Osmium is a chemical element with symbol Os and atomic number 76. It is a hard, bluish-white transition metal in the platinum group, found as a trace element in alloys in platinum ores. Osmium is the densest occurring element, with an experimentally measured density of 22.59 g/cm3. Manufacturers use its alloys with platinum and other platinum-group metals to make fountain pen nib tipping, electrical contacts, in other applications that require extreme durability and hardness; the element's abundance in the Earth's crust is among the rarest. Osmium is the densest stable element. Calculations of density from the X-ray diffraction data may produce the most reliable data for these elements, giving a value of 22.587±0.009 g/cm3 for osmium denser than the 22.562±0.009 g/cm3 of iridium. Osmium is a hard but brittle metal that remains lustrous at high temperatures, it has a low compressibility. Correspondingly, its bulk modulus is high, reported between 395 and 462 GPa, which rivals that of diamond; the hardness of osmium is moderately high at 4 GPa.
Because of its hardness, low vapor pressure, high melting point, solid osmium is difficult to machine, form, or work. Osmium forms compounds with oxidation states ranging from −2 to +8; the most common oxidation states are +2, +3, +4, +8. The +8 oxidation state is notable for being the highest attained by any chemical element aside from iridium's +9 and is encountered only in xenon, ruthenium and iridium; the oxidation states −1 and −2 represented by the two reactive compounds Na2 and Na2 are used in the synthesis of osmium cluster compounds. The most common compound exhibiting the +8 oxidation state is osmium tetroxide; this toxic compound is formed. It is a volatile, water-soluble, pale yellow, crystalline solid with a strong smell. Osmium powder has the characteristic smell of osmium tetroxide. Osmium tetroxide forms. With ammonia, it forms the nitrido-osmates OsO3N−. Osmium tetroxide is a powerful oxidizing agent. By contrast, osmium dioxide is black, non-volatile, much less reactive and toxic.
Only two osmium compounds have major applications: osmium tetroxide for staining tissue in electron microscopy and for the oxidation of alkenes in organic synthesis, the non-volatile osmates for organic oxidation reactions. Osmium pentafluoride is known; the lower oxidation states are stabilized by the larger halogens, so that the trichloride, tribromide and diiodide are known. The oxidation state +1 is known only for osmium iodide, whereas several carbonyl complexes of osmium, such as triosmium dodecacarbonyl, represent oxidation state 0. In general, the lower oxidation states of osmium are stabilized by ligands that are good σ-donors and π-acceptors; the higher oxidation states are stabilized by strong σ- and π-donors, such as O2− and N3−. Despite its broad range of compounds in numerous oxidation states, osmium in bulk form at ordinary temperatures and pressures resists attack by all acids and alkalis, including aqua regia. Osmium has seven occurring isotopes, six of which are stable: 184Os, 187Os, 188Os, 189Os, 190Os, 192Os.
186Os undergoes alpha decay with such a long half-life ×1015 years 140000 times the age of the universe, that for practical purposes it can be considered stable. Alpha decay is predicted for all seven occurring isotopes, but it has been observed only for 186Os due to long half-lives, it is predicted that 184Os and 192Os can undergo double beta decay but this radioactivity has not been observed yet.187Os is the descendant of 187Re and is used extensively in dating terrestrial as well as meteoric rocks. It has been used to measure the intensity of continental weathering over geologic time and to fix minimum ages for stabilization of the mantle roots of continental cratons; this decay is a reason. However, the most notable application of osmium isotopes in geology has been in conjunction with the abundance of iridium, to characterise the layer of shocked quartz along the Cretaceous–Paleogene boundary that marks the extinction of the non-avian dinosaurs 65 million years ago. Osmium was discovered in 1803 by William Hyde Wollaston in London, England.
