Sand is a granular material composed of finely divided rock and mineral particles. It is defined by size, being finer than coarser than silt. Sand can refer to a textural class of soil or soil type; the composition of sand varies, depending on the local rock sources and conditions, but the most common constituent of sand in inland continental settings and non-tropical coastal settings is silica in the form of quartz. The second most common type of sand is calcium carbonate, for example, created, over the past half billion years, by various forms of life, like coral and shellfish. For example, it is the primary form of sand apparent in areas where reefs have dominated the ecosystem for millions of years like the Caribbean. Sand is a non-renewable resource over human timescales, sand suitable for making concrete is in high demand. Desert sand, although plentiful, is not suitable for concrete, 50 billion tons of beach sand and fossil sand is needed each year for construction; the exact definition of sand varies.
The scientific Unified Soil Classification System used in engineering and geology corresponds to US Standard Sieves, defines sand as particles with a diameter of between 0.074 and 4.75 millimeters. By another definition, in terms of particle size as used by geologists, sand particles range in diameter from 0.0625 mm to 2 mm. An individual particle in this range size is termed a sand grain. Sand grains are between silt; the size specification between sand and gravel has remained constant for more than a century, but particle diameters as small as 0.02 mm were considered sand under the Albert Atterberg standard in use during the early 20th century. The grains of sand in Archimedes Sand Reckoner written around 240 BCE, were 0.02 mm in diameter. A 1953 engineering standard published by the American Association of State Highway and Transportation Officials set the minimum sand size at 0.074 mm. A 1938 specification of the United States Department of Agriculture was 0.05 mm. Sand feels gritty when rubbed between the fingers.
Silt, by comparison, feels like flour). ISO 14688 grades sands as fine and coarse with ranges 0.063 mm to 0.2 mm to 0.63 mm to 2.0 mm. In the United States, sand is divided into five sub-categories based on size: fine sand, fine sand, medium sand, coarse sand, coarse sand; these sizes are based on the Krumbein phi scale, where size in Φ = -log2D. On this scale, for sand the value of Φ varies from −1 to +4, with the divisions between sub-categories at whole numbers; the most common constituent of sand, in inland continental settings and non-tropical coastal settings, is silica in the form of quartz, because of its chemical inertness and considerable hardness, is the most common mineral resistant to weathering. The composition of mineral sand is variable, depending on the local rock sources and conditions; the bright white sands found in tropical and subtropical coastal settings are eroded limestone and may contain coral and shell fragments in addition to other organic or organically derived fragmental material, suggesting sand formation depends on living organisms, too.
The gypsum sand dunes of the White Sands National Monument in New Mexico are famous for their bright, white color. Arkose is a sand or sandstone with considerable feldspar content, derived from weathering and erosion of a granitic rock outcrop; some sands contain magnetite, glauconite or gypsum. Sands rich in magnetite are dark to black in color, as are sands derived from volcanic basalts and obsidian. Chlorite-glauconite bearing sands are green in color, as are sands derived from basaltic lava with a high olivine content. Many sands those found extensively in Southern Europe, have iron impurities within the quartz crystals of the sand, giving a deep yellow color. Sand deposits in some areas contain garnets and other resistant minerals, including some small gemstones. Rocks erode/weather over a long period of time by water and wind, their sediments are transported downstream; these sediments continue to break apart into smaller pieces. The type of rock the sediment originated from and the intensity of the environment gives different compositions of sand.
The most common rock to form sand is Granite, where the Feldspar minerals dissolve faster than the Quartz, causing the rock to break apart into small pieces. In high energy environments rocks break apart much faster than in more calm settings. For example, Granite rocks this means more Feldspar minerals in the sand because it wouldn't have had time to dissolve; the term for sand formed by weathering is epiclastic. Sand from rivers are collected either from the river itself or its flood plain, accounts for the majority of the sand used in the construction industry; because if this, many small rivers have been depleted, causing environmental concern and economic losses to adjacent land. The rate of sand mining in such areas outweighs the rate the sand can replenish, making it a non-renewable resource. Sand dunes are a consequence of wind deposition; the Sahara Desert is dry because of its geographic location and is known for its vast sand dunes. They exist here because little vegetation is able to grow and there's not a lot of water.
