The heel is the prominence at the posterior end of the foot. It is based on the projection of one bone, the calcaneus or heel bone, behind the articulation of the bones of the lower leg. To distribute the compressive forces exerted on the heel during gait, the stance phase when the heel contacts the ground, the sole of the foot is covered by a layer of subcutaneous connective tissue up to 2 cm thick; this tissue has a system of pressure chambers that both acts as a shock absorber and stabilises the sole. Each of these chambers contains fibrofatty tissue covered by a layer of tough connective tissue made of collagen fibers; these septa are attached both to the plantar aponeurosis above and the sole's skin below. The sole of the foot is one of the most vascularized regions of the body surface, the dense system of blood vessels further stabilize the septa; the Achilles tendon is the muscle tendon of the triceps surae, a "three-headed" group of muscles—the soleus and the two heads of the gastrocnemius.
The main function of the triceps surae is plantar flexion, i.e. to stretch the foot downward. It is accompanied by a "fourth head", the slight plantaris muscle, the long slender tendon of, attached to the heel bone but not visible; the compressive forces applied to the foot are distributed along five rays, three medial and two lateral. The lateral rays stretch over the cuboid bone to the heel bone and the medial rays over the three cuneiform bones and the navicular bone to the ankle bone; because the ankle bone is placed over the heel bone, these rays are adjacent near the toes but overriding near the heel, together they form the arches of the foot that are optimized to distributed compressive forces across an uneven terrain. In this context the heel thus forms the posterior point of support that together with the balls of the large and little toes bear the brunt of the loads. Cracked heels is a common health problem and it may cause infections, it is caused by dryness of the foot skin, accumulation of dead skin.
Over time it may cause pain and irritations. Various moisturising creams and foot files are available to prevent it. In the long-footed mammals, both the hoofed species and the clawed forms which walk on the toes, the heel is well above the ground at the apex of the angular joint known as the hock. In plantigrade species it rests on the ground. In birds, the heel is the backward-pointing joint, mistaken as the "knee". Achilles' heel a metaphor for weakness Heel of Italy, the SE Ball of the foot Calcaneal spur heel-bone Callus Hard skin which may cause painful cracks in the heel and sole of the foot Plantar fasciitis "policeman's heel" disorder High-heeled footwear fashion Squatting position Thieme Atlas of Anatomy: General Anatomy and Musculoskeletal System. Thieme. 2006. ISBN 1-58890-419-9
Electronic music is music that employs electronic musical instruments, digital instruments and circuitry-based music technology. In general, a distinction can be made between sound produced using electromechanical means, that produced using electronics only. Electromechanical instruments include mechanical elements, such as strings, so on, electric elements, such as magnetic pickups, power amplifiers and loudspeakers. Examples of electromechanical sound producing devices include the telharmonium, Hammond organ, the electric guitar, which are made loud enough for performers and audiences to hear with an instrument amplifier and speaker cabinet. Pure electronic instruments do not have vibrating strings, hammers, or other sound-producing mechanisms. Devices such as the theremin and computer can produce electronic sounds; the first electronic devices for performing music were developed at the end of the 19th century, shortly afterward Italian futurists explored sounds that had not been considered musical.
During the 1920s and 1930s, electronic instruments were introduced and the first compositions for electronic instruments were made. By the 1940s, magnetic audio tape allowed musicians to tape sounds and modify them by changing the tape speed or direction, leading to the development of electroacoustic tape music in the 1940s, in Egypt and France. Musique concrète, created in Paris in 1948, was based on editing together recorded fragments of natural and industrial sounds. Music produced from electronic generators was first produced in Germany in 1953. Electronic music was created in Japan and the United States beginning in the 1950s. An important new development was the advent of computers to compose music. Algorithmic composition with computers was first demonstrated in the 1950s. In the 1960s, live electronics were pioneered in America and Europe, Japanese electronic musical instruments began influencing the music industry, Jamaican dub music emerged as a form of popular electronic music. In the early 1970s, the monophonic Minimoog synthesizer and Japanese drum machines helped popularize synthesized electronic music.
