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Sphalerite

Sphalerite

Sphalerite (ZnS) is a mineral that is the chief ore of zinc. It consists largely of zinc sulfide in crystalline form but almost always contains variable iron. When iron content is high it is an opaque black variety, marmatite ((Zn,Fe)S). It is usually found in association with galena, pyrite, and other sulfides along with calcite, dolomite, and fluorite. Miners have also been known to refer to sphalerite as zinc blende, mock lead, false galena and black-jack. The mineral crystallizes in the cubic crystal system. In the crystal structure, zinc and sulfur atoms are tetrahedrally coordinated. The structure is closely related to the structure of diamond. The hexagonal analog is known as the wurtzite structure. The lattice constant for zinc sulfide in the zincblende crystal structure is 5.420 angstroms. Its color is usually yellow, brown, or gray to gray-black, and it may be shiny or dull. Its luster is resinous. It has a yellow or light brown streak, a hardness of 2.5 - 4, and a specific gravity of 3.9-4.1. Some specimens have a red iridescence within the gray-black crystals; these are called "ruby sphalerite." The pale yellow and red varieties have very little iron and are translucent. The darker more opaque varieties contain more iron. Some specimens are also fluorescent in ultraviolet light. The refractive index of sphalerite (as measured via sodium light, 589.3 nm) is 2.37. Sphalerite crystallizes in the isometric crystal system and possesses perfect dodecahedral cleavage. Gemmy, pale specimens from Franklin, New Jersey (see Franklin Furnace) are highly fluorescent orange and/or blue under longwave ultraviolet light and are known as cleiophane, an almost pure ZnS variety. Crystals of suitable size and transparency have been fashioned into gemstones, usually featuring the brilliant cut to best display sphalerite's high dispersion of 0.156 (B-G interval)—over three times that of diamond. Freshly cut gems are lively with an adamantine luster and could conceivably be mistaken for a fancy-colored diamond in passing, but due to sphalerite's softness and fragility the gems are best left unset as collector's or museum pieces (although some have been set into pendants). Collectors may pay a premium for stones over one carat (200 mg), as clean crystals are usually quite small. Gem-quality material is usually a yellowish to honey brown, red to orange, or green; the two most important sources are the Chivera mine, Cananera, Sonora, Mexico; and the Picos de Europa, Cordillera Cantabrica, near Santander on Spain's northern coast.

References

- Dana's Manual of Mineralogy ISBN 0471032883
- Webster, R., Read, P. G. (Ed.) (2000). Gems: Their sources, descriptions and identification (5th ed.), p. 386. Butterworth-Heinemann, Great Britain. ISBN 0750616741
- [http://www.mindat.org/min-3727.html mindat.org]
- [http://www.minerals.net/mineral/sulfides/sphaleri/sphaleri.htm Minerals.net]
- [http://simplethinking.com/palache/sphalerite.stm Minerals of Franklin, NJ]

External links

- [http://cst-www.nrl.navy.mil/lattice/struk/b3.html The sphalerite structure] Category:Gemstones Category:Sulfide minerals

Ore

)]] An ore is a mineral deposit containing a metal or other valuable resource in economically viable concentrations. Usually, it is used in the context of a mineral deposit from which it is economical to extract its metallic component. Ores are mined. Ore minerals are generally oxides, sulfides, silicates, or native metals that are not commonly concentrated in the Earth's crust. The ores must be processed to extract the metals of interest from the waste rock and from the ore minerals. Ore bodies are formed by a variety of geological processes. The abundance of an ore will directly affect the costs associated with mining the ore and the subsequent cost of the metal extracted.

Important ore minerals

- Argentite: Ag₂S
- Barite: BaSO₄
- Beryl: Be₃Al₂(SiO₃)₆
- Bornite: Cu₅FeS₄
- Cassiterite: SnO₂
- Chalcocite: Cu₂S
- Chalcopyrite: CuFeS₂
- Chromite: (Fe,Mg)Cr₂O₄
- Cinnabar: HgS
- Cobaltite: (Co,Fe)AsS
- Columbite-Tantalite or Coltan: (Fe,Mn)(Nb,Ta)₂O₆
- Galena: PbS
- Gold: Au
- Hematite: Fe₂O₃
- Ilmenite: FeTiO₃
- Magnetite: Fe₃O₄
- Molybdenite: MoS₂
- Pentlandite:(Fe,Ni)₉S₈
- Scheelite: CaWO₄
- Sphalerite: ZnS
- Uraninite: UO₂
- Wolframite: (Fe,Mn)WO₄ Category:Economic geology

Zinc

Zinc (from German Zink, perhaps ultimately from Old Persian) is a chemical element in the periodic table that has the symbol Zn and atomic number 30.

Notable characteristics

Zinc is a moderately reactive metal that will combine with oxygen and other non-metals, and will react with dilute acids to release hydrogen. The one common oxidation state of zinc is +2.

Applications

Zinc is the fourth most common metal in use, trailing only iron, aluminium, and copper in annual production.
- Zinc is used to galvanise metals such as steel to prevent corrosion.
- Zinc is used in alloys such as brass, nickelled silver, typewriter metal, various soldering formulas and German silver.
- Zinc is the primary metal used in making American pennies since 1982.
- Zinc is used in die casting noteably in the automobile industry.
- Zinc is used as part of the containers of batteries.
- Zinc oxide is used as a white pigment in watercolours or paints, and as an activator in the rubber industry. As an over-the-counter ointment, it is applied as a thin coating on the exposed skin of the face or nose to prevent dehydration of the area of skin. It can protect against sunburn in the summer and windburn in the winter. Applied thinly to a baby's diaper area (perineum) with each diaper change, it can protect against rash. As determined in the Age-Related Eye Disease Study, it's part of an effective treatment for age-related macular degeneration in some cases.
- Zinc chloride is used as a deodorant and can be used as a wood preservative.
- Zinc sulfide is used in luminescent pigments such as on the hands of clocks and other items that glow in the dark.
- Zinc methyl (Zn(CH₃)₂) is used in a number of organic syntheses.
- Zinc stearate is a lubricative plastic additive.
- Lotions made of calamine, a mix of Zn-(hydroxy-)carbonates and silicates, are used to treat skin rash.
- Zinc metal is included in most single tablet over-the-counter daily vitamin and mineral suplements. It is believed to possess anti-oxidant properties, which protect against premature aging of the skin and muscles of the body. In larger amounts, taken as zinc alone in other proprietaries, it is believed by some to speed up the healing process after an injury. Preparations include zinc acetate and zinc gluconate.
- Zinc gluconate glycine is used as a lozenge in an attempt to remedy the common cold.

Popular misconceptions

The characteristic metal counters of traditional French bars are often referred to as zinc bars or simply zinc, but in fact zinc has never been used for this purpose and the counters are actually made of an alloy of lead and tin. In Argentina some people wrongly believe that zinc is a poison, and some of them are avoiding food which is known to include zinc. In 1997 a municipality north of the centre of Buenos Aires posted advertisements in popular magazines explaining the usefulness of zinc in the human body.

