Gold
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Reverted edits by [[Special:Contributions/OrgasGirl|OrgasGirl]] to last version by TXiKiBoT (using [[WP:HG|Huggle]])
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{{Two other uses|the metal|the color|Gold (color)||}}
{{infobox gold}}
'''Gold''' ({{pronEng|ˈɡoʊld}}) is a [[chemical element]] with the symbol '''Au''' (from its [[Latin]] name ''aurum'') and [[atomic number]] 79. It is a highly sought-after [[precious metal]] which, for many centuries, has been used as [[money]], a [[store of value]] and in [[jewelry]]. The metal occurs as [[gold nugget|nuggets]] or grains in rocks, underground "veins" and in [[alluvial deposit]]s. It is one of the [[coinage metal]]s. Gold is dense, soft, shiny and the most [[malleable]] and [[ductile]] of the known metals. Pure gold has a bright yellow color traditionally considered attractive.
Gold formed the basis for the [[gold standard]] used before the collapse of the [[Bretton Woods system]]. The [[ISO 4217|ISO currency code]] of gold [[bullion]] is '''XAU'''.
Modern industrial uses include [[dentistry]] and [[electronics]], where gold has traditionally found use because of its good resistance to oxidative corrosion.
Chemically, gold is a [[transition metal]] and can form trivalent and univalent cations upon solvation. Gold does not react with most chemicals, but is attacked by [[chlorine]], [[fluorine]], [[aqua regia]] and [[cyanide]]. Gold dissolves in [[mercury (element)|mercury]], forming [[amalgam]] alloys, but does not react with it. Gold is insoluble in [[nitric acid]], which will dissolve silver and base metals, and this is the basis of the gold refining technique known as "inquartation and parting". Nitric acid has long been used to confirm the presence of gold in items, and this is the origin of the colloquial term "acid test," referring to a ''gold standard'' test for genuine value.
==Characteristics==
Gold is the most [[malleable]] and [[ductility|ductile]] metal; a single [[gram]] can be beaten into a sheet of one [[square meter]], or an [[ounce]] into 300 [[square foot|square feet]]. Gold leaf can be beaten thin enough to become translucent. The transmitted light appears greenish blue, because gold strongly reflects yellow and red.
Gold readily forms alloys with many other metals. These alloys can be produced to increase the hardness or to create exotic colors (see below). Gold is a good conductor of [[heat]] and [[electricity]], and is not affected by [[Earth's atmosphere|air]] and most [[reagent]]s. Heat, moisture, [[oxygen]], and most [[corrosion|corrosive]] agents have very little chemical effect on gold, making it well-suited for use in [[coin]]s and [[jewelry]]; conversely, [[halogen]]s will chemically alter gold, and [[aqua regia]] dissolves it via formation of the chloraurate ion.
Common [[oxidation state]]s of gold include +1 (gold(I) or aurous compounds) and +3 (gold(III) or auric compounds). Gold ions in solution are readily [[reduction (chemistry)|reduced]] and [[Precipitation (chemistry)|precipitated]] out as gold metal by adding any other metal as the reducing agent. The added metal is [[oxidation|oxidized]] and dissolves allowing the gold to be displaced from solution and be recovered as a solid precipitate.
Recent research undertaken by Sir Frank Reith of the Australian National University shows that microbes play an important role in forming gold deposits, transporting and precipitating gold to form grains and nuggets that collect in alluvial deposits.<ref>{{cite web | url=http://www.abc.net.au/science/news/enviro/EnviroRepublish_1032376.htm | title=Environment & Nature News - Bugs grow gold that looks like coral - 28/01/2004 | accessdate=2006-07-22}}</ref>
High quality pure metallic gold is tasteless; in keeping with its resistance to corrosion (it is metal ions which confer taste to metals).
In addition, gold is very dense, a cubic meter weighing 19300 [[kilograms|kg]]. By comparison, the density of [[lead]] is 11340 kg/m³, and the densest element, [[iridium]], is 22650 kg/m³.
=== Color of gold ===
The usual gray color of metals depends on their "[[electron]] sea" that is capable of absorbing and re-emitting [[photon]]s over a wide range of frequencies. Gold behaves differently, depending on subtle [[relativistic quantum chemistry|relativistic effects]] that affect the [[orbital]]s around gold atoms.<ref>[http://math.ucr.edu/home/baez/physics/Relativity/SR/gold_color.html Relativity in Chemistry<!-- Bot generated title -->]</ref>
==Applications==
===As the metal===
====Medium of monetary exchange====
In various countries, gold is used as a standard for [[money|monetary]] exchange, in [[coin]]age and in [[jewelry]]. Pure gold is too soft for ordinary use and is typically hardened by alloying with copper or other base metals. The gold content of gold alloys is measured in [[carat (purity)|carats]] (k), pure gold being designated as 24k.
Gold coins intended for circulation from 1526 into the 1930s were typically a standard 22k alloy called [[crown gold]], for hardness. Modern collector/investment [[bullion coins]] (which do not require good mechanical wear properties) are typically 24k, although the [[American Gold Eagle]] and British [[gold sovereign]] continue to be made at 22k, on historical tradition. The [[Canadian Gold Maple Leaf]] coin contains the highest purity gold of any popular bullion coin, at 99.999% (.99999 fine). Several other 99.99% pure gold coins are currently available, including Australia's Gold Kangaroos (first appearing in 1986 as the [[Australian Gold Nugget]], with the kangaroo theme appearing in 1989), the several coins of the Australian Lunar Calendar series, and the Austrian Philharmonic. In 2006, the U.S. Mint began production of the [[American Buffalo (coin)|American Buffalo]] gold bullion coin also at 99.99% purity.
Today, gold has fallen out of favor for use in coins made for general circulation.