Over time, wind blow
Sodium carbonate, Na2CO3, is the inorganic compound with the formula Na2CO3 and its various hydrates. All forms are white, water-soluble salts. All forms have a alkaline taste and give moderately alkaline solutions in water, it was extracted from the ashes of plants growing in sodium-rich soils. Because the ashes of these sodium-rich plants were noticeably different from ashes of wood, sodium carbonate became known as "soda ash", it is produced in large quantities from sodium limestone by the Solvay process. Sodium carbonate is obtained as three different hydrates and as the anhydrous salt: sodium carbonate decahydrate, Na2CO3·10H2O, which effloresces to form the monohydrate. Sodium carbonate heptahydrate, Na2CO3·7H2O. Sodium carbonate monohydrate, Na2CO3·H2O. Known as crystal carbonate. Anhydrous sodium carbonate known as calcined soda, is formed by heating the hydrates, it is formed when sodium hydrogen carbonate is heated e.g. in the final step of the Solvay process. The decahydrate is formed from water solutions crystallizing in the temperature range -2.1 to +32.0 C, the heptahydrate in the narrow range 32.0 to 35.4 C and above this temperature the monohydrate forms.
In dry air the decahydrate and heptahydrate lose water to give the monohydrate. Other hydrates have been reported. In terms of its largest applications, sodium carbonate is used in the manufacture of glass, rayon and detergents. Sodium carbonate serves as a flux for silica, lowering the melting point of the mixture to something achievable without special materials; this "soda glass" is mildly water-soluble, so some calcium carbonate is added to the melt mixture to make the glass produced insoluble. Bottle and window glass is made by melting such mixtures of sodium carbonate, calcium carbonate, silica sand; when these materials are heated, the carbonates release carbon dioxide. In this way, sodium carbonate is a source of sodium oxide. Soda lime glass has been the most common form of glass for centuries. Sodium carbonate is used to soften water by removing Mg2+ and Ca2+; these ions form insoluble solid precipitates upon treatment with carbonate ions: Ca2+ + CO32- → CaCO3Sodium carbonate is an inexpensive and water-soluble source of carbonate ions.
Sodium carbonate is a food additive used as an acidity regulator, anticaking agent, raising agent, stabilizer. It is one of the components of kansui, a solution of alkaline salts used to give ramen noodles their characteristic flavor and texture, it is used in the production of snus to stabilize the pH of the final product. Sodium carbonate is used in the production of sherbet powder; the cooling and fizzing sensation results from the endothermic reaction between sodium carbonate and a weak acid citric acid, releasing carbon dioxide gas, which occurs when the sherbet is moistened by saliva. In China, it is used to replace lye-water in the crust of traditional Cantonese moon cakes, in many other Chinese steamed buns and noodles. In cooking, it is sometimes used in place of sodium hydroxide for lyeing with German pretzels and lye rolls; these dishes are treated with a solution of an alkaline substance to change the pH of the surface of the food and improve browning. Sodium carbonate is used as a strong base in various fields.
As a common alkali, it is preferred in many chemical processes because it is cheaper than NaOH and far safer to handle. Its mildness recommends its use in domestic applications. For example, it is used as a pH regulator to maintain stable alkaline conditions necessary for the action of the majority of photographic film developing agents. For example, it is a common additive in swimming pools and aquarium water to maintain a desired pH and carbonate hardness. In dyeing with fiber-reactive dyes, sodium carbonate is used to ensure proper chemical bonding of the dye with cellulose fibers before dyeing, mixed with the dye, or after dyeing. Sodium bicarbonate or baking soda a component in fire extinguishers, is generated from sodium carbonate. Although NaHCO3 is itself an intermediate product of the Solvay process, the heating needed to remove the ammonia that contaminates it decomposes some NaHCO3, making it more economic to react finished Na2CO3 with CO2: Na2CO3 + CO2 + H2O → 2NaHCO3In a related reaction, sodium carbonate is used to make sodium bisulphite, used for the "sulfite" method of separating lignin from cellulose.
