Gold (Pronounced /ˈɡoʊld/) is a Chemical Element With the Symbol

8/14/2019 Gold (Pronounced / o ld/) is a Chemical Element With the Symbol

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Gold (pronounced /old/) is a chemical element with the symbol Au (Latin: aurum)and an atomic numberof 79. It has been a highly sought-afterprecious metalfor coinage,jewelry, and other arts since the beginning ofrecorded history. The metal occurs asnuggets or grains in rocks, inveins and inalluvial deposits. Gold is dense, soft, shiny andthe most malleable and ductile pure metal known. Pure gold has a bright yellow color and

luster traditionally considered attractive, which it maintains without oxidizing in air orwater. It is one of the coinage metals and formed the basis for the gold standard usedbefore the collapse of the Bretton Woods system in 1971.

At the end of 2006, it was estimated that all the gold ever mined totaled 158,000 tonnes.[1]

This can be represented by a cube with an edge length of just 20.2 meters. Modernindustrial uses include dentistryand electronics, where gold has traditionally found usebecause of its good resistance to oxidative corrosion and excellent quality as a conductorof electricity.

Chemically, gold is a transition metal and can form trivalentandunivalentcationsupon

solvation. Compared with other metals, pure gold is more chemically unreactive, but it isattacked by aqua regia (a mixture of acids), formingchloroauric acid, and by alkalinesolutions ofcyanide but not by single acids such ashydrochloric, nitricorsulfuric acids.Gold dissolves inmercury, forming amalgam alloys, but does not react with it. Since goldis insoluble innitric acid which will dissolve silver andbase metals, this is exploited asthe basis of the gold refining technique known as "inquartation and parting". Nitric acidhas long been used to confirm the presence of gold in items, and this is the origin of thecolloquial term "acid test", referring to agold standardtest for genuine value.

Characteristics

Native gold nuggets
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Gold nuggets found in Arizona

Gold is the most malleable and ductile of all metals; a single gram can be beaten into asheet of 1 square meter, or anounceinto 300 square feet. Gold leaf can be beaten thinenough to become translucent. The transmitted light appears greenish blue, because goldstrongly reflects yellow and red.[2] Such semi-transparent sheets also strongly reflectinfra-red, making them useful as infrared (radiant heat) shields in visors of heat-resistantsuits, and in sun-visors for spacesuits.[3]

Gold readily creates alloys with many other metals. These alloys can be produced tomodify the hardness and other metallurgical properties, to control melting pointor tocreate exotic colors (see below). Gold is a good conductor ofheatandelectricity andreflects infra redradiation strongly. Chemically, it is unaffected by air,moisture and mostcorrosivereagents, and is therefore well-suited for use in coinsandjewelry and as aprotective coating on other, more reactive, metals. However, it is not chemically inert.Free halogens will react with gold, and aqua regia dissolves it via formation ofchlorinegas which attacks gold to form the chloraurate ion. Gold also dissolves in alkalinesolutions ofpotassium cyanideand in mercury, forming a gold-mercury amalgam.

Common oxidation states of gold include +1 (gold(I) or aurous compounds) and +3(gold(III) or auric compounds). Gold ions in solution are readilyreduced andprecipitatedout as gold metal by adding any other metal as the reducing agent. The added metal isoxidizedand dissolves allowing the gold to be displaced from solution and be recoveredas a solid precipitate.

High quality pure metallic gold is tasteless; in keeping with its resistance to corrosion (itis metal ions which confer taste to metals).

In addition, gold is very dense, a cubic meter weighing 19300kg. By comparison, thedensity oflead is 11340 kg/m, and that of the densest element, osmium, is 22610 kg/m.