====Jewelry====
Because of the softness of pure (24k) gold, it is usually alloyed with base metals for use in jewelry, altering its hardness and ductility, melting point, color and other properties. Alloys with lower caratage, typically 22k, 18k, 14k or 10k, contain higher percentages of copper, silver or other base metals in the alloy. [[Copper]] is the most commonly used base metal, yielding a redder metal, eighteen carat gold containing 25% copper is found in antique and Russian jewellery and has a distinct, though not dominant, copper cast, creating [[rose gold]]. Fourteen carat gold-copper alloy is nearly identical in color to certain [[bronze]] alloys, and both may be used to produce police and other badges. Blue gold can be made by alloying with [[iron]] and purple gold can be made by alloying with [[aluminum]], although rarely done except in specialized jewelry. Blue gold is more brittle and therefore more difficult to work with when making jewelry. Fourteen and eighteen carat gold alloys with [[silver]] alone appear greenish-yellow and are referred to as green gold. White gold alloys can be made with [[palladium]] or [[nickel]]. White 18 carat gold containing 17.3% nickel, 5.5% zinc and 2.2% copper is silver in appearance. Nickel is toxic, however, and its release from nickel white gold is controlled by legislation in Europe. Alternative white gold alloys are available based on palladium, silver and other white metals (World Gold Council), but the palladium alloys are more expensive than those using nickel. High-carat white gold alloys are far more resistant to corrosion than are either pure [[silver]] or [[sterling silver]]. The Japanese craft of [[Mokume-gane]] exploits the color contrasts between laminated colored gold alloys to produce decorative wood-grain effects.
[[Image:Gold ingots.jpg|right|200px]]
[[Image:DSC00710.JPG|thumb|200px|right|The 220 kg Gold brick displayed in Chinkuashi Gold Museum, Taiwan.]]
====Other====
*In medieval times, gold was often seen as beneficial for the health (even though it was not), in the belief that something that rare and beautiful could not be anything but healthy. Even some modern [[Esotericism|esotericists]] and forms of [[alternative medicine]] assign metallic gold a healing power. Some gold salts do have anti-inflammatory properties and are used as pharmaceuticals in the treatment of arthritis and other similar conditions. However, only salts and radioisotopes of gold are of pharmacological value, as elemental (metallic) gold is inert to all chemicals it encounters inside the body.
*[[Gold leaf]], flake or dust is used on and in some gourmet foodstuffs, notably sweets and drinks as decorative ingredient.<ref>{{Cite web | title = The Food Dictionary: Varak | publisher = Barron's Educational Services, Inc | date= 1995 | url = http://www.epicurious.com/cooking/how_to/food_dictionary/entry?id=5061 | accessdate = 2007-05-27}}</ref> Gold flake was used by the nobility in Medieval [[Europe]] as a decoration in foodstuffs and drinks, in the form of leafs, flakes or dust, either to demonstrate the host's wealth or in the belief that something that valuable and rare must be beneficial for one's health.
*Gold [[solder]] is used for joining the components of gold jewelry by high-temperature hard soldering or [[brazing]]. If the work is to be of [[hallmark]]ing quality, gold solder must match the carat weight of the work, and alloy formulas are manufactured in most industry-standard carat weights to color match yellow and white gold. Gold solder is usually made in at least three melting-point ranges referred to as Easy, Medium and Hard. By using the hard, high-melting point solder first, followed by solders with progressively lower melting points, goldsmiths can assemble complex items with several separate soldered joints.
* Gold can be used in food and has the [[E number|E Number]] 175. [[Goldwasser]] ([[German Language|German]]: "Goldwater") is a traditional herbal [[liqueur]] produced in [[Gdańsk]], [[Poland]] and [[Schwabach]], [[Germany]] and contains flakes of gold leaf. There are also some expensive (~$1000) cocktails which contain flakes of gold leaf{{Fact|date=November 2007}}. However, since metallic gold is inert to all body chemistry, it adds no taste nor has it any other nutritional effect and leaves the body unaltered.
*''Dentistry.'' Gold alloys are used in restorative dentistry, especially in tooth restorations, such as [[crown (dentistry)|crowns]] and permanent [[bridge (dentistry)|bridges]]. The gold alloys' slight malleability facilitates the creation of a superior molar mating surface with other teeth and produces results that are generally more satisfactory than those produced by the creation of porcelain crowns. The use of gold crowns in more prominent teeth such as incisors is favored in some cultures and discouraged in others.
*Gold can be made into [[gold thread|thread]] and used in [[embroidery]].
*Gold is [[ductile]] and [[malleable]], meaning it can be drawn into very thin wire and can be beaten into very thin sheets known as gold leaf.
*Gold produces a deep, intense red color when used as a coloring agent in [[cranberry glass]].
*In photography, Gold toners are used to shift the color of silver bromide black and white prints towards brown or blue tones, or to increase their stability. Used on sepia-toned prints, gold toners produce red tones. Kodak publish formulas for several types of gold toners, which use gold as the chloride (Kodak, 2006).
*''Electronics.'' The concentration of free electrons in gold metal is 5.90×10<sup>22</sup> cm<sup>-3</sup>. Gold is highly conductive to electricity, and has been used for electrical wiring in some high energy applications (silver is even more conductive per volume, but gold has the advantage of corrosion resistance). For example, gold electrical wires were used during some of the [[Manhattan Project]]'s atomic experiments, but large high current silver wires were used in the [[calutron]] isotope separator magnets in the project.
**Though gold is attacked by free chlorine, its good conductivity and general resistance to oxidation and corrosion in other environments (including resistance to non-chlorinated acids) has led to its widespread industrial use in the electronic era as a thin layer coating electrical connectors of all kinds, thereby ensuring good connection. For example, gold is used in the connectors of the more expensive electronics cables, such as audio, video and [[USB]] cables. The benefit of using gold over other connector metals such as [[tin]] in these applications, is highly debated. Gold connectors are often criticized by audio-visual experts as unnecessary for most consumers and seen as simply a marketing ploy. However, the use of gold in other applications in electronic sliding contacts in highly humid or corrosive atmospheres, and in use for contacts with a very high failure cost (certain [[computer]]s, communications equipment, [[spacecraft]], [[jet aircraft]] engines) remains very common, and is unlikely to be replaced in the near future by any other metal.