History

human body Zinc alloys have been used for centuries, as brass goods dating to 1000-1400 BC have been found in Palestine and zinc objects with 87% zinc have been found in prehistoric Transylvania. Because of the low boiling point and high chemical reactivity of this metal (isolated zinc would tend to go up the chimney rather than be captured), the true nature of this metal was not understood in ancient times. The manufacture of brass was known to the Romans by about 30 BC, using a technique where calamine and copper were heated together in a crucible. The zinc oxides in calamine were reduced, and the free zinc metal was trapped by the copper, forming an alloy. The resulting calamine brass was either cast or hammered into shape. Smelting and extraction of impure forms of zinc was being accomplished as early as 1000 AD in India and China. By the end of the 14th century, the Hindus were aware of the existence of zinc as a metal separate from the seven known to the ancients. In the West, impure zinc as a remnant in melting ovens was known since Antiquity, but usually thrown away as worthless. Strabo mentions it as pseudo-arguros "mock silver". The Berne Zinc tablet is a votive plaque dating to Roman Gaul, probably made from such zinc remnants. The discovery of pure metallic zinc is most often credited to the German Andreas Marggraf, in the year 1746, though the whole story is considerably more involved. Descriptions of brass manufacture are found in Western Europe in the writings of Albertus Magnus, c. 1248, and by the 16th century, the understanding and awareness of the new metal broadened considerably. Georg Agricola observed, in 1546, that a white metal could be condensed and scraped off the walls of a furnace when zinc ores were smelted. He added in his notes that a similar metal called "zincum" was being produced in Silesia. Paracelsus (died 1541) was the first in the West to say that "zincum" was a new metal and that it had a separate set of chemical properties from other known metals. The upshot is that zinc was known by the time Marggraf made his discoveries and in fact zinc had been isolated two years earlier by another chemist, Anton von Swab. However, Marggraf's reports were exhaustive and methodical and the quality of his research cemented his reputation as the discoverer of zinc. Before the discovery of the zinc sulfide flotation technique, calamine was the mineral source of zinc metal. calamine

Biological role

Zinc is an essential element, necessary for sustaining all life. It is estimated that 3000 of the hundreds of thousands of proteins in the human body contain zinc.

Food Sources

The best and most abundant natural food source of zinc is oysters, although these bottom scavengers also accumulate toxic metals. Zinc is found in most animal proteins such as beef, pork and poultry. Other food sources of zinc include beans, nuts, whole grains, pumpkin seeds and sunflower seeds. Phytates, which are found in whole grain breads, cereals, legumes and other products, have been known to decrease zinc absorption. This, coupled with the fact that the human body absorbs zinc more easily from animal protein than from plant protein means that vegetarians are required to eat many more food sources containing zinc than non-vegetarians.

Zinc Deficiency

Zinc deficiency results from inadequate intake of zinc, or inadequate absorption of zinc into the body. Signs of zinc deficiency includes hair loss, skin lesions, diarrhea, wasting of body tissues, and, eventually, death. Eyesight, taste, smell and memory are also connected with zinc and a deficiency in zinc can cause malfunctions of these organs and functions. Obtaining a sufficient zinc intake during pregnancy and in young children is a very real problem, especially among those who cannot afford a good supply of meat and a varied diet. Brain development is stunted by zinc insufficiency in utero and in youth. There is zinc in semen. As much as 0.25 milligram of zinc will be found in 1 mL of seminal fluid.

Zinc Toxicity

Even though zinc is almost an essential requirement for a healthy body, too much zinc can be harmful. Excessive absorption of zinc can also suppress copper and iron absorption.

Psoriasis

Ionic zinc is a potent antimicrobial, used since 2500 BC in topical creams. Calomine lotion, diaper creams, and dandruff treatments are just some of the common antimicrobial applications. At low concentrations, zinc ions promote wound healing. Zinc ions also directly stimulate zinc receptors on skin cells, promoting wound healing.

Immune System

Zinc salts are effective against pathogens in direct application. Gastrointestinal infections are also strongly attenuated by ingestion of zinc, and this effect could be due to direct antimicrobial action of the zinc ions in the GI tract, or to absorption of the zinc and re-release from immune cells (all granulocytes secrete zinc) or both. The direct effect of zinc (as in lozenges) on bacteria and viruses is also well-established, and has been used since at least 2000 BC, from when zinc salts in palliative salves are documented. However, exactly how to deliver zinc salts against pathogens without injuring one's own tissues is still being investigated.

Abundance

Zinc is the 23rd most abundant element in the Earth's crust. The most heavily mined ores (sphalerite) tend to contain roughly 10% iron as well as 40-50% zinc. Minerals from which zinc is extracted include sphalerite (zinc sulfide), smithsonite (zinc carbonate), hemimorphite (zinc silicate), and franklinite (a zinc spinel).

Zinc production

There are zinc mines throughout the world, with the largest producers being Australia, Canada, China, Peru and the U.S.A. Mines in Europe include Vieille Montagne in Belgium, Tara in Ireland, and Zinkgruvan in Sweden. Zinc metal is produced using extractive metallurgy. Zinc sulfide (sphalerite) minerals are concentrated using the froth flotation method and then usually roasted using pyrometallurgy to oxidise the zinc sulfide to zinc oxide. The zinc oxide is leached in sulfuric acid and the resulting solution is purified using zinc dust. The metal is then extracted by electrowinning as cathodic deposits. Zinc cathodes can be directly cast or alloyed with aluminium. Another process to produce zinc is flash smelting, a pyrometallurgical process. Then zinc oxide is obtained, usually producing zinc of lesser quality than the hydrometallurgical process. Zinc oxide treatment has much fewer applications, but high grade deposits have been successful in producing zinc from zinc oxides and zinc carbonates using hydrometallurgy.

Compounds

Zinc oxide is perhaps the best known and most widely used zinc compound, as it makes a good base for white pigments in paint. It also finds industrial use in the rubber industry, and is sold as opaque sunscreen. A variety of other zinc compounds find use industrially, such as zinc chloride (in deodorants), zinc sulfide (in luminescent paints), and zinc methyl in the organic laboratory. Roughly one quarter of all zinc output is consumed in the form of zinc compounds.

Isotopes

Naturally occurring zinc is composed of the 5 stable isotopes Zn-64, Zn-66, Zn-67, Zn-68, and Zn-70 with 64 being the most abundant (48.6% natural abundance). 21 radioisotopes have been characterised with the most being Zn-65 with a half-life of 244.26 days, and Zn-72 with a half-life of 46.5 hours. All of the remaining radioactive isotopes have half-lives that are less than 14 hours and the majority of these have half lives that are less than 1 second. This element also has 4 meta states.

Precautions

Metallic zinc is not considered to be toxic, but free zinc ions in solution (like copper or iron ions) are highly toxic. There is also a condition called zinc shakes or zinc chills that can be induced by the inhalation of freshly formed zinc oxide. Excessive intake of zinc can promote deficiency in other dietary minerals.

References

- [http://periodic.lanl.gov/elements/30.html Los Alamos National Laboratory - Zinc]

External links

- [http://www.vanderkrogt.net/elements/elem/zn.html History & Etymology of Zinc]
- [http://www.best-home-remedies.com/minerals/zinc.htm Zinc Information - Benefits, Deficiency Symptoms And Food Sources]
- [http://www.webelements.com/webelements/elements/text/Zn/index.html WebElements.com – Zinc]
- [http://chinese-school.netfirms.com/Zinc-information.html Zinc – History, sources, production, uses, health, and Zinc deficiency]
- [http://www.iza.com/zwo_org/Publications/Discovering/0202.htm Discovering the 8th metal]
- [http://minerals.er.usgs.gov/minerals/pubs/commodity/zinc/ Statistics and Information from the U.S. Geological Survey] Category:Chemical elements Category:Transition metals Category:Pyrotechnic chemicals ja:亜鉛 simple:Zinc th:สังกะสี