Isotopes

Main article: Isotopes of gold

Gold has only one stable isotope,197Au, which is also its only naturally-occurring isotope.36 radioisotopes have been synthesized ranging in atomic mass from 169 to 205. The
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most stable of these is 195Au with a half-life of 186.1 days. 195Au is also the only goldisotope to decay by electron capture. The least stable is 171Au, which decays byprotonemission with a half-life of 30 s. Most of gold's radioisotopes with atomic masses below197 decay by some combination ofproton emission, decay, and + decay. Theexceptions are 195Au, which decays by electron capture, and 196Au, which has a minor-

decay path. All of gold's radioisotopes with atomic masses above 197 decay by - decay.[4]

At least 32 nuclear isomers have also been characterized, ranging in atomic mass from170 to 200. Within that range, only 178Au, 180Au, 181Au, 182Au, and 188Au do not haveisomers. Gold's most stable isomer is 198 m2Au with a half-life of 2.27 days. Gold's leaststable isomer is 177 m2Au with a half-life of only 7 ns. 184 m1Au has three decay paths: +decay, isomeric transition, and alpha decay. No other isomer or isotope of gold has threedecay paths.[4]

Color of gold

Different colors of Ag-Au-Cu alloys

The color of pure gold is metallic yellow. Gold, caesium and copperare the only metallicelements with a natural color other than gray or white. The usual gray color of metalsdepends on their "electron sea" that is capable of absorbing and re-emittingphotons overa wide range of frequencies. Gold reacts differently, depending on subtle relativisticeffects that affect the orbitals around gold atoms.[5][6]

Common colored gold alloys such as rose gold can be created by the addition of variousamounts of copper and silver, as indicated in the triangular diagram on the right. Alloyscontaining palladium or nickel are also important in commercial jewelry as these producewhite gold alloys. Less commonly, addition ofmanganese,aluminium,iron, indium andother elements can produce more unusual colors of gold for various applications.[7]

Applications

As the metal

Medium of monetary exchange

Main article: Gold standard
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Special issue Canadian Gold Maple Leafcoin with the highest purity of anygold coin ata guaranteed 99.999%

Throughout the world gold was widely used as a standard formonetaryexchange, but hasbeen abandoned by world governments which have issuedfiat currency in its stead. Theamount of gold in the world is finite, but there is no limit to the quantity of papercurrency which can be issued. At the beginning ofWorld War Ithe warring nationsmoved to a fractional gold standard, inflating their currencies to finance the war effort.

AfterWorld War II gold was replaced by a system ofconvertible currency following theBretton Woods system. The last country to tie its currency to gold was Switzerland,which backed 40% of its value until the Swiss joined the International Monetary Fund in1999.[8]

Pure gold is too soft for day to day monetary use and was typically hardened by alloyingwith copper, silver or other base metals prior to the advent of paper money. The goldcontent of gold alloys is measured incarats (k), pure gold being designated as 24k. Manyholders of gold in storage (as bullion coin or bullion) hold it as a hedge against inflationor other economic disruptions. (The ISO currency codeof goldbullion isXAU).

Gold coins intended for circulation from 1526 into the 1930s were typically a standard22k alloy called crown gold, for hardness. Modern collector/investmentbullion coins(which do not require good mechanical wear properties) are typically 24k, although theAmerican Gold Eagle, the British gold sovereign and the South African Krugerrandcontinue to be made at 22k, on historical tradition. The special issue Canadian GoldMaple Leafcoin contains the highest purity gold of anybullion coin, at 99.999% (.99999fine). The popular issue Canadian Gold Maple Leaf coin has a purity of 99.99%. Severalother 99.99% pure gold coins are currently available, including Australia's GoldKangaroos (first appearing in 1986 as theAustralian Gold Nugget, with the kangarootheme appearing in 1989), the several coins of the Australian Lunar Calendar series, andthe Austrian Philharmonic. In 2006, the U.S. Mint began production of the American

Buffalo gold bullion coin also at 99.99% purity.