The discovery of osmium is intertwined with that of platinum and the other metals of the platinum group. Platinum reached Europe as platina, first encountered in the late 17th century in silver mines around the Chocó Department, in Colombia; the discovery that this metal was not an alloy, but a distinct new element, was published in 1748. Chemists who studied platinum dissolved it in aqua regia to create soluble salts, they always observed a small amount of a insoluble residue. Joseph Louis Proust thought. Victor Collet-Descotils, Antoine François, comte de Fourcroy, Louis Nicolas Vauquelin observed iridium in the black platinum residue in 1803, but did not obtain enough material for further experiments; the two French ch
Rhenium is a chemical element with symbol Re and atomic number 75. It is a silvery-gray, third-row transition metal in group 7 of the periodic table. With an estimated average concentration of 1 part per billion, rhenium is one of the rarest elements in the Earth's crust. Rhenium has the third-highest melting point and second-highest boiling point of any element at 5903 K. Rhenium resembles manganese and technetium chemically and is obtained as a by-product of the extraction and refinement of molybdenum and copper ores. Rhenium shows in its compounds a wide variety of oxidation states ranging from −1 to +7. Discovered in 1908, rhenium was the second-last stable element to be discovered, it was named after the river Rhine in Europe. Nickel-based superalloys of rhenium are used in the combustion chambers, turbine blades, exhaust nozzles of jet engines; these alloys contain up to 6% rhenium, making jet engine construction the largest single use for the element. The second-most important use is as a catalyst: rhenium is an excellent catalyst for hydrogenation and isomerization, is used for example in catalytic reforming of naphtha for use in gasoline.
Because of the low availability relative to demand, rhenium is expensive, with price reaching an all-time high in 2008/2009 US$10,600 per kilogram. Due to increases in rhenium recycling and a drop in demand for rhenium in catalysts, the price of rhenium has dropped to US$2,844 per kilogram as of July 2018. Rhenium was the second last-discovered of the elements; the existence of a yet-undiscovered element at this position in the periodic table had been first predicted by Dmitri Mendeleev. Other calculated information was obtained by Henry Moseley in 1914. In 1908, Japanese chemist Masataka Ogawa announced that he had discovered the 43rd element and named it nipponium after Japan. However, recent analysis indicated the presence of rhenium, not element 43, although this reinterpretation has been questioned by Eric Scerri; the symbol Np was used for the element neptunium, the name "nihonium" named after Japan, along with symbol Nh, was used for element 113. Element 113 was discovered by a team of Japanese scientists and was named in respectful homage to Ogawa's work.
Rhenium is considered to have been discovered by Walter Noddack, Ida Noddack, Otto Berg in Germany. In 1925 they reported that they had detected the element in platinum ore and in the mineral columbite, they found rhenium in gadolinite and molybdenite. In 1928 they were able to extract 1 g of the element by processing 660 kg of molybdenite, it was estimated in 1968 that 75% of the rhenium metal in the United States was used for research and the development of refractory metal alloys. It took several years from that point before the superalloys became used. Rhenium is a silvery-white metal with one of the highest melting points of all elements, exceeded by only tungsten and carbon, it has one of the highest boiling points of all elements. It is one of the densest, exceeded only by platinum and osmium. Rhenium has a hexagonal close-packed crystal structure, with lattice parameters a = 276.1 pm and c = 445.6 pm. Its usual commercial form is a powder, but this element can be consolidated by pressing and sintering in a vacuum or hydrogen atmosphere.
This procedure yields a compact solid having a density above 90% of the density of the metal. When annealed this metal is ductile and can be bent, coiled, or rolled. Rhenium-molybdenum alloys are superconductive at 10 K. Rhenium metal superconducts at 1.697±0.006 K. In bulk form and at room temperature and atmospheric pressure, the element resists alkalis, sulfuric acid, hydrochloric acid, dilute nitric acid, aqua regia. Rhenium has one stable isotope, rhenium-185, which occurs in minority abundance, a situation found only in two other elements. Occurring rhenium is only 37.4% 185Re, 62.6% 187Re, unstable but has a long half-life. This lifetime can be affected by the charge state of rhenium atom; the beta decay of 187Re is used for rhenium-osmium dating of ores. The available energy for this beta decay is one of the lowest known among all radionuclides; the isotope rhenium-186m is notable as being one of the longest lived metastable isotopes with a half-life of around 200,000 years. There are twenty-five other recognized radioactive isotopes of rhenium.