Crystal

:This article is about the form of solid matter. For other uses of this word, see Crystal (disambiguation). Crystal (disambiguation) A crystal is a solid in which the constituent atoms, molecules, or ions are packed in a regularly ordered, repeating pattern extending in all three spatial dimensions. Generally, fluid substances form crystals when they undergo a process of solidification. Under ideal conditions, the result may be a single crystal, where all of the atoms in the solid fit into the same lattice or crystal structure but, generally, many crystals form simultaneously during solidification, leading to a polycrystalline solid. For example, most metals encountered in everyday life are polycrystals. Crystals are often symmetrically intergrown to form crystal twins. Which crystal structure the fluid will form depends on the chemistry of the fluid, the conditions under which it is being solidified, and also on the ambient pressure. The process of forming a crystalline structure is often referred to as crystallization. pressure While the process of cooling usually results in the generation of a crystalline material, under certain conditions the fluid may be frozen in a noncrystalline state. In most cases, this involves cooling the fluid so rapidly that atoms cannot travel to their lattice sites before they lose mobility. A noncrystalline material, which has no long-range order, is called an amorphous, vitreous, or glassy material. It is also often referred to as an amorphous solid, although there are distinct differences between solids and glasses: most notably, the process of forming a glass does not release the latent heat of fusion. For this reason, many scientists consider glassy materials to be viscous liquids rather than solids, although this is a controversial topic; see the entry on glass for more details. glass Crystalline structures occur in all classes of materials, with all types of chemical bonds. Almost all metal exists in a polycrystalline state; amorphous or single-crystal metals must be produced synthetically, often with great difficulty. Ionically bonded crystals can form upon solidification of salts, either from a molten fluid or when it condenses from a solution. Covalently bonded crystals are also very common, notable examples being diamond, silica, and graphite. Polymer materials generally will form crystalline regions, but the lengths of the molecules usually prevents complete crystallization. Weak Van der Waals forces can also play a role in a crystal structure; for example, this type of bonding loosely holds together the hexagonal-patterned sheets in graphite. Most crystalline materials have a variety of crystallographic defects. The types and structures of these defects can have a profound effect on the properties of the materials. crystallographic defect crystallographic defect.]] While the term "crystal" has a precise meaning within materials science and solid-state physics, colloquially "crystal" refers to solid objects that exhibit well-defined and often pleasing geometric shapes. Various shapes of such crystals are found in nature. The shape of these crystals is dependent on the types of molecular bonds between the atoms to determine the structure, as well as on the conditions under which they formed. Snowflakes, diamonds, and common salt are common examples of crystals. Some crystalline materials may exhibit special electrical properties such as the ferroelectric effect or the piezoelectric effect. The behaviour of light in crystals is described by crystal optics. In periodic dielectric structures a range of unique optical properties can be expected as described in photonic crystals. Crystallography is the scientific study of crystals and crystal formation.

External links

- [http://www.rockhounds.com/rockshop/xtal/index.html Introduction to Crystallography and Mineral Crystal Systems]
- [http://www.iucr.ac.uk/iucr-top/comm/cteach/pamphlets.html Crystallographic Teaching Pamphlets]
- [http://cst-www.nrl.navy.mil/lattice/spcgrp/ Crystal Lattice Structures] ja:結晶

Galena

:This article is about the mineral. For various cities and towns of that name, see Galena (disambiguation). Galena is a lead ore. This article describes Galena's mineral properties. In its chemically purified form, galena is known as lead sulfide; refer to that article for chemical and industrial uses. Galena is one of the most abundant and widely distributed sulfide minerals, and is the most common ore of lead. Crystals are usually cubic, sometimes octahedral. It is often associated with the minerals sphalerite and fluorite. Galena deposits sometimes contain significant amounts of silver as an impurity, and these galenas have long been the most important ore of silver in mining. mining Galena deposits are found in Germany, France, Romania, Austria, Belgium, Italy, Spain, Scotland, England, Australia, and Mexico. In the United States it occurs in Missouri, Illinois, Iowa, Kansas, Oklahoma, Colorado, Idaho, Utah, Montana, and Wisconsin. Galena is the official state mineral of Missouri and Wisconsin, USA. It was once used as a semiconductor (i.e. the crystal) in crystal radio sets; combined with a safety pin or similar sharp wire (known as a "cat's whisker"), the galena crystal became part of a point-contact diode used to detect radio signals.

Calcite

The carbonate mineral calcite is a calcium carbonate corresponding to the formula CaCO₃ and is one of the most widely distributed minerals on the Earth's surface. It is a common constituent of sedimentary rocks, limestone in particular. It is also the primary mineral in metamorphic marble. It also occurs as a vein mineral in deposits from hot springs, and also occurs in caverns as stalactites and stalagmites. Calcite is often the primary constituent of the shells of marine organisms (e.g. plankton, bivalves, etc.). Calcite represents the stable form of calcium carbonate; aragonite will change to calcite at 470°C.

Properties

Calcite crystals are hexagonal-rhombohedral, though actual calcite rhombohedrons are rare as natural crystals. However, they show a remarkable variety of habit including acute to obtuse rhombohedrons, tabular forms, prisms, or various scalenohedrons. Calcite exhibits several twinning types adding to the variety of observed forms. It may occur as fibrous, granular, lamellar, or compact. Cleavage is usually in three directions parallel to the rhombohedron form. Its fracture is conchoidal, but difficult to obtain. It has a Mohs hardness of 3, a specific gravity of 2.71, and its luster is vitreous in crystallized varieties. Colour is white or colourless, though shades of gray, red, yellow, green, blue, violet, brown, or even black can occur when the mineral is charged with impurities. Calcite reacts when it comes into contact with dilute hydrochloric acid causing effervescence and the release of carbon dioxide gas. This is the only mineral to do so, and limestone and marble, rocks composed of calcite, will also react with acid. marble Calcite is transparent to opaque and may occasionally show phosphorescence or fluorescence. It is perhaps best known because of its power to produce strong double refraction of light, such that objects viewed through a clear piece of calcite appear doubled in all of their parts - a phenomenon first described by Rasmus Bartholin. A beautifully transparent variety used for optical purposes comes from Iceland, called Iceland spar. Acute scalenohedral crystals are sometimes referred to as "dogtooth spar". Iceland Iceland

External link

- [http://mineral.galleries.com/minerals/carbonat/calcite/calcite.htm Calcite information and images] Category:Carbonate minerals ja:方解石

Dolomite

:For the 18th century French naturalist Deodat de Dolomieu. For the 1975 blaxploitation film, see Dolemite. Dolemite Dolomite is the name of both a carbonate rock and a mineral (formula: CaMg(CO₃)₂) consisting of a calcium magnesium carbonate found in crystals. Dolomite rock (also dolostone) is composed predominantly of the mineral dolomite. Limestone which is partially replaced by dolomite is referred to as dolomitic limestone, or in old U. S. geologic literature as magnesian limestone. Dolomite mineral crystallizes in the trigonal - rhombohedral system. It forms white, gray to pink, commonly curved, crystals although it is usually massive. It has physical properties similar to those of the mineral calcite, but does not rapidly dissolve or effervesce (fizz) in dilute hydrochloric acid. The Mohs hardness is 3.5 to 4 and the specific gravity is 2.85. Dolomite was first described in 1791 as the rock by the French naturalist and geologist, Deodat Guy Tancrede de Gratet de Dolomieu (1750-1801) for exposures in the Dolomite Alps of northern Italy.

The dolomite problem

There is some uncertainty as to the cause of dolomite formation, as vast deposits are present in the geological record, but it is relatively rare in modern environments. This is referred to as the "Dolomite Problem". Dolomite accounts for about 10% of all sedimentary rock, including much that would have been produced near the surface of the Earth. However, laboratory synthesis of undisputed dolomite has been carried out only at temperatures of greater than 100 degrees Celsius, conditions typical of burial in sedimentary basins. Reproducible laboratory syntheses of dolomite (and magnesite) leads first to the initial precipitation of a metastable "precursor" (such as magnesium calcite), to be changed gradually into more and more of the stable phase (such as dolomite or magnesite) during periodical intervals of dissolution and reprecipitation. The general principle governing the course of this irreversible geochemical reaction has been coined "Ostwald's Step Rule". Recent research has found modern dolomite formation under anaerobic conditions in supersaturated saline lagoons along the Rio de Janeiro coast of Brazil, namely, Lagoa Vermelha and Brejo do Espinho. Similar processes have been discovered in the Coorong region of South Australia. One interesting reported case was the formation of dolomite in the kidneys of a dalmatian dog. This was believed to be due to chemical processes triggered by bacteria. Dolomite is now thought to develop under these conditions with the help of sulfate-reducing bacteria. This joins other research in pointing out many new interesting links between large-scale geology and small-scale microbiology (see geomicrobiology). The actual role of bacteria in the low-temperature formation of dolomite remains to be demonstrated. Despite considerable confusion the very mechanism invoked for example by sulfate-reducing bacteria has not yet been demonstrated.

Uses

Dolomite is used as an ornamental stone and as a raw material for the manufacture of cement. It is also a source of magnesium oxide. It is an important petroleum reservoir rock and a host rock for large strata-bound base metal (lead, zinc, copper) deposits. It is sometimes used in place of calcite in the production of iron and steel where it serves as a flux to remove impurities and assists in reducing iron ore.