Jewelry
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Moche gold necklace depicting feline heads. Larco Museum Collection. Lima-Peru

Because of the softness of pure (24k) gold, it is usually alloyed with base metals for usein 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 ofcopper, or other base metals or silver or palladium in the alloy.Copperis the mostcommonly used base metal, yielding a redder color. Eighteen carat gold containing 25%copper is found in antique and Russian jewelry and has a distinct, though not dominant,copper cast, creating rose gold. Fourteen carat gold-copper alloy is nearly identical incolor to certainbronzealloys, and both may be used to produce police, as well as other,badges. Blue gold can be made by alloying with ironand purple gold can be made byalloying with aluminium, although rarely done except in specialized jewelry. Blue gold ismore brittle and therefore more difficult to work with when making jewelry. Fourteenand eighteen carat gold alloys withsilveralone appear greenish-yellow and are referredto as green gold. White gold alloys can be made withpalladium ornickel. White 18 caratgold containing 17.3% nickel, 5.5% zinc and 2.2% copper is silver in appearance. Nickelis toxic, however, and its release from nickel white gold is controlled by legislation inEurope. Alternative white gold alloys are available based on palladium, silver and otherwhite metals (World Gold Council), but the palladium alloys are more expensive thanthose using nickel. High-carat white gold alloys are far more resistant to corrosion thanare either pure silver orsterling silver. The Japanese craft ofMokume-gane exploits thecolor contrasts between laminated colored gold alloys to produce decorative wood-graineffects.

Medicine

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In medieval times, gold was often seen as beneficial for the health, in the beliefthat something that rare and beautiful could not be anything but healthy. Evensome modern esotericistsand forms ofalternative medicineassign metallic gold ahealing power.[9] Some gold salts do have anti-inflammatory properties and areused as pharmaceuticals in the treatment of arthritis and other similar conditions.[10] However, only salts and radioisotopes of gold are of pharmacological value, aselemental (metallic) gold is inert to all chemicals it encounters inside the body.
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In modern times injectable gold has been proven to help to reduce the pain andswelling ofrheumatoid arthritis and tuberculosis.[10][11]

Dentistry. Gold alloys are used in restorative dentistry, especially in toothrestorations, such as crowns and permanentbridges. The gold alloys' slight

malleability facilitates the creation of a superior molar mating surface with otherteeth and produces results that are generally more satisfactory than thoseproduced by the creation of porcelain crowns. The use of gold crowns in moreprominent teeth such as incisors is favored in some cultures and discouraged inothers.

Colloidal goldpreparations (suspensions of gold nanoparticles) in water areintensely red-colored, and can be made with tightly-controlled particle sizes up toa few tens of nanometers across by reduction of gold chloride with citrateorascorbateions. 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 forthe presence and position of antigens on the surfaces of cells (Faulk and Taylor1979). In ultrathin sections of tissues viewed by electron microscopy, theimmunogold labels appear as extremely dense round spots at the position of theantigen(Roth et al. 1980). Colloidal gold is also the form of gold used as goldpaint on ceramics prior to firing.

Gold, or alloys of gold andpalladium, are applied as conductive coating tobiological specimens and other non-conducting materials such as plastics andglass to be viewed in a scanning electron microscope. The coating, which is

usually applied by sputtering with an argonplasma, has a triple role in thisapplication. Gold's very high electrical conductivity drains electrical charge toearth, and its very high density provides stopping power for electrons in theSEM'selectron beam, helping to limit the depth to which the electron beampenetrates the specimen. This improves definition of the position and topographyof the specimen surface and increases thespatial resolution of the image. Goldalso produces a high output ofsecondary electrons when irradiated by an electronbeam, and these low-energy electrons are the most commonly-used signal sourceused in the scanning electron microscope.