This reaction is exploited for removing sulphur dioxide from flue gases in power stations: Na2CO3 + SO2 + H2O → NaHCO3 + NaHSO3This application has become more common where stations have to meet stringent emission controls. Sodium carbonate is used by the cotton industry to neutralize the sulfuric acid needed for acid delinting of fuzzy cottonseed. Sodium carbonate is used by the brick industry as a wetting agent to reduce the amount of water needed to extrude the clay. In casting, it is referred to as "bonding agent" and is used to allow wet alginate to adhere to gelled alginate. Sodium carbonate is used in toothpastes, where it acts as a foaming agent and an abrasive, to temporarily increase mouth pH; the integral enthalpy of solution of sodium carbonate is −28.1 kJ/mol for a 10% w/w aqueous solution. The Mohs hardness of sodium carbonate monohydrate is 1.3. Sodium carbonate is soluble in water, can occur nat
A building, or edifice, is a structure with a roof and walls standing more or less permanently in one place, such as a house or factory. Buildings come in a variety of sizes and functions, have been adapted throughout history for a wide number of factors, from building materials available, to weather conditions, land prices, ground conditions, specific uses, aesthetic reasons. To better understand the term building compare the list of nonbuilding structures. Buildings serve several societal needs – as shelter from weather, living space, privacy, to store belongings, to comfortably live and work. A building as a shelter represents a physical division of the outside. Since the first cave paintings, buildings have become objects or canvasses of much artistic expression. In recent years, interest in sustainable planning and building practices has become an intentional part of the design process of many new buildings; the word building is the act of making it. As a noun, a building is'a structure that has a roof and walls and stands more or less permanently in one place'.
In the broadest interpretation a fence or wall is a building. However, the word structure is used more broadly than building including natural and man-made formations and does not have walls. Structure is more to be used for a fence. Sturgis' Dictionary included that " differs from architecture in excluding all idea of artistic treatment; as a verb, building is the act of construction. Structural height in technical usage is the height to the highest architectural detail on building from street-level. Depending on how they are classified and masts may or may not be included in this height. Spires and masts used as antennas are not included; the definition of a low-rise vs. a high-rise building is a matter of debate, but three storeys or less is considered low-rise. A report by Shinichi Fujimura of a shelter built 500 000 years ago is doubtful since Fujimura was found to have faked many of his findings. Supposed remains of huts found at the Terra Amata site in Nice purportedly dating from 200 000 to 400 000 years ago have been called into question.
There is clear evidence of homebuilding from around 18 000 BC. Buildings became common during the Neolithic. Single-family residential buildings are most called houses or homes. Multi-family residential buildings containing more than one dwelling unit are called a duplex or an apartment building. A condominium is an apartment rather than rents. Houses may be built in pairs, in terraces where all but two of the houses have others either side. Houses which were built as a single dwelling may be divided into apartments or bedsitters. Building types may range from huts to multimillion-dollar high-rise apartment blocks able to house thousands of people. Increasing settlement density in buildings is a response to high ground prices resulting from many people wanting to live close to work or similar attractors. Other common building materials are concrete or combinations of either of these with stone. Residential buildings have different names for their use depending if they are seasonal include holiday cottage or timeshare.
If the residents are in need of special care such as a nursing home, orphanage or prison. Many people lived in communal buildings called longhouses, smaller dwellings called pit-houses and houses combined with barns sometimes called housebarns. Buildings are defined to be substantial, permanent structures so other dwelling forms such as houseboats and motorhomes are dwellings but not buildings. Sometimes a group of inter-related builds are referred to as a complex – for example a housing complex, educational complex, hospital complex, etc; the practice of designing and operating buildings is most a collective effort of different groups of professionals and trades. Depending on the size and purpose of a particular building project, the project team may include: A real estate developer who secures funding for the project. Other possible design Engineer specialists may be involved such as Fire, facade engineers, building physics, Telecomms, AV (Audio V
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
A stalactite is a type of formation that hangs from the ceiling of caves, hot springs, or manmade structures such as bridges and mines. Any material, soluble, can be deposited as a colloid, or is in suspension, or is capable of being melted, may form a stalactite. Stalactites may be composed of lava, mud, pitch, sand and amberat. A stalactite is not a speleothem, though speleothems are the most common form of stalactite because of the abundance of limestone caves; the corresponding formation on the floor of the cave is known as a stalagmite. The most common stalactites are speleothems, they form through deposition of calcium carbonate and other minerals, precipitated from mineralized water solutions. Limestone is the chief form of calcium carbonate rock, dissolved by water that contains carbon dioxide, forming a calcium bicarbonate solution in underground caverns; the chemical formula for this reaction is: CaCO3 + H2O + CO2 → Ca2This solution travels through the rock until it reaches an edge and if this is on the roof of a cave it will drip down.