In the 1970s, electronic music began having a significant influence on popular music, with the adoption of polyphonic synthesizers, electronic drums, drum machines, turntables, through the emergence of genres such as disco, new wave, synth-pop, hip hop and EDM. In the 1980s, electronic music became more dominant in popular music, with a greater reliance on synthesizers, the adoption of programmable drum machines such as the Roland TR-808 and bass synthesizers such as the TB-303. In the early 1980s, digital technologies for synthesizers including digital synthesizers such as the Yamaha DX7 were popularized, a group of musicians and music merchants developed the Musical Instrument Digital Interface. Electronically produced music became prevalent in the popular domain by the 1990s, because of the advent of affordable music technology. Contemporary electronic music includes many varieties and ranges from experimental art music to popular forms such as electronic dance music. Today, pop electronic music is most recognizable in its 4/4 form and more connected with the mainstream culture as opposed to its preceding forms which were specialized to niche markets.
At the turn of the 20th century, experimentation with emerging electronics led to the first electronic musical instruments. These initial inventions were not sold, but were instead used in demonstrations and public performances; the audiences were presented with reproductions of existing music instead of new compositions for the instruments. While some were considered novelties and produced simple tones, the Telharmonium synthesized the sound of orchestral instruments, it achieved viable public interest and made commercial progress into streaming music through telephone networks. Critics of musical conventions at the time saw promise in these developments. Ferruccio Busoni encouraged the composition of microtonal music allowed for by electronic instruments, he predicted the use of machines in future music, writing the influential Sketch of a New Esthetic of Music. Futurists such as Francesco Balilla Pratella and Luigi Russolo began composing music with acoustic noise to evoke the sound of machinery.
They predicted expansions in timbre allowed for by electronics in the influential manifesto The Art of Noises. Developments of the vacuum tube led to electronic instruments that were smaller and more practical for performance. In particular, the theremin, ondes Martenot and trautonium were commercially produced by the early 1930s. From the late 1920s, the increased practicality of electronic instruments influenced composers such as Joseph Schillinger to adopt them, they were used within orchestras, most composers wrote parts for the theremin that could otherwise be performed with string instruments. Avant-garde composers criticized the predominant use of electronic instruments for conventional purposes; the instruments offered expansions in pitch resources that were exploited by advocates of microtonal music such as Charles Ives, Dimitrios Levidis, Olivier Messiaen and Edgard Varèse. Further, Percy Grainger used the theremin to abandon fixed tonation while Russian composers such as Gavriil Popov treated it as a source of noise in otherwise-acoustic noise music.
Developments in early recording technology paralleled that of electronic instruments. The first means of recording and reproducing audio was invented in the late 19th century with the mechanical phonograph. Record players became a common household item, by the 1920s comp
An oxide is a chemical compound that contains at least one oxygen atom and one other element in its chemical formula. "Oxide" itself is the dianion of an O2 -- atom. Metal oxides thus contain an anion of oxygen in the oxidation state of −2. Most of the Earth's crust consists of solid oxides, the result of elements being oxidized by the oxygen in air or in water. Hydrocarbon combustion affords the two principal carbon oxides: carbon monoxide and carbon dioxide. Materials considered pure elements develop an oxide coating. For example, aluminium foil develops a thin skin of Al2O3 that protects the foil from further corrosion. Individual elements can form multiple oxides, each containing different amounts of the element and oxygen. In some cases these are distinguished by specifying the number of atoms as in carbon monoxide and carbon dioxide, in other cases by specifying the element's oxidation number, as in iron oxide and iron oxide. Certain elements can form many different oxides, such as those of nitrogen.
Due to its electronegativity, oxygen forms stable chemical bonds with all elements to give the corresponding oxides. Noble metals are prized because they resist direct chemical combination with oxygen, substances like gold oxide must be generated by indirect routes. Two independent pathways for corrosion of elements are oxidation by oxygen; the combination of water and oxygen is more corrosive. All elements burn in an atmosphere of oxygen or an oxygen-rich environment. In the presence of water and oxygen, some elements— sodium—react to give the hydroxides. In part, for this reason and alkaline earth metals are not found in nature in their metallic, i.e. native, form. Cesium is so reactive with oxygen that it is used as a getter in vacuum tubes, solutions of potassium and sodium, so-called NaK are used to deoxygenate and dehydrate some organic solvents; the surface of most metals consists of hydroxides in the presence of air. A well-known example is aluminium foil, coated with a thin film of aluminium oxide that passivates the metal, slowing further corrosion.