The isotope gold-198, (half-life: 2.7 days) is used in some cancertreatments andfor treating other diseases.[12]

Food and drink

Gold can be used in food and has the E Number175.[13] Gold leaf, flake or dust is used on and in some gourmet foods, notably sweets and

drinks as decorative ingredient.[14]Gold flake was used by the nobility inMedieval Europe as a decoration in food and drinks, in the form of leaf, flakes or
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dust, either to demonstrate the host's wealth or in the belief that something thatvaluable and rare must be beneficial for one's health.

Goldwasser(English: Goldwater) is a traditional herbal liqueurproduced inGdask, Poland, andSchwabach, Germany, and contains flakes of gold leaf.There are also some expensive (~$1000) cocktails which contain flakes of gold

leaf[15]

. However, since metallic gold is inert to all body chemistry, it adds no tastenor has it any other nutritional effect and leaves the body unaltered.[16]

Industry

The 220 kg gold brick displayed in Chinkuashi Gold Museum,Taiwan,Republic ofChina

The world's largest gold bar weighs 250 kg. Toi museum,Japan.
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A gold nugget of 5 mm in diameter (bottom) can be expanded through hammering into agold foil of about 0.5 square meter. Toi museum, Japan.

Gold solderis used for joining the components of gold jewelry by high-temperature hard soldering orbrazing. If the work is to be ofhallmarkingquality,gold solder must match the carat weight of the work, and alloy formulas aremanufactured in most industry-standard carat weights to color match yellow andwhite gold. Gold solder is usually made in at least three melting-point rangesreferred to as Easy, Medium and Hard. By using the hard, high-melting pointsolder first, followed by solders with progressively lower melting points,goldsmiths can assemble complex items with several separate soldered joints.

Gold can be made into 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 ofsilver bromide black and

white prints towards brown or blue tones, or to increase their stability. Used onsepia-toned prints, gold toners produce red tones. Kodak published formulas forseveral types of gold toners, which use gold as the chloride (Kodak, 2006).

As gold is a good reflector ofelectromagnetic radiation such as infrared andvisible light as well as radio waves, it is used for the protective coatings on manyartificial satellites, in infrared protective faceplates in thermal protection suits andastronauts' helmets and in electronic warfare planes like the EA-6B Prowler.

Gold is used as the reflective layer on some high-end CDs. Automobiles may use gold forheat insulation.McLarenuses gold foil in the

engine compartment of its F1 model.[17] Gold can be manufactured so thin that it appears transparent. It is used in some

aircraft cockpit windows forde-icing or anti-icing by passing electricity throughit. The heat produced by the resistance of the gold is enough to deter ice fromforming.[18]
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Electronics

The concentration of free electrons in gold metal is 5.901022 cm3. Gold is highlyconductive to electricity, and has been used forelectrical wiringin some highenergy applications (silver is even more conductive per volume, but gold has the

advantage of corrosion resistance). For example, gold electrical wires were usedduring some of the Manhattan Project's atomic experiments, but large high currentsilver wires were used in the calutronisotope separator magnets in the project.

Though gold is attacked by free chlorine, its good conductivity and generalresistance to oxidation and corrosion in other environments (including resistanceto non-chlorinated acids) has led to its widespread industrial use in the electronicera as a thin layer coating electrical connectors of all kinds, thereby ensuring goodconnection. For example, gold is used in the connectors of the more expensiveelectronics cables, such as audio, video andUSB cables. The benefit of using goldover other connector metals such astinin 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 inother applications in electronic sliding contacts in highly humid or corrosiveatmospheres, and in use for contacts with a very high failure cost (certaincomputers, communications equipment, spacecraft,jet aircraftengines) remainsvery common, and is unlikely to be replaced in the near future by any other metal.

Besides sliding electrical contacts, gold is also used in electrical contacts becauseof its resistance to corrosion,electrical conductivity,ductilityand lack oftoxicity.[19] Switch contacts are generally subjected to more intense corrosion stress thanare sliding contacts.

Fine gold wires are used to connect semiconductor devices to their packagesthrough a process known as wire bonding.