Rhenium compounds are known for all the oxidation states between −3 and +7 except −2. The oxidation states +7, +6, +4, +2 are the most common. Rhenium is most available commercially as salts of perrhenate, including sodium and ammonium perrhenates; these are white, water-soluble compounds. The most common rhenium chlorides are ReCl6, ReCl5, ReCl4, ReCl3; the structures of these compounds feature extensive Re-Re bonding, characteristic of this metal in oxidation states lower than VII. Salts of 2− feature a quadruple metal-metal bond. Although the highest rhenium chloride features Re, fluorine gives the d0 Re derivative rhenium heptafluoride. Bromides and iodides of rhenium are well known. Like tungsten and molybdenum, with which it shares chemical similarities, rhenium forms a variety of oxyhalides; the oxychlorides are most common, include ReOCl4, ReOCl3. The most common oxide is the volatile colourless Re2O7. Rhenium trioxide ReO3 adopts a perovskite-like structure. Other oxides include
The mineral pyrite, or iron pyrite known as fool's gold, is an iron sulfide with the chemical formula FeS2. Pyrite is considered the most common of the sulfide minerals. Pyrite's metallic luster and pale brass-yellow hue give it a superficial resemblance to gold, hence the well-known nickname of fool's gold; the color has led to the nicknames brass and Brazil used to refer to pyrite found in coal. The name pyrite is derived from the Greek πυρίτης, "of fire" or "in fire", in turn from πύρ, "fire". In ancient Roman times, this name was applied to several types of stone that would create sparks when struck against steel. By Georgius Agricola's time, c. 1550, the term had become a generic term for all of the sulfide minerals. Pyrite is found associated with other sulfides or oxides in quartz veins, sedimentary rock, metamorphic rock, as well as in coal beds and as a replacement mineral in fossils, but has been identified in the sclerites of scaly-foot gastropods. Despite being nicknamed fool's gold, pyrite is sometimes found in association with small quantities of gold.
Gold and arsenic occur as a coupled substitution in the pyrite structure. In the Carlin–type gold deposits, arsenian pyrite contains up to 0.37% gold by weight. Pyrite enjoyed brief popularity in the 16th and 17th centuries as a source of ignition in early firearms, most notably the wheellock, where a sample of pyrite was placed against a circular file to strike the sparks needed to fire the gun. Pyrite has been used since classical times to manufacture copperas. Iron pyrite was allowed to weather; the acidic runoff from the heap was boiled with iron to produce iron sulfate. In the 15th century, new methods of such leaching began to replace the burning of sulfur as a source of sulfuric acid. By the 19th century, it had become the dominant method. Pyrite remains in commercial use for the production of sulfur dioxide, for use in such applications as the paper industry, in the manufacture of sulfuric acid. Thermal decomposition of pyrite into FeS and elemental sulfur starts at 540 °C. A newer commercial use for pyrite is as the cathode material in Energizer brand non-rechargeable lithium batteries.
Pyrite is a semiconductor material with a band gap of 0.95 eV. Pure pyrite is n-type, in both crystal and thin-film forms due to sulfur vacancies in the pyrite crystal structure acting as n-dopants. During the early years of the 20th century, pyrite was used as a mineral detector in radio receivers, is still used by crystal radio hobbyists; until the vacuum tube matured, the crystal detector was the most sensitive and dependable detector available – with considerable variation between mineral types and individual samples within a particular type of mineral. Pyrite detectors occupied a midway point between galena detectors and the more mechanically complicated perikon mineral pairs. Pyrite detectors can be as sensitive as a modern 1N34A germanium diode detector. Pyrite has been proposed as an abundant, non-toxic, inexpensive material in low-cost photovoltaic solar panels. Synthetic iron sulfide was used with copper sulfide to create the photovoltaic material.. More recent efforts are working toward thin-film solar cells made of pyrite.