**Besides sliding electrical contacts, gold is also used in [[Switch#Contacts|electrical contacts]] because of its resistance to [[corrosion]], [[electrical conductivity]], [[ductile|ductility]] and lack of [[toxicity]].<ref>{{cite web |title = General Electric Contact Materials | work = Electrical Contact Catalog (Material Catalog) | publisher = Tanaka Precious Metals | date= 2005 | url = http://www.tanaka-precious.com/catalog/material.html| accessdate = 2007-02-21 }}</ref> Switch contacts are generally subjected to more intense corrosion stress than are sliding contacts.
*[[Colloidal gold]] (Colloidal sols of gold [[nanoparticles]]) in water are intensely red-[[color]]ed, and can be made with tightly-controlled particle sizes up to a few tens of nm across by reduction of gold chloride with [[citrate]] or [[ascorbate]] ions. Colloidal gold is used in research applications in medicine, biology and materials science. The technique of immunogold labeling exploits the ability of the gold particles to adsorb protein molecules onto their surfaces. Colloidal gold particles coated with specific antibodies can be used as probes for the presence and position of antigens on the surfaces of cells (Faulk and Taylor 1979). In ultrathin sections of tissues viewed by electron microscopy, the immunogold labels appear as extremely dense round spots at the position of the [[antigen]] (Roth et al. 1980). Colloidal gold is also the form of gold used as gold paint on [[ceramic]]s prior to firing.
* Gold, or alloys of gold and [[palladium]], are applied as conductive coating to biological specimens and other non-conducting materials such as plastics and glass to be viewed in a [[scanning electron microscope]]. The coating, which is usually applied by [[sputtering]] with an [[argon]] [[plasma (physics)|plasma]], has a triple role in this application. Gold's very high electrical conductivity drains electrical charge to earth, and its very high density provides stopping power for electrons in the SEM's [[electron beam]], helping to limit the depth to which the electron beam penetrates the specimen. This improves definition of the position and topography of the specimen surface and increases the spatial resolution of the image. Gold also produces a high output of [[secondary emission|secondary electrons]] when irradiated by an electron beam, and these low-energy electrons are the most commonly-used signal source used in the scanning electron microscope.
* Many competitions, and honors, such as the [[Olympic Games|Olympics]] and the [[Nobel Prize]], award a gold [[medal]] to the winner.
*As gold is a good reflector of electromagnetic radiation such as infrared and visible light as well as radio waves, it is used for the protective coatings on many artificial [[satellite]]s, in infrared protective faceplates in thermal protection suits and astronauts' helmets and in electronic warfare planes like the [[EA-6B Prowler]].
*Gold is used as the reflective layer on some [[Gold CD|high-end CDs]].
*The isotope gold-198, ([[half-life]]: 2.7 days) is used in some [[cancer]] treatments and for treating other diseases.<ref>[http://web.missouri.edu/~kattik/katti/katres.html Nanoscience and Nanotechnology in Nanomedicine: Hybrid Nanoparticles In Imaging and Therapy of Prostate Cancer] - Radiopharmaceutical Sciences Institute, University of Missouri-Columbia</ref>
* Automobiles may use gold for heat insulation. [[McLaren F1]] uses gold foil in the engine compartment.<ref>Supercars.net. [http://www.supercars.net/cars/1177.html 1994 McLaren F1]</ref>
===As gold chemical compounds===
Gold is attacked by and dissolves in alkaline solutions of potassium or sodium [[cyanide]], and gold cyanide is the [[electrolyte]] used in commercial [[electroplating]] of gold onto base metals and [[electroforming]]. Gold chloride ([[chloroauric acid]]) solutions are used to make colloidal gold by reduction with [[citrate]] or [[ascorbate]] [[ions]]. Gold chloride and gold oxide are used to make highly-valued cranberry or red-colored glass, which, like [[colloid]]al gold sols, contains evenly-sized spherical gold [[nanoparticle]]s.
==History==
[[Image:TurinPapyrus1.jpg|thumb|350px|The [[Turin Papyrus Map|Turin mining papyrus]]]]
Gold has been known and highly-valued since [[prehistoric]] times. It may have been the first metal used by [[human]]s and was valued for ornamentation and rituals. [[Egyptian hieroglyph]]s from as early as 2600 BC describe gold, which king [[Tushratta]] of the [[Mitanni]] claimed was "more plentiful than dirt" in Egypt.<ref>Nicholas Reeves, Egypt's False Prophet: Akhenaten, Thames & Hudson, p.69</ref> Egypt and especially [[Nubia]] had the resources to make them major gold-producing areas for much of history. The earliest known map is known as the [[Turin papyrus]] and shows the plan of a [[gold mine]] in Nubia together with indications of the local [[geology]]. The primitive working methods are described by [[Strabo]] and included [[fire-setting]]. Large mines also occurred across the [[Red Sea]] in what is now [[Saudi Arabia]].
[[Image:Tutmask.jpg|thumb|left|150px|Funerary mask of [[Tutankhamun]]]]
[[Image:Sanxingduimasks.jpg|thumb|right|150px|gold mask of [[sanxingdui]],12th to 11th BCE century]]
The legend of the [[golden fleece]] may refer to the use of fleeces to trap gold dust from [[placer deposit]]s in the ancient world.
[[Image:Jason Pelias Louvre K127.jpg|thumb|150px|Jason returns with the golden Fleece on an [[Apulia]]n [[Red-figure pottery|red-figure]] [[Krater|calyx krater]], ca. 340–330 BC]]
Gold is mentioned frequently in the [[Old Testament]], starting with [[Genesis]] 2:11 (at [[Havilah]]) and is included with the gifts of the [[magi]] in the first chapters of Matthew [[New Testament]]. The [[Book of Revelation]] 21:21 describes the city of [[New Jerusalem]] as having streets "made of pure gold, clear as crystal". The south-east corner of the [[Black Sea]] was famed for its gold. Exploitation is said to date from the time of [[Midas]], and this gold was important in the establishment of what is probably the world's earliest coinage in [[Lydia]] between [[630s BC|643 and 630 BC]].