As gold chemical compounds

Gold is attacked by and dissolves in alkaline solutions of potassium or sodium cyanide,and gold cyanide is the electrolyteused in commercial electroplatingof gold onto basemetals and electroforming. Gold chloride (chloroauric acid) solutions are used to makecolloidal gold by reduction with citrate orascorbateions. Gold chloride and gold oxideare used to make highly-valued cranberry or red-colored glass, which, like colloidalgoldsuspensions, contains evenly-sized spherical goldnanoparticles.[20]

History

The Sun symbol has long represented gold
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The Turin Papyrus Map

Funerary mask ofTutankhamun

Jason returns with the golden fleece on anApulianred-figurecalyx krater, ca. 340330BC.

Gold has been known and highly valued sinceprehistorictimes. It may have been thefirst metal used by humans and was valued for ornamentation and rituals. Egyptianhieroglyphs from as early as 2600 BC describe gold, which king Tushrattaof the Mitanni

claimed was "more plentiful than dirt" in Egypt.[21] Egypt and especiallyNubia had theresources to make them major gold-producing areas for much of history. The earliestknown map is known as the Turin Papyrus Map and shows the plan of a gold mineinNubia together with indications of the localgeology. The primitive working methods aredescribed by Strabo and included fire-setting. Large mines also occurred across the RedSea in what is now Saudi Arabia.
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The legend of the golden fleece may refer to the use of fleeces to trap gold dust fromplacer deposits in the ancient world. 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 thefirst chapters of MatthewNew Testament. The Book of Revelation21:21 describes thecity ofNew Jerusalem as having streets "made of pure gold, clear as crystal". The south-

east corner of the Black Seawas famed for its gold. Exploitation is said to date from thetime ofMidas, and this gold was important in the establishment of what is probably theworld's earliest coinage in Lydia around 610 BC.[22] From 6th or 5th century BC, Chu(state) circulated Ying Yuan, one kind of square gold coin.

The Romans developed new methods for extracting gold on a large scale usinghydraulicmining methods, especially in Spain from 25 BC onwards and in Romaniafrom 150 ADonwards. One of their largest mines was atLas MedulasinLen (Spain), where sevenlong aqueducts enabled them to sluice most of a large alluvial deposit. The mines atRoia Montan in Transylvania were also very large, and until very recently, still minedby opencast methods. They also exploited smaller deposits in Britain, such as placer and

hard-rock deposits at Dolaucothi. The various methods they used are well described byPliny the Elderin hisencyclopediaNaturalis Historiawritten towards the end of the firstcentury AD.

The Mali Empire in Africa was famed throughout theold world for its large amounts ofgold. Mansa Musa, ruler of the empire (13121337) became famous throughout the oldworld for his great hajj to Meccain 1324. When he passed through Cairo in July 1324, hewas reportedly accompanied by acamel trainthat included thousands of people andnearly a hundred camels. He gave away so much gold that it depressed the price in Egyptfor over a decade.[23] A contemporary Arab historian remarked:

Gold was at a high price in Egypt until they came in that year. The mithqaldid not go below 25 dirhams and was generally above, but from that time itsvalue fell and it cheapened in price and has remained cheap till now. Themithqal 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 [...] Chihab Al-Umari[24]

The European exploration of the Americas was fueled in no small part by reports of thegold ornaments displayed in great profusion byNative American peoples, especially in

Central America, Peru, Ecuadorand Colombia. The Aztecs regarded gold as literally theproduct of the gods, calling it "god excrement" (teocuitlatlinNahuatl).[25]