Media References

A cousin of Dolomite, a ficitious element called Dolemite, featured in an episode of Futurama entitled 'Jurassic Bark'. In this episode Fry discovers his old pet dog petrified in dolemite ("the tough black mineral that won't cop out when there's heat all about," as professor Farnsworth says) and attempted to have it cloned. The purported 'indestructible' properties of dolomite's badder, blacker cousin played a part in the storyline of that episode. [http://www.gotfuturama.com/Information/Capsules/4ACV07/ Fansite Synopsys]

External links

- [http://webmineral.com/data/Dolomite.shtml Webmineral]
- [http://mineral.galleries.com/minerals/carbonat/dolomite/dolomite.htm Mineral galleries]
- [http://www.the-conference.com/JConfAbs/5/1038.pdf Role of Sulfate Reducing Bacteria During Microbial Dolomite Precipitation as Deduced from Culture Experiments]
- [http://www.jcdeelman.demon.nl/dolomite/bookprospectus.html Low temperature formation of dolomite and magnesite] Category:Sedimentary rocks Category:Rocks Category:Carbonate minerals

Fluorite

Fluorite (also called fluor-spar) is a mineral composed of calcium fluoride, CaF₂. It is an isometric mineral with a cubic habit, though octahedral and more complex isometric forms are not uncommon. Crystal twinning is common and adds complexity to the observed crystal habits.

Occurrence

Fluorite may occur as a vein deposit, especially with metallic minerals, where it often forms a part of the gangue (the worthless "host-rock" in which valuable minerals occur) and may be associated with galena, sphalerite, barite, quartz, 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. Fluorite is a widely occurring mineral which is found in large deposits in many areas. Notable deposits occur in Germany, Austria, Switzerland, England, Norway, Mexico, and Ontario in Canada. In the United States deposits are found in Missouri, Oklahoma, Illinois, Kentucky, Colorado, New Mexico, Arizona, Ohio, New Hampshire, and New York.

Fluorescence

Fluorite gives its name to the property of Fluorescence, as many samples fluoresce strongly in ultra-violet light. The fluorescence may be due to impurities such as yttrium in the crystal lattice.

Blue John

One of the most famous of the older localities of fluorite is Castleton in Derbyshire, England, where, under the name of Derbyshire Blue John, beautiful purple-blue fluorite was used for ornamental purposes, especially in the 19th century. The name derives from french "bleu et jaune" (blue and yellow) characterising its colour. It is now scarce, and only a few hundred kilograms are mined each year for ornamental and lapidary use. Recent deposits in China have produced fluorite with similar colouring and banding to the classic Blue John stone.

Uses

As well as ornamental uses, fluorite is also used as a flux in the manufacture of steel, in the making of opalescent glass, enamels for cooking utensils, and for hydrofluoric acid. Fluorite is also used in some high performance telescopes and camera lens elements instead of glass. It has a very low dispersion so light diffraction is far less than ordinary glass and in telescopes it allows crisp images of astronomical objects even at high power. Most optical material is now synthetic. The name fluorite is derived from the Latin fluo, flow, in reference to its use as a flux. Fluorite is slightly soluble in water, and is decomposed by sulfuric acid and forms free hydrofluoric acid.

References

- Hurlbut, Cornelius S.; Klein, Cornelis, 1985, Manual of Mineralogy, pp. 324 - 325, 20th ed., ISBN 0471805807
- [http://mineral.galleries.com/minerals/halides/fluorite/fluorite.htm Mineral Galleries]
- [http://webmineral.com/data/Fluorite.shtml Webmineral]
- [http://www.mindat.org/min-1576.html Mindat.org]
- [http://www.museum.state.il.us/exhibits/symbols/mineral.html Illinois state mineral]

Gallery

Image:pig.fluorite.750pix.jpg|Pig carved in fluorite, 5 cm (2 inches) long Image:USDA Mineral Flourite 93c3962.jpg|Mineral fluorite Image:Fluorite USA.jpg|Octahedral fluorite crystals from New Mexico, USA image:Fluorite_crystals_270x444.jpg|Cleaved fluorite octahedra. Category:Halide minerals Category:Derbyshire ja:蛍石

Crystal system

A crystal system is a category of space groups, which characterize symmetry of structures in three dimensions with translational symmetry in three directions, having a discrete symmetry group. A major application is in crystallography, to categorize crystals, but by itself the topic is one of 3D Euclidean geometry. There are 7 crystal systems:
- Triclinic, all cases not satisfying the requirements of any other system; thus there is no other symmetry than translational symmetry, or the only extra kind is inversion.
- Monoclinic, requires either 1 two-fold axis of rotation or 1 mirror plane.
- Orthorhombic, requires either 3 two-fold axes of rotation or 1 two fold axis of rotation and two mirror planes.
- Tetragonal, requires 1 four-fold axis of rotation.
- Rhombohedral, also called trigonal, requires 1 three-fold axis of rotation.
- Hexagonal, requires 1 six-fold axis of rotation.
- Isometric or cubic, requires 4 three-fold axes of rotation. There are 2, 13, 59, 68, 25, 27, and 36 space groups per crystal system, respectively, together 230. Within a crystal system there are two ways of categorizing space groups:
- by the linear parts of symmetries, i.e. by crystal class, also called crystallographic point group; each of the 32 crystal classes applies for one of the 7 crystal systems
- by the symmetries in the translation lattice, i.e. by Bravais lattice; each of the 14 Bravais lattices applies for one of the 7 crystal systems. The 73 symmorphic space groups (see space group) are largely combinations, within each crystal system, of each applicable point group with each applicable Bravais lattice: there are 2, 6, 12, 14, 5, 7, and 15 combinations, respectively, together 61.

Crystallographic point group

A symmetry group consists of isometric affine transformations; each is given by an orthogonal matrix and a translation vector (which may be the zero vector). Space groups can be grouped by the matrices involved, i.e. ignoring the translation vectors (see also Euclidean group). This corresponds to discrete symmetry groups with a fixed point. There are infinitely many of these point groups in three dimensions. However, only part of these are compatible with translational symmetry: the crystallographic point groups. This is expressed in the crystallographic restriction theorem. (In spite of these names, this is a geometric limitation, not just a physical one.) The point group of a crystal, among other things, determines the symmetry of the crystal's optical properties. For instance, one knows whether it is birefringent, or whether it shows the Pockels effect, simply by knowing its point group.

Overview of point groups by crystal system

The crystal structures of biological molecules (such as protein structures) can only occur in the 11 enantiomorphic point groups, as biological molecules are invariably chiral. The protein assemblies themselves may have symmetries other than those given above, because they are not intrinsically restricted by the Crystallographic restriction theorem. For example the Rad52 DNA binding protein has an 11-fold rotational symmetry (in human), however, it must form crystals in one of the 11 enantiomorphic point groups given above.

Classification of lattices

In geometry and crystallography, a Bravais lattice is a category of symmetry groups for translational symmetry in three directions, or correspondingly, a category of translation lattices. Such symmetry groups consist of translations by vectors of the form

\mathbf = n_1 \mathbf_1 + n_2 \mathbf_2 + n_3 \mathbf_3,

where n₁, n₂, and n₃ are integers and a₁, a₂, and a₃ are three non-coplanar vectors, called primitive vectors. These lattices are classified by space group of the translation lattice itself; there are 14 Bravais lattices in three dimensions; each can apply in one crystal system only. They represent the maximum symmetry a structure with the translational symmetry concerned can have. All crystalline materials recognised till now (not including quasicrystals) fit in one of these arrangements. For convenience a Bravais lattice is depicted by a unit cell which is a factor 1, 2, 3 or 4 larger than the primitive cell. Depending on the symmetry of a crystal or other pattern, the fundamental domain is again smaller, up to a factor 48. The Bravais lattices were studied by M. L. Frankenheim in 1842, who found that there were 15 Bravais lattices. This was corrected to 14 by A. Bravais in 1848.