When the solution comes into contact with air the chemical reaction that created it is reversed and particles of calcium carbonate are deposited. The reversed reaction is: Ca2 → CaCO3 + H2O + CO2An average growth rate is 0.13 mm a year. The quickest growing stalactites are those formed by a constant supply of slow dripping water rich in calcium carbonate and carbon dioxide, which can grow at 3 mm per year; the drip rate must be slow enough to allow the CO2 to degas from the solution into the cave atmosphere, resulting in deposition of CaCO3 on the stalactite. Too fast a drip rate and the solution, still carrying most of the CaCO3, falls to the cave floor where degassing occurs and CaCO3 is deposited as a stalagmite. All limestone stalactites begin with a single mineral-laden drop of water; when the drop falls, it deposits the thinnest ring of calcite. Each subsequent drop that forms and falls deposits another calcite ring; these rings form a narrow, hollow tube known as a "soda straw" stalactite.
Soda straws can grow quite long, but are fragile. If they become plugged by debris, water begins flowing over the outside, depositing more calcite and creating the more familiar cone-shaped stalactite; the same water drops that fall from the tip of a stalactite deposit more calcite on the floor below resulting in a rounded or cone-shaped stalagmite. Unlike stalactites, stalagmites never start out as hollow "soda straws". Given enough time, these formations can meet and fuse to create pillars of calcium carbonate known as a "column". Stalactite formation begins over a large area, with multiple paths for the mineral rich water to flow; as minerals are dissolved in one channel more than other competing channels, the dominant channel begins to draw more and more of the available water, which speeds its growth resulting in all other channels being choked off. This is one reason; the larger the formation, the greater the interformation distance. Another type of stalactite is formed in lava tubes; the mechanism of formation is the deposition of material on the ceilings of caves, however with lava stalactites formation happens quickly in only a matter of hours, days, or weeks, whereas limestone stalactites may take up to thousands of years.
A key difference with lava stalactites is that once the lava has ceased flowing, so too will the stalactites cease to grow. This means; the generic term lavacicle has been applied to lava stalactites and stalagmites indiscriminately and evolved from the word icicle. Like limestone stalactites, they can leave lava drips on the floor that turn into lava stalagmites and may fuse with the corresponding stalactite to form a column. Shark tooth stalactites, it may begin as a small driblet of lava from a semi-solid ceiling, but grows by accreting layers as successive flows of lava rise and fall in the lava tube and recoating the stalactite with more material. They can vary from a few millimeters to over a meter in length. Splash stalactites As lava flows through a tube, material will be splashed up on the ceiling and ooze back down, hardening into a stalactite; this type of formation results in a irregularly shaped stalactite, looking somewhat like stretched taffy. They may be of a different color than the original lava that formed the cave.
Tubular lava stalactites When the roof of a lava tube is cooling, a skin will form that traps semi-molten material inside. Trapped gases force lava to extrude out through small openings that result in hollow, tubular stalactites analogous to the soda straws formed as depositional speleothems in solution caves, The longest known is 2 meters in length; these are common in Hawaiian lava tubes and are associated with a drip stalagmite that forms below as material is carried through the tubular stalactite and piles up on the floor beneath. Sometimes the tubular form collapses near the distal end, most when the pressure of escaping gases decreased and still-molten portions of the stalactites deflated and cooled; these tubular stalactites will acquire a twisted, vermiform appearance as bits of lava crystallize and force the flow in different directions. These tubular lava helictites may be influenced by air
Germany the Federal Republic of Germany, is a country in Central and Western Europe, lying between the Baltic and North Seas to the north, the Alps to the south. It borders Denmark to the north and the Czech Republic to the east and Switzerland to the south, France to the southwest, Luxembourg and the Netherlands to the west. Germany includes 16 constituent states, covers an area of 357,386 square kilometres, has a temperate seasonal climate. With 83 million inhabitants, it is the second most populous state of Europe after Russia, the most populous state lying in Europe, as well as the most populous member state of the European Union. Germany is a decentralized country, its capital and largest metropolis is Berlin, while Frankfurt serves as its financial capital and has the country's busiest airport. Germany's largest urban area is the Ruhr, with its main centres of Essen; the country's other major cities are Hamburg, Cologne, Stuttgart, Düsseldorf, Dresden, Bremen and Nuremberg. Various Germanic tribes have inhabited the northern parts of modern Germany since classical antiquity.