Pyrite is used to make marcasite jewelry. Marcasite jewelry, made from small faceted pieces of pyrite set in silver, was known since ancient times and was popular in the Victorian era. At the time when the term became common in jewelry making, "marcasite" referred to all iron sulfides including pyrite, not to the orthorhombic FeS2 mineral marcasite, lighter in color and chemically unstable, thus not suitable for jewelry making. Marcasite jewelry does not contain the mineral marcasite. China represents the main importing country with an import of around 376,000 tonnes, which resulted at 45% of total global imports. China is the fastest growing in terms of the unroasted iron pyrites imports, with a CAGR of +27.8% from 2007 to 2016. In value terms, China constitutes the largest market for imported unroasted iron pyrites worldwide, making up 65% of global imports. From the perspective of classical inorganic chemistry, which assigns formal oxidation states to each atom, pyrite is best described as Fe2+S22−.
This formalism recognizes. These persulfide units can be viewed as derived from hydrogen disulfide, H2S2, thus pyrite would be more descriptively, not iron disulfide. In contrast, molybdenite, MoS2, features isolated sulfide centers and the oxidation state of molybdenum is Mo4+; the mineral arsenopyrite has the formula FeAsS. Whereas pyrite has S2 subunits, arsenopyrite has units, formally derived from deprotonation of H2AsSH. Analysis of classical oxidation states would recommend the description of arsenopyrite as Fe3+3−. Iron-pyrite FeS2 represents the prototype compound of the crystallographic pyrite structure; the structure is simple cubic and was among the first crystal structures solved by X-ray diffraction. It belongs to the crystallographic space group Pa3 and is denoted by the Strukturbericht notation C2. Under thermodynamic standard conditions the lattice constant a of stoichiometric iron pyrite FeS2 amounts to 541.87 pm. The unit cell is composed of a Fe face-centered cubic sublattice into.
The pyrite structure is used by other compounds MX2 of trans
Graphite, archaically referred to as plumbago, is a crystalline form of the element carbon with its atoms arranged in a hexagonal structure. It occurs in this form and is the most stable form of carbon under standard conditions. Under high pressures and temperatures it converts to diamond. Graphite is used in lubricants, its high conductivity makes it useful in electronic products such as electrodes and solar panels. The principal types of natural graphite, each occurring in different types of ore deposits, are Crystalline small flakes of graphite occurs as isolated, plate-like particles with hexagonal edges if unbroken; when broken the edges can be angular. Ordered pyrolytic graphite refers to graphite with an angular spread between the graphite sheets of less than 1°; the name "graphite fiber" is sometimes used to refer to carbon fibers or carbon fiber-reinforced polymer. Graphite occurs in metamorphic rocks as a result of the reduction of sedimentary carbon compounds during metamorphism, it occurs in igneous rocks and in meteorites.
Minerals associated with graphite include quartz, calcite and tourmaline. The principal export sources of mined graphite are in order of tonnage: China, Canada and Madagascar. In meteorites, graphite occurs with silicate minerals. Small graphitic crystals in meteoritic iron are called cliftonite; some microscopic grains have distinctive isotopic compositions, indicating that they were formed before the Solar system. They are one of about 12 known types of mineral that predate the Solar System and have been detected in molecular clouds; these minerals were formed in the ejecta when supernovae exploded or low- to intermediate-sized stars expelled their outer envelopes late in their lives. Graphite may be the third oldest mineral in the Universe. Solid carbon comes in different forms known as allotropes depending on the type of chemical bond; the two most common are graphite. In diamond the bonds are sp3 and the atoms form tetrahedra with each bound to four nearest neighbors. In graphite they are sp2 orbital hybrids and the atoms form in planes with each bound to three nearest neighbors 120 degrees apart.