External links

- [http://newton.ex.ac.uk/research/qsystems/people/goss/symmetry/Solids.html Overview of the 32 groups]
- [http://img.cryst.bbk.ac.uk/sgp/MISC/POINTGRP.HTM Ditto] - uses a different notation
- [http://mineral.galleries.com/minerals/symmetry/symmetry.htm Mineral galleries - Symmetry] Category:Symmetry Category:Euclidean geometry Category:Crystallography Category:Mineralogy

Hexagonal (crystal system)

In crystallography, the hexagonal crystal system is one of the 7 lattice point groups. It has the same symmetry as a right prism with a hexagonal base. There is only one hexagonal Bravais lattice. Graphite is an example of a crystal that crystallizes in the hexagonal crystal system. The point groups (crystal classes) that fall under this crystal system are listed below, followed by their representations in international notation and Schoenflies notation, and mineral examples, if they exist. Category:Crystallography

Luster

:For the file system called Lustre, see Lustre (file system). For the municipality in Norway, see Luster, Norway. :There is also a color grading software called Lustre, developed by Autodesk Media and Entertainment. Lustre (American English: luster) is a description of the way light interacts with the surface of a crystal, rock or mineral. For example, a diamond is said to have an adamantine lustre and pyrite is said to have a metallic lustre. The word lustre traces its origins back to the Latin word lux, meaning "light", and generally implies radiance, gloss, or brilliance. Other descriptive terms used for gems include vitreous, like glass; resinous, like amber; waxy, like jade; greasy, like soapstone; pearly; and silky. The term is also used to describe other items with a particular sheen (for example, fabric, especially silk and satin, or metals).
See also

- Hornblende for a description of lustrous ores. Category:Mineralogy

Streak

The term streak is used in several ways:
- Streaking, the act of running around nude in public places.
- "The Streak", a 1974 hit record by Ray Stevens about the streaking fad.
- Streak, in any mineral, refers to the color of its powder left after rubbing on an unglazed porcelain streak plate.
- The Streak, a moth of the family Geometridae.
- Streaking, in biology, is a method to transfer colonies from one Petri plate to another.
- Primitive streak is structure in animal embryo.

Specific gravity

Relative density (also known as specific gravity) is a measure of the density of a material. It is dimensionless, equal to the density of the material divided by the density of water (or, sometimes used for gases, of air). Since water's density is 1.0 × 10³ kg/m³ in SI units, the relative density of a material is approximately the density of the material measured in kg / m³ divided by 1000 (the density of water). There are no units of measurement. Water's density can also be measured as nearly one gram per cubic centimetre (at maximum density) in metric units. The relative density therefore has nearly the same value as density of the material expressed in grams per cubic centimetre, but without any units of measurement. Relative density or specific gravity are often ambiguous terms. This quantity is often stated for a certain temperature. Sometimes when this is done, it is a comparison of the density of the commodity being measured at that temperature, with the density of water at the same temperature. But they are also often compared to water at a different temperature. Relative density is often expressed in forms similar to this: :relative density:

8.15_^ \,\,

or specific gravity:

2.432_0^

The superscripts indicate the temperature at which the density of the material is measured, and the subscripts indicate the temperature of the water to which it is compared. Density of water calculated from formula in 68th CRC Handbook of Chemistry and Physics 1987–1988. Water is nearly incompressible. But it does compress a little; it takes pressures over about 400 kPa or 4 atmospheres before water can reach a density of 1000.000 kg/m³ at any temperature. Relative density is often used by geologists and mineralogists to help determine the mineral content of a rock or other sample. Gemologists use it as an aid in the identification of gemstones. The reason that relative density is measured in terms of the density of water is because that that is the easiest way to measure it in the field. Basically, density is defined as the mass of a sample divided by its volume. With an irregularly shaped rock, the volume can be very difficult to accurately measure. The most accurate way is to put it in a water-filled graduated cylinder and see how much water it displaces. It is also possible to simply suspend the sample from a spring scale and weigh it under water. Solving Isaac Newton's equations yields the following formula for measuring specific gravity: :

G = \frac

where G is the relative density, W is the weight of the sample (measured in pounds-force, newtons, or some other unit of force), and F is the force, measured in the same units, while the sample was submerged. Note that with this technique it is difficult to measure relative densities less than one, because in order to do so, the sign of F must change, requiring the measurement of the downward force needed to keep the sample underwater. Another practical method uses three measurements. The mineral sample is weighed dry. Then a container of water is weighed, and weighed again with the sample immersed. Subtracting the last two readings gives the weight of the displaced water. The relative density result is the dry sample weight divided by that of the displaced water. This method works with scales that can't easily accommodate a suspended sample, and also allows for measurement of samples that are less dense than water. Category:Physical quantity ja:比重

Ultraviolet

Ultraviolet (UV) radiation is electromagnetic radiation of a wavelength shorter than that of the visible region, but longer than that of soft X-rays. It can be subdivided into near UV (380–200 nm wavelength), far or vacuum UV (200–10 nm; abbrev. FUV or VUV), and extreme UV (1–31 nm; abbrev. EUV or XUV). When considering the effect of UV radiation on human health and the environment, the range of UV wavelengths is often subdivided into UVA (380–315 nm), also called Long Wave or "blacklight"; UVB (315–280 nm), also called Medium Wave; and UVC (< 280 nm), also called Short Wave or "germicidal". See 1 E-7 m for a list of objects of comparable sizes. In photolithography, in laser technology, etc., the term deep ultraviolet or DUV refers to wavelengths below 300nm. The name means "beyond violet" (from Latin ultra, "beyond"), violet being the color of the shortest wavelengths of visible light. Some of the UV wavelengths are colloquially called black light, as it is invisible to the human eye. Some animals, including birds, reptiles, and insects such as bees, can see into the near ultraviolet. Many fruits, flowers, and seeds stand out more strongly from the background in ultraviolet wavelengths as compared to human color vision. Many birds have patterns in their plumage that are invisible at usual wavelengths but seen in ultraviolet, and the urine of some animals is much easier to spot with ultraviolet. The Sun emits ultraviolet radiation in the UVA, UVB, and UVC bands, but because of absorption in the atmosphere's ozone layer, 99% of the ultraviolet radiation that reaches the Earth's surface is UVA. (Some of the UVC light is responsible for the generation of the ozone.) Ordinary glass is transparent to UVA but is opaque to shorter wavelengths. Silica or quartz glass, depending on quality, can be transparent even to vacuum UV wavelengths. The onset of vacuum UV, 200 nm, is defined by the fact that ordinary air is opaque below this wavelength. This opacity is due to the strong absorption of light of these wavelengths by oxygen in the air. Pure nitrogen (less than about 10 ppm oxygen) is transparent to wavelengths in the range of about 150–200 nm. This has wide practical significance now that semiconductor manufacturing processes are using wavelengths shorter than 200 nm. By working in oxygen-free gas, the equipment does not have to be built to withstand the pressure differences required to work in a vacuum. Some other scientific instruments, such as circular dichroism spectrometers, are also commonly nitrogen purged and operate in this spectral region. Extreme UV is characterized by a transition in the physics of interaction with matter: wavelengths longer than about 30 nm interact mainly with the chemical valence electrons of matter, while wavelengths shorter than that interact mainly with inner shell electrons and nuclei. The long end of the EUV/XUV spectrum is set by a prominent He⁺ spectral line at 30.4nm. XUV is strongly absorbed by most known materials, but it is possible to synthesize multilayer optics that reflect up to about 50% of XUV radiation at normal incidence. This technology has been used to make telescopes for solar imaging (pioneered by the NIXT and MSSTA sounding rockets in the 1990s; current examples are SOHO/EIT and TRACE) and microphotolithography (printing of traces and devices on microchips). microchips as seen in deep ultraviolet light at 17.1 nm by the Extreme ultraviolet Imaging Telescope instrument aboard the SOHO spacecraft]]

Discovery

Soon after infrared radiation had been discovered, the German physicist Johann Wilhelm Ritter began to look for radiation at the opposite end of the spectrum, at the short wavelengths beyond violet. In 1801 he used silver chloride, a light-sensitive chemical, to show that there was a type of invisible light beyond violet, which he called chemical rays. At that time, many scientists, including Ritter, concluded that light was composed of three separate components: an oxidising or calorific component (infrared), an illuminating component (visible light), and a reducing or hydrogenating component (ultraviolet). The unity of the different parts of the spectrum was not understood until about 1842, with the work of Macedonio Melloni, Alexandre-Edmond Becquerel and others. During that time, UV radiation was also called "actinic radiation".