A region named Germania was documented before 100 AD. During the Migration Period, the Germanic tribes expanded southward. Beginning in the 10th century, German territories formed a central part of the Holy Roman Empire. During the 16th century, northern German regions became the centre of the Protestant Reformation. After the collapse of the Holy Roman Empire, the German Confederation was formed in 1815; the German revolutions of 1848–49 resulted in the Frankfurt Parliament establishing major democratic rights. In 1871, Germany became a nation state when most of the German states unified into the Prussian-dominated German Empire. After World War I and the revolution of 1918–19, the Empire was replaced by the parliamentary Weimar Republic; the Nazi seizure of power in 1933 led to the establishment of a dictatorship, the annexation of Austria, World War II, the Holocaust. After the end of World War II in Europe and a period of Allied occupation, Austria was re-established as an independent country and two new German states were founded: West Germany, formed from the American and French occupation zones, East Germany, formed from the Soviet occupation zone.
Following the Revolutions of 1989 that ended communist rule in Central and Eastern Europe, the country was reunified on 3 October 1990. Today, the sovereign state of Germany is a federal parliamentary republic led by a chancellor, it is a great power with a strong economy. As a global leader in several industrial and technological sectors, it is both the world's third-largest exporter and importer of goods; as a developed country with a high standard of living, it upholds a social security and universal health care system, environmental protection, a tuition-free university education. The Federal Republic of Germany was a founding member of the European Economic Community in 1957 and the European Union in 1993, it is part of the Schengen Area and became a co-founder of the Eurozone in 1999. Germany is a member of the United Nations, NATO, the G7, the G20, the OECD. Known for its rich cultural history, Germany has been continuously the home of influential and successful artists, musicians, film people, entrepreneurs, scientists and inventors.
Germany has a large number of World Heritage sites and is among the top tourism destinations in the world. The English word Germany derives from the Latin Germania, which came into use after Julius Caesar adopted it for the peoples east of the Rhine; the German term Deutschland diutisciu land is derived from deutsch, descended from Old High German diutisc "popular" used to distinguish the language of the common people from Latin and its Romance descendants. This in turn descends from Proto-Germanic *þiudiskaz "popular", derived from *þeudō, descended from Proto-Indo-European *tewtéh₂- "people", from which the word Teutons originates; the discovery of the Mauer 1 mandible shows that ancient humans were present in Germany at least 600,000 years ago. The oldest complete hunting weapons found anywhere in the world were discovered in a coal mine in Schöningen between 1994 and 1998 where eight 380,000-year-old wooden javelins of 1.82 to 2.25 m length were unearthed. The Neander Valley was the location where the first non-modern human fossil was discovered.