The individual layers are called graphene. In each layer, the carbon atoms are arranged in a honeycomb lattice with separation of 0.142 nm, the distance between planes is 0.335 nm. Atoms in the plane are bonded covalently, with only three of the four potential bonding sites satisfied; the fourth electron is free to migrate in the plane. However, it does not conduct in a direction at right angles to the plane. Bonding between layers is via weak van der Waals bonds, which allows layers of graphite to be separated, or to slide past each other; the two known forms of graphite and beta, have similar physical properties, except that the graphene layers stack differently. The alpha graphite may be either buckled; the alpha form can be converted to the beta form through mechanical treatment and the beta form reverts to the alpha form when it is heated above 1300 °C. The equilibrium pressure and temperature conditions for a transition between graphite and diamond is well established theoretically and experimentally.
The pressure changes linearly between 1.7 GPa at 0 K and 12 GPa at 5000 K. However, the phases have a wide region about this line where they can coexist. At normal temperature and pressure, 20 °C and 1 standard atmosphere, the stable phase of carbon is graphite, but diamond is metastable and its rate of conversion to graphite is negligible. However, at temperatures above about 4500 K, diamond converts to graphite. Rapid conversion of graphite to diamond requires pressures well above the equilibrium line: at 2000 K, a pressure of 35 GPa is needed; the acoustic and thermal properties of graphite are anisotropic, since phonons propagate along the bound planes, but are slower to travel from one plane to another. Graphite's high thermal stability and electrical and thermal conductivity facilitate its widespread use as electrodes and refractories in high temperature material processing applications. However, in oxygen-containing atmospheres graphite oxidizes to form carbon dioxide at temperatures of 700 °C and above.
Graphite is hence useful in such applications as arc lamp electrodes. It can conduct electricity due to the vast electron delocalization within the carbon layers; these valence electrons are free to move. However, the electricity is conducted within the plane of the layers; the conductive properties of powdered graphite allow its use as pressure sensor in carbon microphones. Graphite and graphite powder are valued in industrial applications for their self-lubricating and dry lubricating properties. There is a common belief that graphite's lubricating properties are due to the loose interlamellar coupling between sheets in the structure. However, it has been shown that in a vacuum environment, graphite degrades as a lubricant, due to the hypoxic conditions; this observation led to the hypothesis that the lubrication is due to the presence of fluids between the layers, such as air and water, which are adsorbed from the
A diode is a two-terminal electronic component that conducts current in one direction. A diode vacuum tube or thermionic diode is a vacuum tube with two electrodes, a heated cathode and a plate, in which electrons can flow in only one direction, from cathode to plate. A semiconductor diode, the most common type today, is a crystalline piece of semiconductor material with a p–n junction connected to two electrical terminals. Semiconductor diodes were the first semiconductor electronic devices; the discovery of asymmetric electrical conduction across the contact between a crystalline mineral and a metal was made by German physicist Ferdinand Braun in 1874. Today, most diodes are made of silicon, but other materials such as gallium arsenide and germanium are used; the most common function of a diode is to allow an electric current to pass in one direction, while blocking it in the opposite direction. As such, the diode can be viewed as an electronic version of a check valve; this unidirectional behavior is called rectification, is used to convert alternating current to direct current.
Forms of rectifiers, diodes can be used for such tasks as extracting modulation from radio signals in radio receivers. However, diodes can have more complicated behavior than this simple on–off action, because of their nonlinear current-voltage characteristics. Semiconductor diodes begin conducting electricity only if a certain threshold voltage or cut-in voltage is present in the forward direction; the voltage drop across a forward-biased diode varies only a little with the current, is a function of temperature. Diodes' high resistance to current flowing in the reverse direction drops to a low resistance when the reverse voltage across the diode reaches a value called the breakdown voltage. A semiconductor diode's current–voltage characteristic can be tailored by selecting the semiconductor materials and the doping impurities introduced into the materials during manufacture; these techniques are used to create special-purpose diodes. For example, diodes are used to regulate voltage, to protect circuits from high voltage surges, to electronically tune radio and TV receivers, to generate radio-frequency oscillations, to produce light.