Health concerns and protection

In humans, prolonged exposure to solar UV radiation may result in acute and chronic health effects on the skin, eye, and immune system [http://www.who.int/uv/health/en/]. Tungsten-halogen lamps have bulbs made of quartz, not of ordinary glass. Tungsten-halogen lamps that are not filtered by an additional layer of ordinary glass are a common, useful, and possibly dangerous, source of UVB light. UVC rays are the highest energy, most dangerous type of ultraviolet light. Little attention has been given to UVC rays in the past since they are filtered out by the atmosphere. However, their use in equipment such as pond sterilization units may pose an exposure risk, if the lamp is switched on outside of its enclosed pond sterilization unit. sterilization

Skin

UVA, UVB and UVC all can damage collagen fibers and thereby accelerate aging of the skin. In general, UVA is the least harmful, but can contribute to the aging of skin, DNA damage and possibly skin cancer. It penetrates deeply and does not cause sunburn. Because it does not cause reddening of the skin (erythema) it cannot be measured in the SPF testing. There is no good clinical measurement of the blocking of UVA radiation, but it is important that sunscreen block both UVA and UVB. UVA light is known as "dark-light" and, because of its longer wavelength, can penetrate most windows. It also penetrates deeper into the skin than UVB light and is thought to be a prime cause of wrinkles. UVB light in particular has been linked to skin cancers such as melanoma. The radiation excites DNA molecules in skin cells, causing covalent bonds to form between adjacent thymine bases, producing thymidine dimers. Thymidine dimers do not base pair normally, which can cause distortion of the DNA helix, stalled replication, gaps, and misincorporation. These can lead to mutations, which can result in cancerous growths. The mutagenicity of UV radiation can be easily observed in bacteria cultures. This cancer connection is one reason for concern about ozone depletion and the ozone hole. As a defense against UV radiation, the body tans when exposed to moderate (depending on skin type) levels of radiation by releasing the brown pigment melanin. This helps to block UV penetration and prevent damage to the vulnerable skin tissues deeper down. Suntan lotion that partly blocks UV is widely available (often referred to as "sun block" or "sunscreen"). Most of these products contain an "SPF rating" that describes the amount of protection given. This protection applies only to UVB light. In any case, most dermatologists recommend against prolonged sunbathing.

Eye

High intensities of UVB light are hazardous to the eyes, and exposure can cause welder's flash (photokeratitis or arc eye) and may lead to cataracts, pterygium[http://ajp.amjpathol.org/cgi/content/abstract/162/2/567] [http://ajp.amjpathol.org/cgi/content/abstract/167/2/489], and pinguecula formation. Protective eyewear is beneficial to those who are working with or those who might be exposed to ultraviolet radiation, particularly short wave UV. Given that light may reach the eye from the sides, full coverage eye protection is usually warranted if there is an increased risk of exposure as in high altitude mountaineering. Mountaineers are exposed to higher than ordinary levels of UV radiation, both because there is less atmospheric filtering and because of reflection from snow and ice. Ordinary eyeglasses give some protection, and most plastic lenses give more protection than glass lenses. Some plastic lens materials, such as polycarbonate, block most UV. There are protective treatments available for eyeglass lenses that need it to give better protection. Most intraocular lenses help to protect the retina by absorbing UV radiation.

Immune system

Beneficial effects

A positive effect of UV light is that it induces the production of vitamin D in the skin. Grant (2002) claims tens of thousands of premature deaths occur in the US annually from cancer due to insufficient UVB exposures (apparently via vitamin D deficiency). Another effect of vitamin D deficiency is osteomalacia, which can result in bone pain, difficulty in weight bearing and sometimes fractures. Ultraviolet radiation has other medical applications, in the treatment of skin conditions such as psoriasis. UVB and UVA radiation can be used, in conjunction with psoralens (PUVA treatment).

Uses

UV light has many various uses.

Black lights

PUVA A black light is the name commonly given to a lamp emitting almost entirely long wave UV radiation and very little visible light. Fluorescent black lights are typically made in the same fashion as normal fluorescent lights except that only one phosphor is used instead of the typical 2 or 3 which produce a full spectrum light and the normally clear glass envelope of the bulb is replaced by a deep bluish purple glass called Wood's glass. Wood's glass is a nickel oxide, cobalt oxide-doped glass which blocks virtually all visible light above 400 nanometers. The phosphor typically used for a near 368 to 371 nanometer emission peak is either europium-doped strontium fluoroborate (SrB₄O₇F:Eu²⁺) or europium-doped strontium borate (SrB₄O₇:Eu²⁺) while the phosphor used to produce a peak around 350 to 353 nanometers is lead-doped barium silicate (BaSi₂O₅:Pb⁺). The ultraviolet radiation itself is invisible to the human eye, but illuminating certain materials with UV radiation prompts the visible effects of fluorescence and phosphorescence. Black light testing is commonly used to authenticate antiques and bank notes. It is extensively used in non-destructive testing (NDT); fluorescing fluids are applied to metal structures and illuminated with a black light. Cracks and other artefacts can easily be detected. It is also used to illuminate pictures painted with fluorescent colors (preferably on black velvet to intensify the illusion of self-illumination). The fluorescence it prompts from certain textile fibers is also used as a recreational effect (as seen for instance in the opening credits of the James Bond film A View to a Kill). A View to a Kill In forensic investigations, black lights are used to reveal the presence of trace evidence, such as blood, urine, semen and saliva, by causing visible fluorescence in these substances. The use of this technique by exposé style television news magazines for reporting on the various unsanitary and mysterious stains found in hotel rooms has become such an oft-repeated stunt that it has been lampooned on comedy shows such as The Family Guy.

Fluorescent lamps

Fluorescent lamps produce UV radiation by the emission of low-pressure mercury gas. A phosphorescent coating on the inside of the tubes absorbs the UV and becomes visible. The main mercury emission wavelength is in the UVC range. Unshielded exposure of the skin or eyes to mercury arc lamps that do not have a conversion phosphor is quite dangerous. The light from a mercury lamp is predominantly at discrete wavelengths. Other practical UV sources with more continuous emission spectra include xenon arc lamps (commonly used as sunlight simulators), deuterium arc lamps, mercury-xenon arc lamps, metal-halide arc lamps, and tungsten-halogen incandescent lamps.

Pest control

Ultraviolet fly traps are used for the elimination of various small flying insects. They are attracted to the UV light and are killed using an electrical shock or trapped once they come into contact with the device.

Spectrophotometry

UV/VIS spectroscopy is widely used as a technique in chemistry, for analysis of chemical structure, most notably conjugated systems. UV radiation is often used in visible spectrophotometry to determine the existence of fluorescence a given sample.

Astronomy

spectrophotometry's north pole as seen in ultraviolet light by the Hubble Space Telescope.]] In astronomy, very hot objects preferentially emit UV radiation (see Wien's law). However, the same ozone layer that protects us causes difficulties for astronomers observing from the Earth, so most UV observations are made from space. (see UV astronomy, space observatory)

Analyzing minerals

Ultraviolet lamps are also used in analyzing minerals, gems, and in other detective work including authentication of various collectibles. Materials may look the same under visible light, but fluoresce to different degrees under ultraviolet light; or may fluoresce differently under short wave ultraviolet versus long wave ultraviolet. UV fluorescent dyes are used in many applications (for example, biochemistry and forensics). The fluorescent protein Green Fluorescent Protein (GFP) is often used in genetics as a marker. Many substances, proteins for instance, have significant light absorption bands in the ultraviolet that are of use and interest in biochemistry and related fields. UV-capable spectrophotometers are common in such laboratories.