The Neanderthal 1 fossils are known to be 40,000 years old. Evidence of modern humans dated, has been found in caves in the Swabian Jura near Ulm; the finds included 42,000-year-old bird bone and mammoth ivory flutes which are the oldest musical instruments found, the 40,000-year-old Ice Age Lion Man, the oldest uncontested figurative art discovered, the 35,000-year-old Venus of Hohle Fels, the oldest uncontested human figurative art discovered. The Nebra sky disk is a bronze artefact created during the European Bronze Age attributed to a site near Nebra, Saxony-Anhalt, it is part of UNESCO's Memory of the World Programme. The Germanic tribes are thought to date from the Pre-Roman Iron Age. From southern Scandinavia and north Germany, they expanded south and west from the 1st century BC, coming into contact with the Celtic tribes of Gaul as well
Polymorphism (materials science)
In materials science, polymorphism is the ability of a solid material to exist in more than one form or crystal structure. Polymorphism can be found in any crystalline material including polymers and metals, is related to allotropy, which refers to chemical elements; the complete morphology of a material is described by polymorphism and other variables such as crystal habit, amorphous fraction or crystallographic defects. Polymorphism is relevant to the fields of pharmaceuticals, pigments, dyestuffs and explosives; when polymorphism exists as a result of a difference in crystal packing, it is called packing polymorphism. Polymorphism can result from the existence of different conformers of the same molecule in conformational polymorphism. In pseudopolymorphism the different crystal types are the result of solvation; this is more referred to as solvomorphism as different solvates have different chemical formulae. An example of an organic polymorph is glycine, able to form monoclinic and hexagonal crystals.
Silica is known to form many polymorphs. A classical example is the pair of minerals and aragonite, both forms of calcium carbonate. An analogous phenomenon for amorphous materials is polyamorphism, when a substance can take on several different amorphous modifications. In terms of thermodynamics, there are two types of polymorphic behaviour. For a monotropic system, a plot of the free energy of the various polymorphs against temperature do not cross before all polymorphs melt—in other words, any transition from one polymorph to another below melting point will be irreversible. For an enantiotropic system, a plot of the free energy against temperature shows a crossing point threshold before the various melting points, it may be possible to revert interchangeably between the two polymorphs by heating or cooling, or through physical contact with a lower energy polymorph. The first observation of polymorphism in organic materials is attributed to Friedrich Wöhler and Justus von Liebig when in 1832 they examined a boiling solution of benzamide: upon cooling, the benzamide crystallised as silky needles, but when standing these were replaced by rhombic crystals.
Present-day analysis identifies three polymorphs for benzamide: the least stable one, formed by flash cooling is the orthorhombic form II. This type is followed by the monoclinic form III; the most stable form is monoclinic form I. The hydrogen bonding mechanisms are the same for all three phases. Polymorphs have different stabilities and may spontaneously convert from a metastable form to the stable form at a particular temperature. Most polymorphs of organic molecules only differ by a few kJ/mol in lattice energy. 50% of known polymorph pairs differ by less than 2 kJ/mol and stability differences of more than 10 kJ/mol are rare. They exhibit different melting points, solubilities, X-ray crystal and diffraction patterns. Various conditions in the crystallisation process is the main reason responsible for the development of different polymorphic forms; these conditions include: Solvent effects Certain impurities inhibiting growth pattern and favour the growth of a metastable polymorphs The level of supersaturation from which material is crystallised Temperature at which crystallisation is carried out Geometry of covalent bonds Change in stirring conditionsDespite the potential implications, polymorphism is not always well understood.
In 2006 a new crystal form of maleic acid was discovered 124 years after the first crystal form was studied. Maleic acid is a chemical manufactured on a large scale in the chemical industry and is a salt forming component in medicine; the new crystal type is produced when a co-crystal of caffeine and maleic acid is dissolved in chloroform and when the solvent is allowed to evaporate slowly. Whereas form I has monoclinic space group P21/c, the new form has space group Pc. Both polymorphs consist of sheets of molecules connected through hydrogen bonding of the carboxylic acid groups. 1,3,5-Trinitrobenzene is more than 125 years old and was used as an explosive before the arrival of the safer 2,4,6-trinitrotoluene. Only one crystal form of 1,3,5-trinitrobenzene was known in the space group Pbca. In 2004, a second polymorph was obtained in the space group Pca21 when the compound was crystallised in the presence of an additive, trisindane; this experiment shows. Walter McCrone has stated that "every compound has different polymorphic forms, that, in general, the number of forms known for a given compound is proportional to the time and money spent in research on that compound."
Ostwald's rule or Ostwald's step rule, conceived by Wilhelm Ostwald, states that in general it is not the most stable but the least stable polymorph that crystallises first. See for examples the aforementioned benzamide, dolomite or phosphorus, which on sublimation first forms the less stable white and the more stable red allotrope. Ostwald suggested that the solid first formed on crystallisation of a solution or a melt would be the least s