Detractors said the project would not be able to find funding, other nations might not attend, U. S. exhibitions might compare poorly to foreign exhibits. The Franklin Institute became an early supporter of the exposition and asked the Philadelphia City Council for use of Fairmount Park. With reference to the numerous events of national importance that were held in the past and related to the City of Philadelphia, the City Council resolved in January 1870, to hold the Centennial Exposition in the city in 1876; the Philadelphia City Council and the Pennsylvania General Assembly created a committee to study the project and seek support of the U.
S. Congress. Congressman William D. Kelley spoke for the city and state and Daniel Johnson Morrell introduced a bill to create a United States Centennial Commission; the bill, which passed on March 3, 1871, provided that the U. S. government would not be liable for any expenses. The United States Centennial Commission organized on March 3, 1872, with Joseph R. Hawley of Connecticut as president; the Centennial Commission's commissioners included one representative from each state and territory in the United States. On June 1, 1872, Congress created a Centennial Board of Finance to help raise money; the board's president was John Welsh, brother of philanthropist William Welsh, who had raised funds for The Great Sanitary Fair in 1864. The board was authorized to sell up to $10 million in stock via $10 shares; the board sold $1,784,320 worth of shares by February 22, 1873. Philadelphia contributed $1.5 million and Pennsylvania gave $1 million. On February 11, 1876, Congress appropriated $1.5 million in a loan.
The board thought it was a subsidy, but after the Centennial ended, the federal government sued for the money back, the United States Supreme Court forced repayment. John Welsh enlisted help from the women of Philadelphia who had helped him in The Great Sanitary Fair. A Women's Centennial Executive Committee was formed with Elizabeth Duane Gillespie, a descendant of Benjamin Franklin, as president. In its first few months, the group raised $40,000; when the group learned the planning commission was not doing much to display the work of women, the group raised $30,000 for a women's exhibition building. In 1873, the Centennial Commission named Alfred T. Goshorn as the director general of the Exposition; the Fairmount Park Commission set aside 450 acres of West Fairmount Park for the exposition, dedicated on July 4, 1873, by Secretary of the Navy George M. Robeson; the Commission decided to classify the exhibits into seven departments: agriculture, art and science, machinery and mining and metallurgy.
Newspaper publisher John W. Forney agreed to head and pay for a Philadelphia commission sent to Europe to invite nations to exhibit at the exposition. Despite fears of a European boycott and high American tariffs making foreign goods not worthwhile, no European country declined the invitation. To accommodate out-of-town visitors, temporary hotels were constructed near the Centennial's grounds. A Centennial Lodging-House Agency made a list of rooms in hotels, boarding houses and private homes and sold tickets for the available rooms in cities promoting the Centennial or on trains heading for Philadelphia. Philadelphia streetcars increased service and the Pennsylvania Railroad ran special trains from Philadelphia's Market Street, New York City and Pittsburgh; the Philadelphia and Reading Railroad ran special trains from the Center City part of Philadelphia. A small hospital was built on the Exposition's grounds by the Centennial's Medical Bureau, but despite a heat wave during the summer, no mass deaths or epidemics occurred.
Philadelphia passed an ordinance that authorized Mayor William S. Stokley to appoint five hundred men as centennial guards for the exposition. Among soldiers and local men hired by the city was Frank Geyer, best known for investigating one of America's first serial killers, H. H. Holmes. Centennial guards policed exhibits, kept the peace, reunited lost children, received and when possible, returned lost items, the most unusual of which were front hair pieces and false te
Calcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks as the minerals calcite and aragonite and is the main component of pearls and the shells of marine organisms and eggs. Calcium carbonate is the active ingredient in agricultural lime and is created when calcium ions in hard water react with carbonate ions to create limescale, it is medicinally used as a calcium supplement or as an antacid, but excessive consumption can be hazardous. Calcium carbonate shares the typical properties of other carbonates. Notably it reacts with acids, releasing carbon dioxide:CaCO3 + 2 H+ → Ca2+ + CO2 + H2Oreleases carbon dioxide upon heating, called a thermal decomposition reaction, or calcination, to form calcium oxide called quicklime, with reaction enthalpy 178 kJ/mol:CaCO3 → CaO + CO2Calcium carbonate will react with water, saturated with carbon dioxide to form the soluble calcium bicarbonate. CaCO3 + CO2 + H2O → Ca2This reaction is important in the erosion of carbonate rock, forming caverns, leads to hard water in many regions.