Tunnel, Gunn and IMPATT diodes exhibit negative resistance, useful in microwave and switching circuits. Diodes, both vacuum and semiconductor, can be used as shot-noise generators. Thermionic diodes and solid-state diodes were developed separately, at the same time, in the early 1900s, as radio receiver detectors; until the 1950s, vacuum diodes were used more in radios because the early point-contact semiconductor diodes were less stable. In addition, most receiving sets had vacuum tubes for amplification that could have the thermionic diodes included in the tube, vacuum-tube rectifiers and gas-filled rectifiers were capable of handling some high-voltage/high-current rectification tasks better than the semiconductor diodes that were available at that time. In 1873, Frederick Guthrie observed that a grounded, white hot metal ball brought in close proximity to an electroscope would discharge a positively charged electroscope, but not a negatively charged electroscope. In 1880, Thomas Edison observed unidirectional current between heated and unheated elements in a bulb called Edison effect, was granted a patent on application of the phenomenon for use in a dc voltmeter.
About 20 years John Ambrose Fleming realized that the Edison effect could be used as a radio detector. Fleming patented the first true thermionic diode, the Fleming valve, in Britain on November 16, 1904. Throughout the vacuum tube era, valve diodes were used in all electronics such as radios, sound systems and instrumentation, they lost market share beginning in the late 1940s due to selenium rectifier technology and to semiconductor diodes during the 1960s. Today they are still used in a few high power applications where their ability to withstand transient voltages and their robustness gives them an advantage over semiconductor devices, in musical instrument and audiophile applications. In 1874, German scientist Karl Ferdinand Braun discovered the "unilateral conduction" across a contact between a metal and a mineral. Indian scientist Jagadish Chandra Bose was the first to use a crystal for detecting radio waves in 1894; the crystal detector was developed into a practical device for wireless telegraphy by Greenleaf Whittier Pickard, who invented a silicon crystal detector in 1903 and received a patent for it on November 20, 1906.
Other experimenters tried a variety of other minerals as detectors. Semiconductor principles were unknown to the developers of these early rectifiers. During the 1930s understanding of physics advanced and in the mid 1930s researchers at Bell Telephone Laboratories recognized the potential of the crystal detector for application in microwave technology. Researchers at Bell Labs, Western Electric, MIT, Purdue and in the UK intensively developed point-contact diodes during World War II for application in ra
Molybdenum is a chemical element with symbol Mo and atomic number 42. The name is from Neo-Latin molybdaenum, from Ancient Greek Μόλυβδος molybdos, meaning lead, since its ores were confused with lead ores. Molybdenum minerals have been known throughout history, but the element was discovered in 1778 by Carl Wilhelm Scheele; the metal was first isolated in 1781 by Peter Jacob Hjelm. Molybdenum does not occur as a free metal on Earth; the free element, a silvery metal with a gray cast, has the sixth-highest melting point of any element. It forms hard, stable carbides in alloys, for this reason most of world production of the element is used in steel alloys, including high-strength alloys and superalloys. Most molybdenum compounds have low solubility in water, but when molybdenum-bearing minerals contact oxygen and water, the resulting molybdate ion MoO2−4 is quite soluble. Industrially, molybdenum compounds are used in high-pressure and high-temperature applications as pigments and catalysts. Molybdenum-bearing enzymes are by far the most common bacterial catalysts for breaking the chemical bond in atmospheric molecular nitrogen in the process of biological nitrogen fixation.
At least 50 molybdenum enzymes are now known in bacteria and animals, although only bacterial and cyanobacterial enzymes are involved in nitrogen fixation. These nitrogenases contain molybdenum in a form different from other molybdenum enzymes, which all contain oxidized molybdenum in a molybdenum cofactor; these various molybdenum cofactor enzymes are vital to the organisms, molybdenum is an essential element for life in all higher eukaryote organisms, though not in all bacteria. In its pure form, molybdenum is a silvery-grey metal with a Mohs hardness of 5.5, a standard atomic weight of 95.95 g/mol. It has a melting point of 2,623 °C, it has one of the lowest coefficients of thermal expansion among commercially used metals. The tensile strength of molybdenum wires increases about 3 times, from about 10 to 30 GPa, when their diameter decreases from ~50–100 nm to 10 nm. Molybdenum is a transition metal with an electronegativity of 2.16 on the Pauling scale. It does not visibly react with water at room temperature.