[[Image:Panorámica de Las Médulas.jpg|thumb|right|200px|Panoramic view of Las Médulas]]
From 6th or 5th century BCE,[[Chu (state)]] circulated [[Ying Yuan]],one kind of square gold coin.
The [[Ancient Rome|Romans]] developed new methods for extracting gold on a large scale using [[hydraulic mining]] methods, especially in [[Spain]] from 25 BC onwards and in [[Romania]] from 150 AD onwards. One of their largest mines was at [[Las Medulas]] in [[León (province)|León (Spain)]], where seven long [[aqueduct]]s enabled them to sluice most of a large alluvial deposit. The mines at [[Roşia Montană]] in [[Transylvania]] were also very large, and until very recently, still mined by opencast methods. They also exploited smaller deposits in [[Wales]], such as placer and hard-rock deposits at [[Dolaucothi]]. The various methods they used are well described by [[Pliny the Elder]] in his [[encyclopedia]] [[Naturalis Historia]] written towards the end of the first century AD.
The [[Mali Empire]] in Africa was famed throughout the old world for its large amounts of gold. [[Mansa Musa]], ruler of the empire (1312–1337) became famous throughout the old world for his great [[hajj]] to [[Mecca]] in 1324. When he passed through [[Cairo]] in July of 1324, he was reportedly accompanied by a [[camel train]] that included thousands of people and nearly a hundred camels. He gave away so much gold that it took over a decade for the economy across North Africa to recover, due to the rapid inflation that it initiated.<ref>[http://blackhistorypages.net/pages/mansamusa.php Mansa Musa] - Black History Pages</ref> A contemporary Arab historian remarked;
{{cquote|''Gold was at a high price in Egypt until they came in that year. The mithqal did not go below 25 dirhams and was generally above, but from that time its value fell and it cheapened in price and has remained cheap till now. The mithqal does not exceed 22 dirhams or less. This has been the state of affairs for about twelve years until this day by reason of the large amount of gold which they brought into Egypt and spent there [...]''|20px|20px|[[Chihab Al-Umari]]<ref>[http://www.bu.edu/africa/outreach/materials/handouts/k_o_mali.html Kingdom of Mali] - Boston University: African Studies Center</ref>}}
The European exploration of the Americas was fueled in no small part by reports of the gold ornaments displayed in great profusion by [[Indigenous peoples of the Americas|Native American]] peoples, especially in [[Central America]], [[Peru]], and [[Colombia]].
Although the price of some platinum group metals can be much higher, gold has long been considered the most desirable of [[precious metal]]s, and its value has been used as the standard for many [[currency|currencies]] (known as the [[gold standard]]) in history. Gold has been used as a symbol for purity, value, royalty, and particularly roles that combine these properties. Gold as a sign of wealth and prestige was made fun of by [[Thomas More]] in his treatise [[Utopia (book)|''Utopia'']]. On that imaginary island, gold is so abundant that it is used to make chains for slaves, tableware and lavatory-seats. When ambassadors from other countries arrive, dressed in ostentatious gold jewels and badges, the Utopians mistake them for menial servants, paying homage instead to the most modestly-dressed of their party.
There is an age-old tradition of biting gold in order to test its authenticity. Although this is certainly not a professional way of examining gold, the ''bite test'' should score the gold because gold is considered a soft metal according to the [[Mohs' scale of mineral hardness]]. The purer the gold the easier it should be to mark it. Painted lead can cheat this test because lead is softer than gold (and may invite a small risk of [[lead poisoning]] if sufficient lead is absorbed by the biting).[[Image:Stringer156 nugget.jpg|thumb|right|250px|This 156 ounce (4,42 kg) nugget was found by an individual prospector in the Southern California Desert using a metal detector.]]
Gold in antiquity was relatively easy to obtain [[Geology|geologically]]; however, 75% of all gold ever produced has been extracted since 1910.<ref name="goldsheetlinks">{{cite web | url=http://www.goldsheetlinks.com/production2.htm | title=GOLDSHEET - YEARLY and CUMULATIVE WORLD GOLD PRODUCTION CHARTS | accessdate=2006-07-22}}</ref> It has been estimated that all the gold in the world that has ever been refined would form a single cube 20 m (66 ft) on a side (equivalent to 8000 m³).<!--Can be proved easily by converting the cumulative gold production tonnage to the volume using the density.--><ref name="goldsheetlinks"/>
One main goal of the [[alchemy|alchemists]] was to produce gold from other substances, such as [[lead]] — presumably by the interaction with a mythical substance called the [[philosopher's stone]]. Although they never succeeded in this attempt, the alchemists promoted an interest in what can be done with substances, and this laid a foundation for today's [[chemistry]]. Their symbol for gold was the [[circle with a point at its centre|circle with a point at its center]] (☉), which was also the [[astrology|astrological]] symbol, the [[Egypt]]ian [[Egyptian hieroglyph|hieroglyph]] and the ancient [[Chinese character]] for the [[Sun]]. For modern attempts to produce artificial gold, see [[gold synthesis]].
During the 19th century, [[gold rush]]es occurred whenever large gold deposits were discovered. The first major gold strike in the United States occurred in a small north Georgia town called Dahlonega.<ref>{{cite web
| title=Road to adventure
| publisher=Georgia Magazine
| last=Garvey
| first = Jane A.
| url=http://www.georgiamagazine.org/archives_view.asp?mon=7&yr=2006&ID=1344
| year= 2006
| accessdate=2007-01-23}}</ref> Further gold rushes occurred in [[California Gold Rush|California]], [[Pike's Peak Gold Rush|Colorado]], [[Central Otago Gold Rush|Otago]], [[Australian gold rushes|Australia]], [[Witwatersrand Gold Rush|Witwatersrand]], [[Black Hills Gold Rush|Black Hills]], and [[Klondike Gold Rush|Klondike]].