An unusual form of calcium carbonate is the hexahydrate, ikaite, CaCO3·6H2O. Ikaite is stable only below 8 °C; the vast majority of calcium carbonate used in industry is extracted by quarrying. Pure calcium carbonate, can be produced from a pure quarried source. Alternatively, calcium carbonate is prepared from calcium oxide. Water is added to give calcium hydroxide carbon dioxide is passed through this solution to precipitate the desired calcium carbonate, referred to in the industry as precipitated calcium carbonate: CaO + H2O → Ca2 Ca2 + CO2 → CaCO3↓ + H2O The thermodynamically stable form of CaCO3 under normal conditions is hexagonal β-CaCO3. Other forms can be prepared, the denser orthorhombic λ-CaCO3 and μ-CaCO3, occurring as the mineral vaterite; the aragonite form can be prepared by precipitation at temperatures above 85 °C, the vaterite form can be prepared by precipitation at 60 °C. Calcite contains calcium atoms coordinated by six oxygen atoms, in aragonite they are coordinated by nine oxygen atoms.
The vaterite structure is not understood. Magnesium carbonate has the calcite structure, whereas strontium carbonate and barium carbonate adopt the aragonite structure, reflecting their larger ionic radii. Calcite and vaterite are pure calcium carbonate minerals. Industrially important source rocks which are predominantly calcium carbonate include limestone, chalk and travertine. Eggshells, snail shells and most seashells are predominantly calcium carbonate and can be used as industrial sources of that chemical. Oyster shells have enjoyed recent recognition as a source of dietary calcium, but are a practical industrial source. Dark green vegetables such as broccoli and kale contain dietarily significant amounts of calcium carbonate, they are not practical as an industrial source. Beyond Earth, strong evidence suggests the presence of calcium carbonate on Mars. Signs of calcium carbonate have been detected at more than one location; this provides some evidence for the past presence of liquid water.
Carbonate, is found in geologic settings and constitutes an enormous carbon reservoir. Calcium carbonate occurs as aragonite and dolomite as significant constituents of the calcium cycle; the carbonate minerals form the rock types: limestone, marble, travertine and others. In warm, clear tropical waters corals are more abundant than towards the poles where the waters are cold. Calcium carbonate contributors, including plankton, coralline algae, brachiopods, echinoderms and mollusks, are found in shallow water environments where sunlight and filterable food are more abundant. Cold-water carbonates do exist at higher latitudes but have a slow growth rate; the calcification processes are changed by ocean acidification. Where the oceanic crust is subducted under a continental plate sediments will be carried down to warmer zones in the asthenosphere and lithosphere. Under these conditions calcium carbonate decomposes to produce carbon dioxide which, along with other gases, give rise to explosive volcanic eruptions.
The carbonate compensation depth is the point in the ocean where the rate of precipitation of calcium carbonate is balanced by the rate of dissolution due to the conditions present. Deep in the ocean, the temperature pressure increases. Calcium carbonate is unusual in. Increasing pressure increases the solubility of calcium carbonate; the carbonate compensation depth can range from 4,000 to 6,000 meters below sea level. Calcium carbonate can preserve fossils through permineralization. Most of the vertebrate fossils of the Two Medicine Formation—a geologic formation known for its duck-billed dinosaur eggs—are preserved by CaCO3 permineralization; this type of preservation conserves high levels of detail down to the microscopic level. However, it leaves specimens vulnerable to weathering when exposed to the surface. Trilobite populations were once thought to have composed the majority of aquatic life during the Cambrian, due to the fact that their calcium carbonate-rich shells were more preserved than those of other species, which had purely chitinous shells.
The main use of calcium ca
Chalk is a soft, porous, sedimentary carbonate rock, a form of limestone composed of the mineral calcite. Calcite is an ionic salt called calcium carbonate or CaCO3, it forms under reasonably deep marine conditions from the gradual accumulation of minute calcite shells shed from micro-organisms called coccolithophores. Flint is common as bands parallel to the bedding or as nodules embedded in chalk, it is derived from sponge spicules or other siliceous organisms as water is expelled upwards during compaction. Flint is deposited around larger fossils such as Echinoidea which may be silicified. Chalk as seen in Cretaceous deposits of Western Europe is unusual among sedimentary limestones in the thickness of the beds. Most cliffs of chalk have few obvious bedding planes unlike most thick sequences of limestone such as the Carboniferous Limestone or the Jurassic oolitic limestones; this indicates stable conditions over tens of millions of years. Chalk has greater resistance to weathering and slumping than the clays with which it is associated, thus forming tall, steep cliffs where chalk ridges meet the sea.
Chalk hills, known as chalk downland 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 through dry seasons. Chalk is mined from chalk deposits both above underground. Chalk mining boomed during the Industrial Revolution, due to the need for chalk products such as quicklime and bricks; some abandoned chalk mines remain tourist destinations due to their massive expanse and natural beauty. The Chalk Group is a European stratigraphic unit, 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 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.