The aluminum oxide layer can be built to greater thickness by the process of electrolytic anodizing. Though solid magnesium and aluminum react with oxygen at STP—they, like most metals, burn in air, generating high temperatures. Finely grained powders of most metals can be dangerously explosive in air, they are used in solid-fuel rockets. In dry oxygen, iron forms iron oxide, but the formation of the hydrated ferric oxides, Fe2O3−x2x, that comprise rust requires oxygen and water. Free oxygen production by photosynthetic bacteria some 3.5 billion years ago precipitated iron out of solution in the oceans as Fe2O3 in the economically important iron ore hematite. Oxides have a range of different structures, from individual molecules to polymeric and crystalline structures. At standard conditions, oxides may range from solids to gases. Oxides of most metals adopt polymeric structures; the oxide links three metal atoms or six metal atoms. Because the M-O bonds are strong and these compounds are crosslinked polymers, the solids tend to be insoluble in solvents, though they are attacked by acids and bases.
The formulas are deceptively simple. Many are nonstoichiometric compounds; some important gaseous oxides. Examples of molecular oxides are carbon monoxide. All simple oxides of nitrogen are molecular, e.g. NO, N2O, NO2 and N2O4. Phosphorus pentoxide is a more complex molecular oxide with a deceptive name, the real formula being P4O10; some polymeric oxides depolymerize when heated to give molecules, examples being selenium dioxide and sulfur trioxide. Tetroxides are rare; the more common examples: ruthenium tetroxide, osmium tetroxide, xenon tetroxide. Many oxyanions are known, such as polyoxometalates. Oxycations are rarer, some examples being nitrosonium and uranyl. Of course many compounds are known with other groups. In organic chemistry, these include many related carbonyl compounds. For the transition metals, many oxo complexes are known as well as oxyhalides. Conversion of a metal oxide to the metal is called reduction; the reduction can be induced by many reagents. Many metal oxides convert to metals by heating.
Metals are "won" from their oxides by chemical reduction, i.e. by the addition of a chemical reagent. A common and cheap reducing agent is carbon in the form of coke; the most prominent example is that of iron ore smelting. Many reactions are involved, but the simplified equation is shown as: 2 Fe2O3 + 3 C → 4 Fe + 3 CO2Metal oxides can be reduced by organic compounds; this redox process is the basis for many important transformations in chemistry, such as the detoxification of drugs by the P450 enzymes and the production of ethylene oxide, converted to antifreeze. In such systems, the metal center transfers an oxide ligand to the organic compound followed by regeneration of the metal oxide by oxygen in the air. Metals that are lower in the reactivity series can be reduced by heating alone. For example, silver oxide decomposes at 200 °C: 2 Ag2O → 4 Ag + O2 Metals that are more reactive displace the oxide of the metals that are less reactive. For example, zinc is more reactive than copper, so it displaces copper oxide to form zinc oxide: Zn + CuO → ZnO + Cu Apart from metals, hydrogen can displace metal oxides to form hydrogen oxide
Combustion, or burning, is a high-temperature exothermic redox chemical reaction between a fuel and an oxidant atmospheric oxygen, that produces oxidized gaseous products, in a mixture termed as smoke. Combustion in a fire produces a flame, the heat produced can make combustion self-sustaining. Combustion is a complicated sequence of elementary radical reactions. Solid fuels, such as wood and coal, first undergo endothermic pyrolysis to produce gaseous fuels whose combustion supplies the heat required to produce more of them. Combustion is hot enough that incandescent light in the form of either glowing or a flame is produced. A simple example can be seen in the combustion of hydrogen and oxygen into water vapor, a reaction used to fuel rocket engines; this reaction releases 242 kJ/mol of heat and reduces the enthalpy accordingly: 2H2 + O2 → 2H2OCombustion of an organic fuel in air is always exothermic because the double bond in O2 is much weaker than other double bonds or pairs of single bonds, therefore the formation of the stronger bonds in the combustion products CO2 and H2O results in the release of energy.