Because of its historically high value, much of the gold mined throughout history is still in circulation in one form or another.
==Occurrence==
In nature, gold most often occurs in its native state (that is, as a metal), though usually alloyed with silver. Native gold contains usually eight to ten percent silver, but often much more — alloys with a silver content over 20% are called [[electrum]]. As the amount of silver increases, the color becomes whiter and the specific gravity becomes lower.
[[Ore]]s bearing native gold consist of grains or microscopic particles of metallic gold embedded in rock, often in association with veins of [[quartz]] or [[sulfide mineral]]s like [[pyrite]]. These are called "lode" deposits. Native gold is also found in the form of free flakes, grains or larger nuggets that have been eroded from rocks and end up in [[alluvial]] deposits (called [[placer deposit]]s). Such free gold is always richer at the surface of gold-bearing veins owing to the oxidation of accompanying minerals followed by weathering, and washing of the dust into streams and rivers, where it collects and can be welded by water action to form [[nugget]]s.
Gold sometimes occurs in minerals in chemical composition with other elements, especially in association with [[tellurium]]. Examples are [[calaverite]], [[sylvanite]], [[nagyagite]], [[petzite]] and [[krennerite]]. Gold also occurs rarely as a mercury-gold [[amalgam]], and in very low [[concentration]]s in [[seawater]].
==Production==
[[Image:Nugsrandt.jpg|thumb|left|Gold Nuggets found in Arizona]]
{{main|Gold prospecting|Gold mining|Gold extraction}}
[[Image:Gold mine.jpg|thumb|200px|right|The entrance to an underground gold mine in Victoria, Australia]]
[[Image:GoldOreUSGOV.jpg|200px|right|thumb|Gold ore]]
[[Image:Gold - world production trend.svg|thumb|left|World gold production trend]]
[[Image:Gold (mined)2.PNG|thumb|right|Gold output in 2005]]
Economic [[gold extraction]] can be achieved from ore grades as little as 0.5 g/1000 kg (0.5 parts per million, ppm) on average in large easily mined deposits. Typical ore grades in [[open-pit mining|open-pit]] mines are 1–5 g/1000 kg (1–5 ppm), ore grades in underground or [[Underground mining (hard rock)|hard rock]] mines are usually at least 3 g/1000 kg (3 ppm). Since ore grades of 30 g/1000 kg (30 ppm) are usually needed before gold is visible to the naked eye, in most gold mines the gold is invisible.
Since the 1880s, [[South Africa]] has been the source for a large proportion of the world’s gold supply, with about 50% of all gold ever produced having come from South Africa. Production in 1970 accounted for 79% of the world supply, producing about 1,000 [[tonne]]s. However by 2007 production was just 272 tonnes. This sharp decline was due to the increasing difficulty of extraction, changing economic factors affecting the industry, and tightened safety auditing. In 2007 China (with 276 tonnes) overtook South Africa as the world's largest gold producer, the first time since 1905 that South Africa has not been the largest.<ref>[http://www.marketwatch.com/news/story/china-now-worlds-largest-gold/story.aspx?guid=%7B8C528CE8%2D0262%2D485D%2DACEB%2D2247D18282CB%7D China now world's largest gold producer; foreign miners at door - MarketWatch<!-- Bot generated title -->]</ref>
The city of [[Johannesburg]] located in South Africa was founded as a result of the [[Witwatersrand Gold Rush]] which resulted in the discovery of some of the largest gold deposits the world has ever seen. Gold fields located within the basin in the [[Free State]] and [[Gauteng]] provinces are extensive in strike and dip requiring some of the world's deepest mines, with the Savuka and [[TauTona]] mines being currently the world's deepest gold mine at 3,777 m. The [[Second Boer War]] of 1899–1901 between the [[British Empire]] and the [[Afrikaner]] [[Boers]] was at least partly over the rights of miners and possession of the gold wealth in South Africa.
Other major producers are [[United States]], [[Australia]], [[China]], [[Russia]] and [[Peru]]. Mines in [[South Dakota]] and [[Nevada]] supply two-thirds of gold used in the United States. In South America, the controversial project [[Pascua Lama]] aims at exploitation of rich fields in the high mountains of [[Atacama Desert]], at the border between [[Chile]] and [[Argentina]]. Today about one-quarter of the world gold output is estimated to originate from artisanal or small scale mining.<ref>{{cite journal | title=Removal of Barriers to the Abatement of Global Mercury Pollution from Artisanal Gold Mining | url=http://www.unido.org/file-storage/download/?file_id=10644 | last=Beinhoff | first=Christian | format={{Dead link|date=June 2008}} – <sup>[http://scholar.google.co.uk/scholar?hl=en&lr=&q=author%3ABeinhoff+intitle%3ARemoval+of+Barriers+to+the+Abatement+of+Global+Mercury+Pollution+from+Artisanal+Gold+Mining&as_publication=&as_ylo=&as_yhi=&btnG=Search Scholar search]</sup>}}</ref>
After initial production, gold is often subsequently refined industrially by the [[Wohlwill process]] or the [[Miller process]]. Other methods of assaying and purifying smaller amounts of gold include [[parting and inquartation]] as well as [[cuppelation]], or refining methods based on the dissolution of gold in [[aqua regia]].
The world's oceans hold a vast amount of gold, but in very low concentrations (perhaps 1–2 parts per 10 billion). A number of people have claimed to be able to economically recover gold from sea water, but so far they have all been either mistaken or crooks. Reverend Prescott Jernegan ran a gold-from-seawater swindle in America in the 1890s. A British fraud ran the same scam in England in the early 1900s.<ref>Dan Plazak, ''A Hole in the Ground with a Liar at the Top'' (Salt Lake: Univ. of Utah Press, 2006) (contains a chapter on gold-from seawater swindles)</ref>
[[Fritz Haber]] (the German inventor of the [[Haber process]]) attempted commercial extraction of gold from sea water in an effort to help pay Germany's reparations following the [[First World War]]. Unfortunately, his assessment of the concentration of gold in sea water was unduly high, probably due to sample contamination. The effort produced little gold and cost the German government far more than the commercial value of the gold recovered. No commercially viable mechanism for performing gold extraction from sea water has yet been identified. [[Gold synthesis]] is not economically viable and is unlikely to become so in the foreseeable future.