Although the price of someplatinum group metals can be much higher, gold has longbeen considered the most desirable ofprecious metals, and its value has been used as thestandard for many currencies (known as the gold standard) in history. Gold has been usedas a symbol for purity, value, royalty, and particularly roles that combine theseproperties. Gold as a sign of wealth and prestige was made fun of by Thomas More in his
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iki/Nahuatlhttp://en.wikipedia.org/wiki/Gold#cite_note-24%23cite_note-24http://en.wikipedia.org/wiki/Platinumhttp://en.wikipedia.org/wiki/Precious_metalhttp://en.wikipedia.org/wiki/Currencyhttp://en.wikipedia.org/wiki/Gold_standardhttp://en.wikipedia.org/wiki/Thomas_More


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treatise Utopia. On that imaginary island, gold is so abundant that it is used to makechains for slaves, tableware and lavatory-seats. When ambassadors from other countriesarrive, dressed in ostentatious gold jewels and badges, the Utopians mistake them formenial 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 thisis certainly not a professional way of examining gold, the bite testshould score the goldbecause gold is a soft metal, as indicated by its score on theMohs' scale of mineralhardness. The purer the gold the easier it should be to mark it. Painted lead can cheat thistest because lead is softer than gold (and may invite a small risk oflead poisoning ifsufficient lead is absorbed by the biting).

Gold in antiquity was relatively easy to obtaingeologically; however, 75% of all goldever produced has been extracted since 1910.[26]It has been estimated that all the gold inthe world that has ever been refined would form a single cube 20 m (66 ft) on a side(equivalent to 8000 m).[26]

One main goal of the alchemists was to produce gold from other substances, such as lead presumably by the interaction with a mythical substance called thephilosopher'sstone. Although they never succeeded in this attempt, the alchemists promoted an interestin 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 center( ), which was also the astrological symbol and the ancient Chinese characterfor the Sun. For modern creation ofartificial gold by neutron capture, see gold synthesis.

During the 19th century, gold rushes occurred whenever large gold deposits werediscovered. The first documented discovery of gold in the United States was at the Reed

Gold Mine near Georgeville, North Carolina in 1803.

[27]

The first major gold strike in theUnited States occurred in a small north Georgia town calledDahlonega.[28]Further goldrushes occurred in California,Colorado, the Black Hills,Otago,Australia,Witwatersrand, and the Klondike.

Because of its historically high value, much of the gold mined throughout history is stillin circulation in one form or another.

Occurrence

Gold ore
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This 156-ounce (4.85 kg) nugget was found by an individual prospector in the SouthernCalifornia Desert using a metal detector.

Gold's atomic numberof 79 makes it one of the higheratomic numberelements whichoccur naturally. Like all elements with atomic numbers larger than iron, gold is thoughtto have been formed from a supernova nucleosynthesisprocess. Their explosionsscattered metal-containing dusts (including heavy elementslike gold) into the region of

space in which they later condensed into oursolar systemand the Earth.

On Earth, whenever elemental gold occurs, it appears most often as a metal solid solutionof gold with silver, i.e. a gold silver alloy. Such alloys usually have a silver content of 810%. Electrum is elemental gold with more than 20% silver. Electrum's color runs fromgolden silvery to silvery, dependent upon the silver content. The more silver, the lowerthe specific gravity.

Relative sizes of a an 860kg rock ore, and the 30g of gold that can be extracted from it.Toi gold mine, Japan.

Gold is found in ores made up of rock with very small or microscopic particles of gold.This gold ore is often found together with quartzorsulfide mineralssuch asFool's Gold,which is apyrite.[29]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 upin alluvial deposits (calledplacer deposits). Such free gold is always richer at the surfaceof gold-bearing veins owing to the oxidation of accompanying minerals followed byweathering, and washing of the dust into streams and rivers, where it collects and can bewelded by water action to form nuggets.

Gold sometimes occurs combined with telluriumas the mineralscalaverite, krennerite,nagyagite,petzite and sylvanite, and as the rare bismuthide maldonite(Au2Bi) and
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antimonide aurostibite (AuSb2). Gold also occurs in rare alloys with copper, lead, andmercury: the minerals auricupride (Cu3Au), novodneprite (AuPb3) and weishanite((Au,Ag)3Hg2).