Ninety million years ago what is now the chalk downland of Northern Europe was ooze accumulating at the bottom of a great sea. Chalk was one of the earliest rocks made up of microscopic particles to be studied under the microscope, when it was found to be composed entirely of coccoliths, their shells were made of calcite extracted from the rich seawater. As they died, a substantial layer built up over millions of years and, through the weight of overlying sediments became consolidated into rock. Earth movements related to the formation of the Alps raised these former sea-floor deposits above sea level; the chemical composition of chalk is calcium carbonate, with minor amounts of clay. It is formed in the sea by sub-microscopic plankton, which fall to the sea floor and are consolidated and compressed during diagenesis into chalk rock. Most people first encounter the word "chalk" in school where it refers to blackboard chalk, made of mineral chalk, since it crumbles and leaves particles that stick loosely to rough surfaces, allowing it to make writing that can be erased.
Blackboard chalk manufacture now may use mineral chalk, other mineral sources of calcium carbonate, or the mineral gypsum. While gypsum-based blackboard chalk is the lowest cost to produce, thus used in the developing world, calcium-based chalk can be made where the crumbling particles are larger and thus produce less dust, is marketed as "dustless chalk". Colored chalks, pastel chalks, sidewalk chalk, used to draw on sidewalks and driveways, are made of gypsum. Chalk is a source of quicklime by thermal decomposition, or slaked lime following quenching of quicklime with water. In southeast England, deneholes are a notable example of ancient chalk pits; such bell pits may 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 the most famous is the extensive complex at Grimes Graves in Norfolk. 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. Builder's putty mainly contains chalk as a filler in linseed oil. Chalk may be used for its properties as a base. In agriculture, chalk is used for raising pH in soils with high acidity; the most common forms are CaCO3 and CaO. Small doses of chalk can be used as an antacid. Additionally, the small particles of chalk make it a substance ideal for polishing. For example, toothpaste contains small amounts of chalk, which serves as a mild abrasive. Polishing chalk is chalk prepared with a controlled grain size, for fine polishing of metals. Chalk can be used as fingerprint powder. Several traditional uses of chalk have been replaced by other substances, although the word "chalk" is still applied to the usual replacements. Tailor's chalk is traditionally a hard chalk used to make temporary markings on cloth by tailors, it is now made of talc. 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 p
Maerl is a collective name for non-geniculate coralline red algae with a certain growth habit. Maerl grows at a rate of c. 1 mm per year. It forms extensive beds in suitable sublittoral sites; the term maerl refers to the branched growth form of Lemoine and rhodolith is a sedimentological or genetic term for both the nodular and branched growth forms. In Europe maerl beds occur throughout the Mediterranean, along most of the Atlantic coast from Portugal to Norway, in the English Channel, Irish Sea and North Sea; the distribution of maerl is dependent on water movement and salinity concentration. Maerl beds occur in the photic zone, can be found to around 30 m depth in the British Isles and up to 120 m deep in the Mediterranean. Maerl deposits can reach up to 10 m thick, but are much thinner. In the British Isles maerl is composed of three species of coralline algae growing loose in beds of fragmented nodules in the sub-littoral; the species involved are: Lithothamnion corallioides, Lithothamnion glaciale and Phymatolithon calcareum.
Maerl is crushed to form a powder. It is still harvested around the coasts of Brittany in France and Bantry Bay, is a popular fertilizer for organic gardening, it was dredged off Falmouth, but this ceased in 2004. Scientists investigated Falmouth maerl and found that L. corallioides predominated down to 6 m and P. calcareum from 6–10 m. Chemical analysis of maerl showed that it contained 32.1% CaCO3 and 3.1% MgCO3. The ecology of maerl habitats has received little attention in contrast to other marine ecosystems such as kelp forests or sea grass beds. Maerl beds provide a complex habitat for a wide range of taxa with a variety of niches that support high associated invertebrate and algal biodiversity. Maerl beds act as nursery areas for the juvenile stages of commercial species such as juvenile cod Gadus morhua, saithe Pollachius virens, Pollack Pollachius pollachius and juvenile scallops Aequipecten opercularis. Maerl beds offer physical refuge and protection from predation as well as productive feeding grounds but are damaged by dredging and towed fishing gear.
Maerl has no tolerance for desiccation. Maerl has been extracted for centuries for use as an agricultural fertilizer; the amount extracted increased in the late 20th century and in 2000, maerl was extracted at c. 5,000 tonnes per year in Ireland and c. 500,000 tonnes per year in France. Large scale maerl extraction over the past 40 years has degraded maerl beds. In Cornwall, maerl has been extracted since the 1970s, but was banned in 2005 by Falmouth Harbour Commissioners. An early reference to maerl was made by John Ray in 1690. In Ireland, maerl is extracted from subfossil beds in Bantry Bay by Celtic Sea Minerals; the maerl-forming species Lithothamion corallioides and Phymatolithon calcareum are listed in Annex V of the EC Habitats Directive. Used as a soil conditioner, it is crushed to a powder; the slow growth of individual nodules and their accumulation in beds over a millennial timescale means that there is no possibility of maerl keeping up with dredging for this purpose. Maerl should be considered as a non-renewable resource, available alternative products make modern day exploitation controversial