The average gold mining and extraction costs are $238 per [[troy ounce]] but these can vary widely depending on mining type and ore quality. In 2001, global mine production amounted to 2,604 tonnes, or 67% of total gold demand in that year. At the end of 2006, it was estimated that all the gold ever mined totaled 158,000 tonnes.<ref>{{cite web | url=http://www.invest.gold.org/sites/en/why_gold/demand_and_supply/ | title=World Gold Council | accessdate=2008-07-04}}</ref>
At current consumption rates, the supply of gold is believed to last 45 years.<ref>{{cite journal | date=[[May 26]], [[2007]] | journal = New Scientist | volume = 194 | issue = 2605 | pages = 38–39 | issn = 0262 4079 | title = How Long Will it Last? }}</ref>
==Price==
{{main|Gold as an investment|Gold standard}}
[[Image:LBMA Au AM USD 2001-06042006.png|thumb|right|200px|LBMA USD morning [[Gold Fix|price fixings]] ($US per troy ounce) since 2001]]
[[Image:Gold price.png|thumb|200px|Gold price per ounce in USD since 1968, in actual US$ and 2006 US$]]
Like other precious metals, gold is measured by [[troy weight]] and by [[gram]]s. When it is alloyed with other metals the term ''[[carat (purity)|carat]]'' or ''karat'' is used to indicate the amount of gold present, with 24 karats being pure gold and lower ratings proportionally less. The purity of a [[gold bar]] can also be expressed as a decimal figure ranging from 0 to 1, known as the [[millesimal fineness]], such as 0.995 being very pure.
The price of gold is determined on the open market, but a procedure known as the [[Gold Fixing]] in [[London]], originating in September 1919, provides a daily benchmark figure to the industry. The afternoon fixing appeared in 1968 to fix a price when US markets are open.
The high price of gold is due to its rare amount. Only three parts out of every billion (0.000000003) in the Earth's crust is gold.
Historically gold was used to back currency; in an economic system known as the [[gold standard]], a certain [[weight]] of gold was given the name of a unit of currency. For a long period, the United States government set the value of the US dollar so that one [[troy weight|troy ounce]] was equal to $20.67 ($664.56/kg), but in 1934 the dollar was revalued to $35.00 per troy ounce ($1125.27/kg). By 1961 it was becoming hard to maintain this price, and a pool of US and European banks agreed to manipulate the market to prevent further currency devaluation against increased gold demand.
On [[17 March]] [[1968]], economic circumstances caused the collapse of the gold pool, and a two-tiered pricing scheme was established whereby gold was still used to settle international accounts at the old $35.00 per troy ounce ($1.13/g) but the price of gold on the private market was allowed to fluctuate; this two-tiered pricing system was abandoned in 1975 when the price of gold was left to find its free-market level. Central banks still hold historical [[official gold reserves|gold reserves]] as a [[store of value]] although the level has generally been declining. The largest gold depository in the world is that of the [[U.S. Federal Reserve Bank]] in [[New York]], which holds about 3% {{Fact|date=June 2008}} of the gold ever mined, as does the similarly-laden [[U.S. Bullion Depository]] at [[Fort Knox]].
In 2005 the [[World Gold Council]] estimated total global gold supply to be 3,859 tonnes and demand to be 3,754 tonnes, giving a surplus of 105 tonnes.<ref>{{cite web | url=http://www.gold.org/value/stats/statistics/gold_demand/index.html | title=World Gold Council > value > research & statistics > statistics > supply and demand statistics | accessdate=2006-07-22}}</ref>
===Long term price trends===
Since April 2001 the gold price has more than tripled in value against the US dollar [http://kitco.com/LFgif/au3650nyb.gif (as seen here)], prompting speculation that this long [[secular bear market]] (or the [[Great Commodities Depression]]) has ended and a [[bull market]] has returned <ref>[http://www.lewrockwell.com/thornton/thornton18.html A Bull Market in Gold – Technically Speaking by Mark Thornton<!-- Bot generated title -->]</ref> <ref>[http://www.ameinfo.com/75511.html Gold starts 2006 well, but this is not a 25-year high! | Financial Planning<!-- Bot generated title -->]</ref>. In March 2008, the gold price increased above $1000 <ref>[http://www.lbma.org.uk/2008dailygold.htm 2008 London Gold Fixings<!-- Bot generated title -->]</ref>, which in [[Real versus nominal value|real terms]] is still well below the $850/oz. peak on 21 January 1980. Indexed for inflation, the 1980 high would equate to a price of around $2400 in 2007 US dollars.
In the last century, major economic crises (such as the [[Great Depression]], [[World War II]], the [[1973 Oil Crisis|first]] and [[1979 energy crisis|second]] oil crisis) lowered the Dow/Gold ratio (which is inherently inflation adjusted) substantially, in most cases to a value well below 4 [http://upload.wikimedia.org/wikipedia/en/8/84/Longtermdowgoldlogtr1800.png (as seen here)]. During these difficult times, investors tried to preserve their assets by investing in precious metals, most notably gold and [[silver]]. The long-term trend in the Dow/Gold ratio since 2001 shows that such a
scenario is currently repeating. Major reasons are, among others, the rapid increase in [[Money supply|money supply M3]] in Europe <ref>[http://www.ecb.int/press/pr/stats/md/html/index.en.html Monetary developments in the euro area<!-- Bot generated title -->]</ref> and the USA <ref>[http://www.federalreserve.gov/Releases/h6/HIST/h6hista.htm Federal Reserve: Money stock measures<!-- Bot generated title -->]</ref> <ref>[http://www.shadowstats.com/alternate_data Money supply in the USA, alternate data series<!-- Bot generated title -->]</ref>
([[monetary inflation]]) and the high [[double deficit (economics)|double deficit]] of the USA.<ref>[https://www.cia.gov/library/publications/the-world-factbook/rankorder/2187rank.html CIA: Rank order - current account balance <!-- Bot generated title -->]</ref> <ref>[http://www.spiegel.de/img/0,1020,1034726,00.jpg Double deficit in the USA <!-- Bot generated title -->]</ref> These severe economic problems have been leading to the [[Subprime mortgage crisis|financial crisis]], high [[price inflation]] and the strong depreciation of major currencies against commodities, most notably of the [[United States Dollar|US-Dollar]].