The bond energies in the fuel play only a minor role, since they are similar to those in the combustion products. The heat of combustion is -418 kJ per mole of O2 used up in the combustion reaction, can be estimated from the elemental composition of the fuel. Uncatalyzed combustion in air requires high temperatures. Complete combustion is stoichiometric with respect to the fuel, where there is no remaining fuel, ideally, no remaining oxidant. Thermodynamically, the chemical equilibrium of combustion in air is overwhelmingly on the side of the products. However, complete combustion is impossible to achieve, since the chemical equilibrium is not reached, or may contain unburnt products such as carbon monoxide and carbon. Thus, the produced smoke is toxic and contains unburned or oxidized products. Any combustion at high temperatures in atmospheric air, 78 percent nitrogen, will create small amounts of several nitrogen oxides referred to as NO x, since the combustion of nitrogen is thermodynamically favored at high, but not low temperatures.
Since combustion is clean, flue gas cleaning or catalytic converters may be required by law. Fires occur ignited by lightning strikes or by volcanic products. Combustion was the first controlled chemical reaction discovered by humans, in the form of campfires and bonfires, continues to be the main method to produce energy for humanity; the fuel is carbon, hydrocarbons or more complicated mixtures such as wood that contains oxidized hydrocarbons. The thermal energy produced from combustion of either fossil fuels such as coal or oil, or from renewable fuels such as firewood, is harvested for diverse uses such as cooking, production of electricity or industrial or domestic heating. Combustion is currently the only reaction used to power rockets. Combustion is used to destroy waste, both nonhazardous and hazardous. Oxidants for combustion have high oxidation potential and include atmospheric or pure oxygen, fluorine, chlorine trifluoride, nitrous oxide and nitric acid. For instance, hydrogen burns in chlorine to form hydrogen chloride with the liberation of heat and light characteristic of combustion.
Although not catalyzed, combustion can be catalyzed by platinum or vanadium, as in the contact process. In complete combustion, the reactant burns in oxygen, produces a limited number of products; when a hydrocarbon burns in oxygen, the reaction will yield carbon dioxide and water. When elements are burned, the products are the most common oxides. Carbon will yield carbon dioxide, sulfur will yield sulfur dioxide, iron will yield iron oxide. Nitrogen is not considered to be a combustible substance when oxygen is the oxidant, but small amounts of various nitrogen oxides form when the air is the oxidant. Combustion is not favorable to the maximum degree of oxidation, it can be temperature-dependent. For example, sulfur trioxide is not produced quantitatively by the combustion of sulfur. NOx species appear in significant amounts above about 2,800 °F, more is produced at higher temperatures; the amount of NOx is a function of oxygen excess. In most industrial applications and in fires, air is the source of oxygen.
In the air, each mole of oxygen is mixed with 3.71 mol of nitrogen. Nitrogen does not take part in combustion, but at high temperatures some nitrogen will be converted to NOx. On the other hand, when there is insufficient oxygen to combust the fuel, some fuel carbon is converted to carbon monoxide and some of the hydrogen remains unreacted. A more complete set of equations for the combustion of a hydrocarbon in the air, requires an additional calculation for the distribution of oxygen between the carbon and hydrogen in the fuel; the amount of air required for complete combustion to take place is known as theoretical air. However, in practice, the air used is 2-3x. Incomplete combustion will occur when there is not enough oxygen to allow the fuel to react to produce carbon dioxide and water, it happens when the combustion is quenched by a heat sink, such as a solid surface or flame trap. Same as complete combustion, water is produced by incomplete combustion. However, carbon monoxide, and/or hydroxide are the products in
The phlogiston theory is a superseded scientific theory that postulated that a fire-like element called phlogiston is contained within combustible bodies and released during combustion. The name comes from φλόξ phlóx, it was first stated in 1667 by Johann Joachim Becher and put together more formally by Georg Ernst Stahl. The theory attempted to explain processes such as combustion and rusting, which are now collectively known as oxidation. Phlogiston theory states that phlogisticated substances are substances that contain phlogiston and dephlogisticate when burned. Dephlogisticating is the process of releasing stored phlogiston, absorbed by the air. Growing plants absorb this phlogiston, why air does not spontaneously combust and why plant matter burns as well as it does, thus phlogiston accounted for combustion via a process, opposite to that of the oxygen theory. In general, substances that burned in air were said to be rich in phlogiston; when air had become phlogisticated it would no longer serve to support combustion of any material, nor would a metal heated in it yield a calx.