Recent research suggests that microbes can sometimes play an important role in forming

gold deposits, transporting and precipitating gold to form grains and nuggets that collectin alluvial deposits.[30]

Production

Main articles: Gold prospecting,Gold mining, and Gold extraction

The entrance to an underground gold mine in Victoria, Australia

World gold production trend

Gold output in 2005

Gold extraction is most economical in large, easily mined deposits. Ore grades as little as0.5 g/1000 kg (0.5 parts per million, ppm) can be economical. Typical ore grades inopen-pit mines are 15 g/1000 kg (15 ppm); ore grades in underground orhard rockmines are usually at least 3 g/1000 kg (3 ppm). Because ore grades of 30 g/1000 kg (30ppm) are usually needed before gold is visible to the naked eye, in most gold mines thegold is invisible.

Since the 1880s, South Africa has been the source for a large proportion of the worldsgold 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 1,000tonnes. 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 asthe world's largest gold producer, the first time since 1905 that South Africa has not beenthe largest.[31]

The city ofJohannesburg located in South Africa was founded as a result of theWitwatersrand Gold Rushwhich resulted in the discovery of some of the largest golddeposits the world has ever seen. Gold fields located within the basin in the Free State
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andGauteng provinces are extensive in strike and diprequiring some of the world'sdeepest mines, with the Savuka and TauTona mines being currently the world's deepestgold mine at 3,777 m. The Second Boer Warof 18991901 between the British Empireand the AfrikanerBoers was at least partly over the rights of miners and possession of thegold wealth in South Africa.

Other major producers are the United States, Australia, Russia andPeru. Mines in SouthDakota andNevada supply two-thirds of gold used in the United States. In SouthAmerica, the controversial project Pascua Lama aims at exploitation of rich fields in thehigh mountains ofAtacama Desert, at the border between Chile and Argentina. Todayabout one-quarter of the world gold output is estimated to originate from artisanal orsmall scale mining.[32]

After initial production, gold is often subsequently refined industrially by the Wohlwillprocess or the Miller process. Other methods of assaying and purifying smaller amountsof gold includeparting and inquartation as well ascuppelation, or refining methods based

on the dissolution of gold in aqua regia.

The world'soceans hold a vast amount of gold, but in very low concentrations (perhaps12 parts per 10 billion, e.g. every cubic kilometerof water could contain 10 to 20 kg ofgold). A number of people have claimed to be able to economically recover gold fromseawater, but so far they have all been either mistaken or crooks. Reverend Prescott Jerneganran a gold-from-seawater swindle in theUnited States in the 1890s. A British fraudsterran the same scam inEngland in the early 1900s.[33]Fritz Haber(the German inventor ofthe Haber process) attempted commercial extraction of gold from sea water in an effort tohelp pay Germany's reparations followingWorld War I. Unfortunately, his assessment ofthe 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 morethan the commercial value of the gold recovered.[citation needed] No commercially viablemechanism for performing gold extraction from sea water has yet been identified.Goldsynthesis is not economically viable and is unlikely to become so in the foreseeablefuture.

The average gold mining and extraction costs[when?] are $238 pertroy ounce but these canvary widely depending on mining type and ore quality. In 2001, global mine productionamounted 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.[1] This can berepresented by a cube with an edge length of just 20.2 meters.

Gold is so stable and so valuable that it is always recovered and recycled. There is no true"consumption" of gold in the economic sense; the stock of gold remains essentiallyconstant while ownership shifts from one party to another.[34]

Consumption
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India is the worlds largest consumer of gold, as Indians buy about 25 per cent of theworlds gold,[35] purchasing approximately 800 tonnes of gold every year. India is also thelargest importer of the yellow metal; in 2008 India imported around 400 tonnes of gold.[36]

Price

Main articles: Gold as an investmentandGold standard

LBMA USD morningprice fixings ($US per troy ounce) from Jan 2001 to Apr 2006

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 grams. When it is

alloyed with other metals the term caratorkaratis used to indicate the amount of goldpresent, with 24 carats being pure gold and lower ratings proportionally less. The purityof a gold barcan also be expressed as a decimal figure ranging from 0 to 1, known as themillesimal fineness, such as 0.995 being very pure.