Photolithography

Ultraviolet radiation is used for very fine resolution photolithography, a procedure where a chemical known as a photoresist is exposed to UV radiation which has passed through a mask. The light allows chemical reactions to take place in the photoresist, and after development (a step that either removes the exposed or unexposed photoresist), a geometric pattern which is determined by the mask remains on the sample. Further steps may then be taken to "etch" away parts of the sample with no photoresist remaining. UV radiation is used extensively in the electronics industry because photolithography is used in the manufacture of semiconductors, integrated circuit components and printed circuit boards.

Checking electrical insulation

printed circuit board and Franklinite containing mineral sample as seen under visible light (top) and fluorescing under UV light (bottom).]] A new application of UV is to detect corona discharge (often simply called "corona") on electrical apparatus. Degradation of insulation of electrical apparatus or pollution causes corona, wherein a strong electric field ionizes the air and excites nitrogen molecules, causing the emission of ultraviolet radiation. The corona degrades the insulation level of the apparatus. Corona produces ozone and to a lesser extent nitrogen oxide which may subsequently react with water in the air to form nitrous acid and nitric acid vapour in the surrounding air. [http://www.seeing-corona.com/]

Sterilization

Ultraviolet lamps are used to sterilize workspaces and tools used in biology laboratories and medical facilities. Commercially-available low pressure mercury-vapor lamps emit about 86% of their light at 254 nanometers (nm) which coincides very well with one of the two peaks of the germicidal effectiveness curve (i.e., effectiveness for UV absorption by DNA). One of these peaks is at about 265 nm and the other is at about 185 nm. Although 185 nm is better absorbed by DNA, the quartz glass used in commercially-available lamps, as well as environmental media such as water, are more opaque to 185 nm than 254 nm (C. von Sonntag et al., 1992). UV light at these germicidal wavelengths causes adjacent thymine molecules on DNA to dimerize, if enough of these defects accumulate on a microorganism's DNA its replication is inhibited, thereby rendering it harmless (even though the organism may not be killed outright). Since microorganisms can be shielded from ultraviolet light in small cracks and other shaded areas, however, these lamps are used only as a supplement to other sterilization techniques.

Disinfecting drinking water

UV radiation can be an effective viricide and bactericide. Disinfection using UV radiation was more commonly used in wastewater treatment applications but is finding increased usage in drinking water treatment. Generally, UV disinfection is more effective for bacteria and virus, which have more exposed genetic material, than for larger pathogens which have outer coatings or that form cyst states (e.g., Giardia) that shield their DNA from the UV light. However, it was recently discovered that ultraviolet radiation can be somewhat effective for treating the microorganism Cryptosporidium. The findings resulted in two [http://www.calgoncarbon.com/company/news/index.cfm?mode=detail&id=DF8B2807-AB22-705E-D9769AEA0B6A744E US patents] and the use of UV radiation as a viable method to treat drinking water.

Food Processing

As consumer demand for fresh and "fresh like" food products increases, the demand for nonthermal methods of food processing is likewise on the rise. In addition, public awareness regarding the dangers of food poisoning is also raising demand for improved food processing methods. Ultraviolet radiation is used in several food processes to remove unwanted microorganisms. UV light can be used to pasteurize fruit juices by pumping the juice over a high intensity ultraviolet light source. The effectiveness of such a process depends on the UV absorbance of the juice (see Beer's law).

Fire detection

Ultraviolet (UV) detectors generally use either a solid-state device, such as one based on silicon carbide or aluminum nitride, or a gas-filled tube as the sensing element. UV detectors which are sensitive to UV light in any part of the spectrum respond to irradiation by sunlight and artificial light. A burning hydrogen flame, for instance, radiates strongly in the 185 to 260 nanometre) range and only very weakly in the IR region, while a coal fire emits very weakly in the UV band yet very strongly at IR wavelengths; thus a fire detector which operates using both UV and IR detectors is more reliable than one with a UV detector alone. Virtually all fires emit some radiation in the UVB band, while the Sun's radiation at this band is absorbed by the Earth's atmosphere. The result is that the UV detector is "solar blind", meaning it will not cause an alarm in response to radiation from the Sun, so it can easily be used both indoors and outdoors. UV detectors are sensitive to most fires, including hydrocarbons, metals, sulfur, hydrogen, hydrazine, and ammonia. Arc welding, electrical arcs, lightning, X-rays used in nondestructive metal testing equipment (though this is highly unlikely), and radioactive materials can produce levels that will activate a UV detection system. The presence of UV-absorbing gases and vapors will attenuate the UV radiation from a fire, adversely affecting the ability of the detector to detect flames. Likewise, the presence of an oil mist in the air or an oil film on the detector window will have the same effect.

Curing of adhesives and coatings

Certain adhesives and coatings are formulated with photoinitiators. When exposed to the correct wavelengths of UV light, polymerisation occurs, and so the adhesives harden or cure. Usually, this reaction is very quick, a matter of a few seconds. Applications include glass and plastic bonding, optical fiber coatings, the coating of flooring, and dental fillings.

External link

- [http://www.iuva.org/ International Ultraviolet Association]

References

- Grant, William B. (2002). [http://www3.interscience.wiley.com/cgi-bin/abstract/91016211/ABSTRACT An estimate of premature cancer mortality in the US due to inadequate doses of solar ultraviolet-B radiation.] Cancer 94 (6), 1867–1875.
- Matsumura Y, Ananthaswamy HN (2004). [http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=15020192 Toxic effects of UV radiation on the skin.] Toxicol. Appl. Pharmacol. 195 (3), 298-308.
- Hu S, et al. (2004). [http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=15262692 UV radiation and melanoma in US Hispanics & blacks.] Arch Dermatol. 140 (7), 819-824. Category:Electromagnetic spectrum ms:Ultraungu ja:紫外線 simple:Ultraviolet

Refractive index

The refractive index of a material is the factor by which the phase velocity of electromagnetic radiation is slowed relative to vacuum. It is usually given the symbol n, and defined for a material by: :

n=\sqrt

where ε_r is the material's relative permittivity, and μ_r is its relative permeability. For a non-magnetic material, μ_r is very close to 1, therefore n is approximately

\sqrt

. The phase velocity is defined as the rate at which the crests of the waveform propagate; that is, the rate at which the phase of the waveform is moving. The group velocity is the rate that the envelope of the waveform is propagating; that is, the rate of variation of the amplitude of the waveform. It is the group velocity that (almost always) represents the rate that information (and energy) may be transmitted by the wave, for example the velocity at which a pulse of light travels down an optical fiber.

The speed of light

The speed of all electromagnetic radiation in vacuum is the same, approximately 3×10⁸ meters per second, and is denoted by c. Therefore, if v is the phase velocity of radiation of a specific frequency in a specific material, the refractive index is given by :

n =\frac

This number is typically greater than one: the higher the index of the material, the more the light is slowed down. However, at certain frequencies (e.g. near absorption resonances, and for x-rays), n will actually be smaller than one. This does not contradict the theory of relativity, which holds that no information-carrying signal can ever propagate faster than c, because the phase velocity is not the same as the group velocity or the signal velocity. Sometimes, a "group velocity refractive index", usually called the group index is defined: :

n_g=\frac

, where v_g is the group velocity. This value should not be confused with n, which is always defined with respect to the phase velocity. At the microscale, an electromagnetic wave's phase velocity is slowed in a material because the electric field creates a disturbance in the charges of each atom (primarily the electrons) proportional to the permittivity. The charges will, in general, oscillate slightly out of phase with respect to the driving electric field. The charges thus radiate their own electromagnetic wave that is at the same frequency but with a phase delay. The macroscopic sum of all such contributions in the material is a wave with the same frequency but shorter wavelength than the original, leading to a slowing of the wave's phase velocity. Most of the radiation from oscillating material charges will modify the incoming wave, changing its velocity. However, some net energy will be radiated in other directions (see scattering). If the refractive indices of two materials are known for a given frequency, then one can compute the angle by which radiation of that frequency will be refracted as it moves from the first into the second material from Snell's law. Recent research has also demonstrated the existence of negative refractive index which can occur if ε and μ are simultaneously negative. Not thought to occur naturally, this can be achieved with so called metamaterials and offers the possibility of perfect lenses and other exotic phenomena such as a reversal of Snell's law.