Breathing was thought to take phlogiston out of the body. Joseph Black's student Daniel Rutherford discovered nitrogen in 1772 and the pair used the theory to explain his results; the residue of air left after burning, in fact a mixture of nitrogen and carbon dioxide, was sometimes referred to as phlogisticated air, having taken up all of the phlogiston. Conversely, when Joseph Priestley discovered oxygen, he believed it to be dephlogisticated air, capable of combining with more phlogiston and thus supporting combustion for longer than ordinary air. Empedocles had formulated the classical theory that there were four elements: water, earth and air, Aristotle reinforced this idea by characterising them as moist, dry and cold. Fire was thus thought of as a substance and burning was seen as a process of decomposition which applied only to compounds. Experience had shown that burning was not always accompanied by a loss of material and a better theory was needed to account for this. In 1667, Johann Joachim Becher published his book Physica subterranea, which contained the first instance of what would become the phlogiston theory.
In his book, Becher eliminated fire and air from the classical element model and replaced them with three forms of earth: terra lapidea, terra fluida, terra pinguis. Terra pinguis was the element that imparted sulphurous, or combustible properties. Becher believed that terra pinguis was a key feature of combustion and was released when combustible substances were burned. Becher did not have much to do with phlogiston theory as we know it now, but he had a large influence on his student Stahl. Becher's main contribution was the start of the theory itself, however much it was changed after him. Becher's idea was that combustible substances contain an ignitable matter, the terra pinguis. In 1703 Georg Ernst Stahl, professor of medicine and chemistry at Halle, proposed a variant of the theory in which he renamed Becher's terra pinguis to phlogiston, it was in this form that the theory had its greatest influence; the term phlogiston. There is evidence that the word was used as early as 1606, in a way, similar to what Stahl was using it for.
The term was derived from a Greek word meaning to inflame. The following paragraph describes Stahl's view of phlogiston:To Stahl, metals were compounds containing phlogiston in combination with metallic oxides; when the oxide was heated with a substance rich in phlogiston, such as charcoal, the calx again took up phlogiston and regenerated the metal. Phlogiston was the same in all its combinations. Stahl's first definition of phlogiston first appeared in his "Zymotechnia fundamentalis", published in 1697, his most quoted definition was found in the treatise on chemistry entitled "Fundamenta chymiae" in 1723. According to Stahl, phlogiston was a substance, not able to be put into a bottle, but could be transferred nonetheless. To him, wood was just a combination of ash and phlogiston, making a metal was as simple as getting a metal calx and adding phlogiston. Soot was pure phlogiston, why heating it with a metallic calx transforms the calx into the metal and Stahl attempted to prove that the phlogiston in soot and sulphur were identical by converting sulphates to liver of sulphur using charcoal.