== Compounds ==
Although gold is a [[noble metal]], it forms many and diverse compounds. The [[oxidation state]] of gold in its compound ranges from −1 to +5 but Au(I) and Au(III) dominate. Gold(I), referred to as the aurous ion, is the most common oxidation state with “soft” [[ligand]]s such as [[thioether]]s, [[thiolate]]s, and tertiary [[phosphine]]s. Au(I) compounds are typically linear. A good example is [[gold cyanidation|Au(CN)<sub>2</sub><sup>−</sup>]], which is the soluble form of gold encountered in mining. Curiously, aurous complexes of water are rare. The binary [[gold halide]]s, such as [[gold(I) chloride|AuCl]], form zig-zag polymeric chains, again featuring linear coordination at Au. Most drugs based on gold are Au(I) derivatives.<ref>{{cite journal |author=Shaw III, C. F. |title=Gold-Based Medicinal Agents |journal=Chemical Reviews |year=1999 |volume=99 |issue=9 |pages=2589–2600 |doi=10.1021/cr980431o}}</ref>
Gold(III) (“auric”) is a common oxidation state and is illustrated by [[gold(III) chloride]], AuCl<sub>3</sub>. Its derivative is chloroauric acid, HAuCl<sub>4</sub>, which forms when Au dissolves in [[aqua regia]]. Au(III) complexes, like other d<sup>8</sup> compounds, are typically [[square planar]].
===Less common oxidation states: Au(-I), Au(II), and Au(V)===
Compounds containing the Au<sup>−</sup> [[anion]] are called aurides. [[Caesium]] auride, CsAu which crystallizes in the [[caesium chloride]] motif.<ref name="relativist_Au_Pt">[http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6W6W-4HDGBTS-4&_user=10&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=ec7c3f8a892a329ac1f747fedf8ce9f4 ScienceDirect: Effects of relativistic motion of electrons on the chemistry of gold and platinum]</ref> Other aurides include those of [[rubidium|Rb]]<sup>+</sup>, [[potassium|K]]<sup>+</sup>, and tetramethylammonium (CH<sub>3</sub>)<sub>4</sub>N<sup>+</sup>.<ref name=Holleman>Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.</ref> Gold(II) compounds are usually [[diamagnetic]] with Au-Au bonds such as [Au(CH<sub>2</sub>)<sub>2</sub>P(C<sub>6</sub>H<sub>5</sub>)<sub>2</sub>]<sub>2</sub>Cl<sub>2</sub>. A noteworthy, legitimate Au(II) complex contains [[xenon]] as a ligand, [AuXe<sub>4</sub>](Sb<sub>2</sub>F<sub>11</sub>)<sub>2</sub>.<ref>{{cite journal |author=Seidel, S.; Seppelt, K. |title=Xenon as a Complex Ligand: The Tetra Xenono Gold(II) Cation in AuXe<sub>4</sub><sup>2+</sup>(Sb<sub>2</sub>F<sub>11</sub><sup>−</sup>)<sub>2</sub> |journal=Science |year=2000 |volume=290 |issue=5489 |pages=117–118 |doi=10.1126/science.290.5489.117 |pmid=11021792}}</ref> [[Gold pentafluoride]] is the sole example of Au(V), the highest verified oxidation state.<ref>{{cite journal |author=Riedel, S.; Kaupp, M. |title=Revising the Highest Oxidation States of the 5d Elements: The Case of Iridium(+VII) |journal=Angewandte Chemie International Edition |year=2006 |volume=45 |issue=22 |pages=3708–3711 |doi=10.1002/anie.200600274}}</ref>
Some gold compounds exhibit ''[[aurophilicity|aurophilic bonding]]'', which describes the tendency of gold ions to interact at distances that are too long to be a conventional Au-Au bond but shorter that van der Waals bonding. The interaction is estimated to be comparable in strength to that of a [[hydrogen bond]].
===Mixed valence compounds===
Well-defined cluster compounds are numerous.<ref name=Holleman/> In such cases, gold has a fractional oxidation state. A representative example is the octahedral species {Au([[Triphenylphosphine|P(C<sub>6</sub>H<sub>5</sub>)<sub>3</sub>]])}<sub>6</sub><sup>2+</sup>. [[Gold chalcogenides]], e.g. "AuS" feature equal amounts of Au(I) and Au(III).
==Isotopes==
{{main|isotopes of gold}}
There is one stable [[isotope]] of gold, and 18 [[radioisotope]]s with <sup>195</sup>Au being the most stable with a [[half-life]] of 186 days.
Gold has been proposed as a "[[Salted bomb|salting]]" material for [[nuclear weapon]]s ([[cobalt]] is another, better-known salting material, as per the proposed [[cobalt bomb]]). A jacket of natural gold, irradiated by the intense high-energy neutron flux from an exploding thermonuclear weapon, would transmute into the radioactive isotope Au-198 with a half-life of 2.697 days and produce approximately .411 [[MeV]] of [[Gamma ray|gamma radiation]], significantly increasing the radioactivity of the weapon's [[Nuclear fallout|fallout]] for several days. Such a weapon is not known to have ever been built, tested, or used.
==Symbolism==
[[Image:3sovriegns.jpg|thumb|right|150px|Three [[gold sovereign|Gold Sovereign]]s with a [[Krugerrand]]]]
[[Image:Goldkey logo removed.jpg|thumb|right|150px|Swiss-cast 1 kg [[gold bar]].]]