Dispersion and Absorption

In real materials, the polarization does not respond instantaneously to an applied field. This causes dielectric loss, which can be expressed by a permittivity that is both complex and frequency dependent. Real materials are not perfect insulators either, i.e. they have non-zero direct current conductivity. Taking both aspects into consideration, we can define a complex index of refraction: :

\tilde=n-i\kappa

Here, n is the refractive index indicating the phase velocity as above, while κ is called the extinction coefficient, which indicates the amount of absorption loss when the electromagnetic wave propagates through the material. Both n and κ are dependent on the frequency (wavelength). The effect that n varies with frequency (except in vacuum, where all frequencies travel at the same speed, c) is known as dispersion, and it is what causes a prism to divide white light into its constituent spectral colors, explains rainbows, and is the cause of chromatic aberration in lenses. In regions of the spectrum where the material does not absorb, the of the refractive index tends to increase with frequency. Near absorption peaks, the curve of the refractive index is a complex form given by the Kramers-Kronig relations, and can decreases with frequency. Since the refractive index of a material varies with the frequency (and thus wavelength) of light, it is usual to specify the corresponding vacuum wavelength at which the refractive index is measured. Typically, this is done at various well-defined spectral emission lines; for example, n_D is the refractive index at the Fraunhofer "D" line, the centre of the yellow sodium doublet emission at 589.29 nm wavelength. The Sellmeier equation is an empirical formula that works well in describing dispersion, and Sellmeier coefficients are often quoted instead of the refractive index in tables. For some representative refractive indices at different wavelengths, see list of indices of refraction. As shown above, dielectric loss and non-zero DC conductivity in materials cause absorption. Good dielectric materials such as glass have extremely low DC conductivity, and at low frequencies the dielectric loss is also negligible, resulting in almost no absorption (κ ≈ 0). However, at higher frequencies (such as visible light), dielectric loss may increase absorption significantly, reducing the material's transparency to these frequencies. The real and imaginary parts of the complex refractive index are related through use of the Kramers-Kronig relations. For example, one can determine a material's full complex refractive index as a function of wavelength from an absorption spectrum of the material.

Anisotropy

The refractive index of certain media may be different depending on the polarization and direction of propagation of the light through the medium. This is known as birefringence or anisotropy and is described by the field of crystal optics. In the most general case, the dielectric constant is a rank-2 tensor (a 3 by 3 matrix), which cannot simply be described by refractive indices except for polarizations along principal axes. In magneto-optic (gyro-magnetic) and optically active materials, the principal axes are complex (corresponding to elliptical polarizations), and the dielectric tensor is complex-Hermitian (for lossless media); such materials break time-reversal symmetry and are used e.g. to construct Faraday isolators.

Nonlinearity

The strong electric field of high intensity light (such as output of a laser) may cause a medium's refractive index to vary as the light passes through it, giving rise to nonlinear optics. If the index varies quadratically with the field (linearly with the intensity), it is called the optical Kerr effect and causes phenomena such as self-focusing and self phase modulation. If the index varies linearly with the field (which is only possible in materials that do not possess inversion symmetry), it is known as the Pockels effect.

Inhomogeneity

If the refractive index of a medium is not constant, but varies gradually with position, the material is known as a gradient-index medium and is described by gradient index optics. Light travelling through such a medium can be bent or focussed, and this effect can be exploited to produce lenses, some optical fibers and other devices. Some common mirages are caused by a spatially-varying refractive index of air.

Applications

The refractive index of a material is the most important property of any optical system that uses refraction. It is used to calculate the focusing power of lenses, and the dispersive power of prisms. For a solution of sugar, the refractive index can be used to determine the sugar content (see Brix). The refractive index is also used in chemistry to determine the purity of chemicals.

External link

- [http://www.tf.uni-kiel.de/matwis/amat/elmat_en/index.html Dielectric materials]

Sodium light

A sodium vapor lamp is a gas discharge lamp which uses sodium in an excited state to produce light. There are two varieties of such lamps: low pressure and high pressure.

Low pressure / LPS / SOX

light LPS Lamps (Low Pressure Sodium), also known as SOX Lamps (Sodium OXide), consist of an outer vacuum envelope of glass coated with an infrared reflecting layer of indium-tin oxide, a semi-conductor material that allows the visible light wavelengths out and keeps the infrared (heat) back. It has an inner borosilicate 2 ply glass U shaped tube containing sodium metal and a small amount of neon and argon gas to start the gas discharge, so when the lamp is turned on it emits a dim red/pink light to warm the sodium metal and within a few minutes it turns into the common bright orange/yellow color as the sodium metal vaporizes. These lamps produce a virtually monochromatic light in the 589 nm wavelength. As a result, objects have no color rendition under a LPS light, only the reflection of the 589 nm light. LPS lamps are the most efficient electrically-powered light source — up to 200 lm/W. As a result they are widely used for outdoor lighting such as streetlights and security lighting where color rendition is less important. LPS lamps are available with power ratings from 18 W up to 180 W.

High pressure / HPS

power High pressure sodium (HPS) lamps are smaller and contain some other elements (for example, mercury), produce a dark pink glow when first struck, and produce a pinkish orange light when warmed up. These lamps produce continuous spectrum light (not monochromatic), hence colors of objects under them can be distinguished. This leads them to be used in areas where good color rendering is important, or desired (such as to identify the color of a fleeing suspect's car). High pressure sodium lamps are quite efficient — about 100 lm/W, up to 150 lm/W. Therefore they are widely used for outdoor lighting such as streetlights and security lighting where color rendition is more important. Because of the extremely high chemical activity of the high pressure sodium arc, the arc tube is typically made of translucent aluminum oxide (alumina). This construction led General Electric to use the tradename "Lucalox" for their line of high-pressure sodium lamps.

Theory of operation

General Electric The operation of a high-pressure sodium lamp is illustrated in the diagram on the right. An amalgam of metallic sodium and mercury lies at the coolest part of the lamp and provides the sodium and mercury vapor in which the arc is drawn. The temperature of the amalgam is determined to a great extent by lamp power, and this temperature in turn determines the sodium and mercury pressures throughout the arc tube. An increase in the metal pressures will cause a decrease in the electrical resistance of the lamp. For a given voltage, there are generally three modes of operation:
- the lamp is extinguished and no current flows
- the lamp is operating, with liquid amalgam in the tube
- the lamp is operating with all amalgam evaporated The first and last states are stable, because the lamp resistance is weakly related to the voltage, but the second state is unstable. Any anomalous increase in current will cause an increase in power, causing an increase in amalgam temperature, which will cause a decrease in resistance, which will cause a further increase in current. This will create a runaway effect, and the lamp will jump to the high-current state. Since the lamp is not designed to handle this much power, this would result in catastrophic failure. Similarly, an anomalous drop in current will drive the lamp to extinction. The lamp is powered by an AC voltage source in series with an inductive "ballast" in order to supply a nearly constant current to the lamp, rather than a constant voltage, thus assuring stable operation. The ballast is usually inductive rather than simply being resistive which minimizes resistive losses. Also, since the lamp effectively extinguishes at each zero-current point in the AC cycle, the inductive ballast assists in the reignition by providing a voltage spike at the zero-current point. The light from the lamp consists of atomic emission lines of mercury and sodium, but is dominated by the sodium D-line emission. This line is extremely pressure (resonance) broadened and is also self-reversed due to absorption in the cooler outer layers of the arc, giving the lamp its improved color rendering characteristics. In addition, the red wing of the D-line emission is further pressure broadened (Van der Waals broadened) by the high mercury pressure in the lamp.

Light pollution considerations

For placements where light pollution is of prime importance (for example an observatory parking lot), low pressure sodium is preferred. As it emits light on only one wavelength, it is the easiest to filter out.

References

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- Category:Lighting

Cubic (crystal system)

In crystallography, the cubic crystal system (or isometric crystal system) is the most symmetric of the 7 crystal systems. The system is composed of the three Bravais lattices whose symmetry group is that of a cube. The three Bravais lattices that form the cubic crystal system are: The cubes drawn are the conventional unit cells. For a cube whose vertices include 000 and 200, bcc has additional lattice point 111, while fcc has 110, 101, and 011. For bcc the