He did not account for the increase in weight on combustion of tin and lead that were known at the time. Johann Heinrich Pott, a student of one of Stahl's students, expanded the theory and attempted to make it much more understandable to a general audience, he compared phlogiston to light or fire, saying that all three were substances whose natures were understood but not defined. He thought that phlogiston should not be considered as a particle but as an essence that permeates substances, arguing that in a pound of any substance one could not pick out the particles of phlogiston. Pott observed the fact that when certain substances are burned they increase in mass instead of losing the mass of the phlogiston as it escapes. Flames were considered to be a mix of phlogiston and water, while a phlogiston-and-earthy mixture could not burn properly. Phlogiston permeating everything in
Chemistry is the scientific discipline involved with elements and compounds composed of atoms and ions: their composition, properties and the changes they undergo during a reaction with other substances. In the scope of its subject, chemistry occupies an intermediate position between physics and biology, it is sometimes called the central science because it provides a foundation for understanding both basic and applied scientific disciplines at a fundamental level. For example, chemistry explains aspects of plant chemistry, the formation of igneous rocks, how atmospheric ozone is formed and how environmental pollutants are degraded, the properties of the soil on the moon, how medications work, how to collect DNA evidence at a crime scene. Chemistry addresses topics such as how atoms and molecules interact via chemical bonds to form new chemical compounds. There are four types of chemical bonds: covalent bonds, in which compounds share one or more electron; the word chemistry comes from alchemy, which referred to an earlier set of practices that encompassed elements of chemistry, philosophy, astronomy and medicine.
It is seen as linked to the quest to turn lead or another common starting material into gold, though in ancient times the study encompassed many of the questions of modern chemistry being defined as the study of the composition of waters, growth, disembodying, drawing the spirits from bodies and bonding the spirits within bodies by the early 4th century Greek-Egyptian alchemist Zosimos. An alchemist was called a'chemist' in popular speech, the suffix "-ry" was added to this to describe the art of the chemist as "chemistry"; the modern word alchemy in turn is derived from the Arabic word al-kīmīā. In origin, the term is borrowed from the Greek χημία or χημεία; this may have Egyptian origins since al-kīmīā is derived from the Greek χημία, in turn derived from the word Kemet, the ancient name of Egypt in the Egyptian language. Alternately, al-kīmīā may derive from χημεία, meaning "cast together"; the current model of atomic structure is the quantum mechanical model. Traditional chemistry starts with the study of elementary particles, molecules, metals and other aggregates of matter.
This matter can be studied in isolation or in combination. The interactions and transformations that are studied in chemistry are the result of interactions between atoms, leading to rearrangements of the chemical bonds which hold atoms together; such behaviors are studied in a chemistry laboratory. The chemistry laboratory stereotypically uses various forms of laboratory glassware; however glassware is not central to chemistry, a great deal of experimental chemistry is done without it. A chemical reaction is a transformation of some substances into one or more different substances; the basis of such a chemical transformation is the rearrangement of electrons in the chemical bonds between atoms. It can be symbolically depicted through a chemical equation, which involves atoms as subjects; the number of atoms on the left and the right in the equation for a chemical transformation is equal. The type of chemical reactions a substance may undergo and the energy changes that may accompany it are constrained by certain basic rules, known as chemical laws.
Energy and entropy considerations are invariably important in all chemical studies. Chemical substances are classified in terms of their structure, phase, as well as their chemical compositions, they can be analyzed using the tools of e.g. spectroscopy and chromatography. Scientists engaged in chemical research are known as chemists. Most chemists specialize in one or more sub-disciplines. Several concepts are essential for the study of chemistry; the particles that make up matter have rest mass as well – not all particles have rest mass, such as the photon. Matter can be a mixture of substances; the atom is the basic unit of chemistry. It consists of a dense core called the atomic nucleus surrounded by a space occupied by an electron cloud; the nucleus is made up of positively charged protons and uncharged neutrons, while the electron cloud consists of negatively charged electrons which orbit the nucleus. In a neutral atom, the negatively charged electrons balance out the positive charge of the protons.