The price of gold is determined on the open market, but a procedure known as the GoldFixing in London, originating in September 1919, provides a daily benchmark figure tothe industry. The afternoon fixing appeared in 1968 to fix a price when US markets areopen.

Historically gold coinagewas widely used as currency; Whenpaper money was

introduced, it typically was a receipt redeemable for gold coin or bullion. In an economicsystem known as the gold standard, a certainweightof gold was given the name of a unitof currency. For a long period, the United States government set the value of the USdollar so that one troy ounce was equal to $20.67 ($664.56/kg), but in 1934 the dollarwas devalued to $35.00 per troy ounce ($1125.27/kg). By 1961 it was becoming hard tomaintain this price, and a pool of US and European banks agreed to manipulate themarket to prevent furthercurrency devaluationagainst increased gold demand.

On March 17, 1968, economic circumstances caused the collapse of the gold pool, and atwo-tiered pricing scheme was established whereby gold was still used to settleinternational 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 wasabandoned in 1975 when the price of gold was left to find its free-market level. Centralbanks still hold historical gold reserves as a store of valuealthough the level hasgenerally been declining. The largest gold depository in the world is that of the U.S.Federal Reserve BankinNew York, which holds about 3%[citation needed] of the gold evermined, as does the similarly-laden U.S. Bullion Depository atFort Knox.
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In 2005 the World Gold Councilestimated total global gold supply to be 3,859 tonnesand demand to be 3,754 tonnes, giving a surplus of 105 tonnes.[37]

Price records

Since 1968 the price of gold on the open market has ranged widely, from a high of$850/oz ($27,300/kg) on January 21, 1980, to a low of $252.90/oz ($8,131/kg) on June21, 1999 (London Gold Fixing).[38]The 1980 high was not overtaken until January 3,2008 when a new maximum of $865.35 pertroy ouncewas set (a.m. London GoldFixing).[39] The current record price was set on March 17, 2008 at $1023.50/oz($32,900/kg)(am. London Gold Fixing).[39]The intraday record was set the day after at$1,033.90/oz.[40]

Long term price trends

Since April 2001 the gold price has more than tripled in value against the US dollar,[41]

prompting speculation that this long secular bear market(or the Great CommoditiesDepression) has ended and abull markethas returned.[42] In March 2008, the gold priceincreased above $1000,[43] which in real terms is still well below the $850/oz. peak onJanuary 21, 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 first andsecond oil crisis) lowered the Dow/Gold ratio (which is inherently inflationadjusted) substantially, in most cases to a value well below 4. [44] During these difficulttimes, investors tried to preserve their assets by investing in precious metals, mostnotably gold and silver.

Compounds

Although gold is a noble metal, it forms many and diverse compounds. Theoxidationstate 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 softligands such as thioethers, thiolates, and tertiaryphosphines. Au(I) compounds aretypically linear. A good example isAu(CN)2

, which is the soluble form of goldencountered in mining. Curiously, aurous complexes of water are rare. The binary goldhalides, such as AuCl, form zig-zag polymeric chains, again featuring linear coordinationat Au. Most drugs based on gold are Au(I) derivatives. [45]

Gold(III) (auric) is a common oxidation state and is illustrated bygold(III) chloride,AuCl3. Its derivative ischloroauric acid, HAuCl4, which forms when Au dissolves inaqua regia. Au(III) complexes, like other d8 compounds, are typically square planar.

Less common oxidation states
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Gold (Pronounced /ˈɡoʊld/) is a Chemical Element With the Symbol