Weak oxidation of molybdenum starts at 300 °C. Like many heavier transition metals, molybdenum shows little inclination to form a cation in aqueous solution, although the Mo3+ cation is known under controlled conditions. There are 35 known isotopes of molybdenum, ranging in atomic mass from 83 to 117, as well as four metastable nuclear isomers. Seven isotopes occur with atomic masses of 92, 94, 95, 96, 97, 98, 100. Of these occurring isotopes, only molybdenum-100 is unstable. Molybdenum-98 is the most abundant isotope, comprising 24.14% of all molybdenum. Molybdenum-100 has a half-life of about 1019 y and undergoes double beta decay into ruthenium-100. Molybdenum isotopes with mass numbers from 111 to 117 all have half-lives of 150 ns. All unstable isotopes of molybdenum decay into isotopes of niobium and ruthenium; as noted below, the most common isotopic molybdenum application involves molybdenum-99, a fission product. It is a parent radioisotope to the short-lived gamma-emitting daughter radioisotope technetium-99m, a nuclear isomer used in various imaging applications in medicine.
In 2008, the Delft University of Technology applied for a patent on the molybdenum-98-based production of molybdenum-99. Molybdenum forms chemical compounds in oxidation states from -II to +VI. Higher oxidation states are more relevant to its terrestrial occurrence and its biological roles, mid-level oxidation states are associated with metal clusters, low oxidation states are associated with organomolybdenum compounds. Mo and W chemistry shows strong similarities; the relative rarity of molybdenum, for example, contrasts with the pervasiveness of the chromium compounds. The highest oxidation state is seen in molybdenum oxide, whereas the normal sulfur compound is molybdenum disulfide MoS2. From the perspective of commerce, the most important compounds are molybdenum disulfide and molybdenum trioxide; the black disulfide is the main mineral. It is roasted in air to give the trioxide: 2 MoS2 + 7 O2 → 2 MoO3 + 4 SO2The trioxide, volatile at high temperatures, is the precursor to all other Mo compounds as well as alloys.
Molybdenum has several oxidation states, the most stable being +4 and +6. Molybdenum oxide is soluble in strong alkaline water, forming molybdates. Molybdates are weaker oxidants than chromates, they tend to form structurally complex oxyanions by condensation at lower pH values, such as 6− and 4−. Polymolybdates can incorporate other ions; the dark-blue phosphorus-containing heteropolymolybdate P3− is used for the spectroscopic detection of phosphorus. The broad range of oxidation states of molybdenum is reflected in various molybdenum chlorides: Molybdenum chloride MoCl2, which exists as the hexamer Mo6Cl12 and the related dianion 2-. Molybdenum chloride MoCl3, a dark red solid, which converts to the anion trianionic complex 3-. Molybdenum chloride MoCl4, a black solid, which adopts a polymeric structure. Molybdenum chloride MoCl5 dark green solid that
Fluorite is the mineral form of calcium fluoride, CaF2. It belongs to the halide minerals, it crystallizes in isometric cubic habit, although octahedral and more complex isometric forms are not uncommon. The Mohs scale of mineral hardness, based on scratch hardness comparison, defines value 4 as Fluorite. Fluorite is a colorful mineral, both in visible and ultraviolet light, the stone has ornamental and lapidary uses. Industrially, fluorite is used as a flux for smelting, in the production of certain glasses and enamels; the purest grades of fluorite are a source of fluoride for hydrofluoric acid manufacture, the intermediate source of most fluorine-containing fine chemicals. Optically clear transparent fluorite lenses have low dispersion, so lenses made from it exhibit less chromatic aberration, making them valuable in microscopes and telescopes. Fluorite optics are usable in the far-ultraviolet and mid-infrared ranges, where conventional glasses are too absorbent for use; the word fluorite is derived from the Latin verb fluere, meaning to flow.