The nucleus is dense. The atom is the smallest entity that can be envisaged to retain the chemical properties of the element, such as electronegativity, ionization potential, preferred oxidation state, coordination number, preferred types of bonds to form. A chemical element is a pure substance, composed of a single type of atom, characterized by its particular number of protons in the nuclei of its atoms, known as the atomic number and represented by the symbol Z; the mass number is the sum of the number of neutrons in a nucleus. Although all the nuclei of all atoms belonging to one element will have the same
Limestone is a carbonate sedimentary rock, composed of the skeletal fragments of marine organisms such as coral and molluscs. Its major materials are the minerals calcite and aragonite, which are different crystal forms of calcium carbonate. A related rock is dolostone, which contains a high percentage of the mineral dolomite, CaMg2. In fact, in old USGS publications, dolostone was referred to as magnesian limestone, a term now reserved for magnesium-deficient dolostones or magnesium-rich limestones. About 10% of sedimentary rocks are limestones; the solubility of limestone in water and weak acid solutions leads to karst landscapes, in which water erodes the limestone over thousands to millions of years. Most cave systems are through limestone bedrock. Limestone has numerous uses: as a building material, an essential component of concrete, as aggregate for the base of roads, as white pigment or filler in products such as toothpaste or paints, as a chemical feedstock for the production of lime, as a soil conditioner, or as a popular decorative addition to rock gardens.
Like most other sedimentary rocks, most limestone is composed of grains. Most grains in limestone are skeletal fragments of marine organisms such as foraminifera; these organisms secrete shells made of aragonite or calcite, leave these shells behind when they die. Other carbonate grains composing limestones are ooids, peloids and extraclasts. Limestone contains variable amounts of silica in the form of chert or siliceous skeletal fragment, varying amounts of clay and sand carried in by rivers; some limestones do not consist of grains, are formed by the chemical precipitation of calcite or aragonite, i.e. travertine. Secondary calcite may be deposited by supersaturated meteoric waters; this produces speleothems, such as stalactites. Another form taken by calcite is oolitic limestone, which can be recognized by its granular appearance; the primary source of the calcite in limestone is most marine organisms. Some of these organisms can construct mounds of rock building upon past generations. Below about 3,000 meters, water pressure and temperature conditions cause the dissolution of calcite to increase nonlinearly, so limestone does not form in deeper waters.
Limestones may form in lacustrine and evaporite depositional environments. Calcite can be dissolved or precipitated by groundwater, depending on several factors, including the water temperature, pH, dissolved ion concentrations. Calcite exhibits an unusual characteristic called retrograde solubility, in which it becomes less soluble in water as the temperature increases. Impurities will cause limestones to exhibit different colors with weathered surfaces. Limestone may be crystalline, granular, or massive, depending on the method of formation. Crystals of calcite, dolomite or barite may line small cavities in the rock; when conditions are right for precipitation, calcite forms mineral coatings that cement the existing rock grains together, or it can fill fractures. Travertine is a banded, compact variety of limestone formed along streams where there are waterfalls and around hot or cold springs. Calcium carbonate is deposited where evaporation of the water leaves a solution supersaturated with the chemical constituents of calcite.
Tufa, a porous or cellular variety of travertine, is found near waterfalls. Coquina is a poorly consolidated limestone composed of pieces of coral or shells. During regional metamorphism that occurs during the mountain building process, limestone recrystallizes into marble. Limestone is a parent material of Mollisol soil group. Two major classification schemes, the Folk and the Dunham, are used for identifying the types of carbonate rocks collectively known as limestone. Robert L. Folk developed a classification system that places primary emphasis on the detailed composition of grains and interstitial material in carbonate rocks. Based on composition, there are three main components: allochems and cement; the Folk system uses two-part names. It is helpful to have a petrographic microscope when using the Folk scheme, because it is easier to determine the components present in each sample; the Dunham scheme focuses on depositional textures. Each name is based upon the texture of the grains. Robert J. Dunham published his system for limestone in 1962.
Dunham divides the rocks into four main groups based on relative proportions of coarser clastic particles. Dunham names are for rock families, his efforts deal with the question of whether or not the grains were in mutual contact, therefore self-supporting, or whether the rock is characterized by the presence of frame builders and algal mats. Unlike the Folk scheme, Dunham deals with the original porosity of the rock; the Dunham scheme is more useful for hand samples because it is based on texture, not the grains in the sample. A revised classification was proposed by Wright, it adds some diagenetic patterns and can be summarized as follows: See: Carbonate platform About 10% of all sedimentary rocks are limestones. Limestone is soluble in acid, therefore forms many erosional landforms; these include limestone pavements, pot holes, cenotes and gorges. Such erosion landscapes are known