Gold has been associated with the extremities of utmost evil and great sanctity throughout history. In the [[Exodus|Book of Exodus]], the [[Golden Calf]] is a symbol of [[idolatry]] and rebellion against God. In [[Communist]] [[propaganda]], the golden [[pocket watch]] and its fastening golden chain were the characteristic accessories of the class enemy, the bourgeois and the industrial [[tycoon]]s. [[Credit card]] companies associate their product with wealth by naming and coloring their top-of-the-range cards “gold;” although, in an attempt to out-do each other, [[platinum]] (and the even-more-elite [[Centurion Card|black card]]) has now overtaken gold.
On the other hand in the [[Genesis|Book of Genesis]], [[Abraham]] was said to be rich in gold and [[silver]], and Moses was instructed to cover the Mercy Seat of the Ark of the Covenant with pure gold. Eminent [[orator]]s such as [[John Chrysostom]] were said to have a “mouth of gold with a silver tongue.” Gold is associated with notable anniversaries, particularly in a 50-year cycle, such as a golden [[wedding anniversary]], [[golden jubilee]], etc.
Great human achievements are frequently rewarded with gold, in the form of medals and decorations. Winners of races and prizes are usually awarded the gold medal (such as the [[Olympic Games]] and the [[Nobel Prize]]), while many award statues are depicted in gold (such as the [[Academy Awards]], the [[Golden Globe Awards]] the [[Emmy Awards]], the [[Palme d'Or]], and the [[British Academy Film Awards]]).
Medieval kings were inaugurated under the signs of [[anointing|sacred oil]] and a golden crown, the latter symbolizing the eternal shining light of heaven and thus a [[Christian]] king's divinely inspired authority. [[Wedding ring]]s are traditionally made of gold; since it is long-lasting and unaffected by the passage of time, it is considered a suitable material for everyday wear as well as a metaphor for the relationship. In [[Eastern Orthodox Church|Orthodox Christianity]], the wedded couple is adorned with a golden crown during the ceremony, an amalgamation of symbolic rites.
The symbolic value of gold varies greatly around the world, even within geographic regions. For example, gold is quite common in [[Turkey]] but considered a most valuable gift in [[Sicily]].
==Toxicity==
Pure gold is non-toxic and non-irritating when ingested<ref>{{cite web|title=Gold MSDS|url=http://www.espi-metals.com/msds's/gold.htm|publisher=Electronic Space Products International|author=S Dierks|date=May 2005}}</ref> and is sometimes used as a food decoration in the form of [[gold leaf]]. It is also a component of the alcoholic drinks [[Goldschläger]], [[Gold Strike (drink)|Gold Strike]], and [[Goldwasser]]. Gold is approved as a food additive in the EU ([[E175]] in the [[Codex Alimentarius]]).
Soluble compounds ([[gold salts]]) such as potassium gold cyanide, used in gold electroplating, are toxic to the liver and kidneys. There are rare cases of lethal gold poisoning from potassium gold cyanide.<ref>I. H. Wright, C. J. Vesey (1986)
Acute poisoning with gold cyanide
Anaesthesia 41 (9), 936–939.
doi:10.1111/j.1365-2044.1986.tb12920.x
[http://www.blackwell-synergy.com/doi/abs/10.1111/j.1365-2044.1986.tb12920.x]
</ref><ref>
Wu, Ming-Ling; Tsai, Wei-Jen; Ger, Jiin; Deng, Jou-Fang; Tsay, Shyh-Haw; Yang, Mo-Hsiung.
J. toxicol., Clin. toxicol. Metals. 2001, vol. 39, no 7, pp. 739-743
ISSN 0731-3810
[http://cat.inist.fr/?aModele=afficheN&cpsidt=13399830]
</ref> Gold toxicity can be ameliorated with [[chelating agent]]s such as [[British anti-Lewisite]].
==See also==
<div style="-moz-column-count:2; column-count:2;">
* [[Carat (purity)]]
* [[ChipGold]]
* [[Colloidal gold]]
* [[White gold]]
* [[Rose gold]]
* [[Black gold (Jewelery)|Black gold]]
* [[Gold as an investment]]
* [[Gold coin]]
* [[Precious metal]]
* [[Digital gold currency]]
* [[Hallmark]]
* [[Altay Mountains]]
* [[Commodity fetishism]]
* [[Pyrite|Fool's Gold]]
* [[Roman mining]]
* [[Roman engineering]]
* [[Gold fingerprinting]]
</div>
==Footnotes==
{{Reflist}}
==Bibliography==
{{Refbegin}}
*Faulk W, Taylor G (1979) ''An Immunocolloid Method for the Electron Microscope'' Immunochemistry '''8''', 1081–1083.
*Kodak (2006) [http://www.kodak.com/global/en/professional/support/techPubs/g23/g23.pdf Toning black-and-white materials]. Technical Data/Reference sheet G-23, May 2006.
*Roth J, Bendayan M, Orci L (1980) ''FITC-Protein A-Gold Complex for Light and Electron Microscopic Immunocytochemistry''. Journal of Histochemistry and Cytochemistry '''28''', 55–57.
*World Gold Council, Jewellery Technology, [http://www.utilisegold.com/jewellery_technology/colours/colour_alloys/ Jewellery Alloys]
*[http://periodic.lanl.gov/elements/79.html Los Alamos National Laboratory – Gold]
{{Refend}}
==External links==
{{Commons|Gold}}
{{wiktionary|gold}}
*[http://www.lateralscience.co.uk/gold/auriferous.html ''Getting Gold'' 1898 book]
*[http://www.epa.gov/epaoswer/other/mining/techdocs/gold.pdf Technical Document on Extraction and Mining of Gold]
*[http://www.pniok.de/au.htm Picture in the Element collection from Heinrich Pniok]
{{Jewellery Materials}}
{{Compact periodic table}}
[[Category:Chemical elements]]
[[Category:Gold| ]]
[[Category:Precious metals]]
[[Category:Noble metals]]
[[Category:Electrical conductors]]
[[Category:Dental materials]]
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