The mineral is used as a flux in iron smelting to decrease the viscosity of slags. The term flux comes from the Latin adjective fluxus, meaning flowing, slack; the mineral fluorite was termed fluorospar and was first discussed in print in a 1530 work Bermannvs sive de re metallica dialogus, by Georgius Agricola, as a mineral noted for its usefulness as a flux. Agricola, a German scientist with expertise in philology and metallurgy, named fluorspar as a neo-Latinization of the German Flussspat from Fluß and Spat. In 1852, fluorite gave its name to the phenomenon of fluorescence, prominent in fluorites from certain locations, due to certain impurities in the crystal. Fluorite gave the name to its constitutive element fluorine. Presently, the word "fluorspar" is most used for fluorite as the industrial and chemical commodity, while "fluorite" is used mineralogically and in most other senses. In the context of archeology, classical studies, egyptology, the Latin terms murrina and myrrhina refer to fluorite.
In book 37 of his Naturalis Historia, Pliny the Elder describes it as a precious stone with purple and white mottling, whose objects carved from it, the Romans prize. Fluorite crystallises in a cubic motif. Crystal twinning adds complexity to the observed crystal habits. Fluorite has four perfect cleavage planes. Element substitution for the calcium cation includes certain rare earth elements, such as yttrium and cerium. Iron and barium are common impurities; some fluorine may be replaced by the chloride anion. Fluorite is a occurring mineral that occurs globally with significant deposits in over 9,000 areas, it may occur as a vein deposit with metallic minerals, where it forms a part of the gangue and may be associated with galena, barite and calcite. It is a common mineral in deposits of hydrothermal origin and has been noted as a primary mineral in granites and other igneous rocks and as a common minor constituent of dolostone and limestone; the world reserves of fluorite are estimated at 230 million tonnes with the largest deposits being in South Africa and China.
China is leading the world production with about 3 Mt annually, followed by Mexico, Russia, South Africa and Namibia. One of the largest deposits of fluorspar in North America is located in the Burin Peninsula, Canada; the first official recognition of fluorspar in the area was recorded by geologist J. B. Jukes in 1843, he noted an occurrence of "galena" or lead ore and fluoride of lime on the west side of St. Lawrence harbour, it is recorded that interest in the commercial mining of fluorspar began in 1928 with the first ore being extracted in 1933. At Iron Springs Mine, the shafts reached depths of 970 feet. In the St. Lawrence area, the veins are persistent for great lengths and several of them have wide lenses; the area with veins of known workable size comprises about 60 square miles. Cubic crystals up to 20 cm across have been found at Russia; the largest documented single crystal of fluorite was a cube weighing ~ 16 tonnes. Fluorite may be found in mines in Caldoveiro Peak, in Asturias, Spain.
One of the most famous of the older-known localities of fluorite is Castleton in Derbyshire, where, under the name of Derbyshire Blue John, purple-blue fluorite was extracted from several mines or caves. During the 19th century, this attractive fluorite was mined for its ornamental value; the mineral Blue John is now scarce, only a few hundred kilograms are mined each year for ornamental and lapidary use. Mining still takes place in Treak Cliff Cavern. Discovered deposits in China have produced fluorite with coloring and banding similar to the classic Blue John stone. George Gabriel Stokes named the phenomenon of fluorescence from fluorite, in 1852. Many samples of fluorite exhibit fluorescence under ultraviolet light, a property that takes its name from fluorite. Many minerals, as well as other substances, fluoresce. Fluorescence involves the elevation of electron energy levels by quanta of ultraviolet light, followed by the progressive falling back of the electrons into their previous energy state, releasing quanta of visible light in the process.
In fluorite, the visible