Tin And Gold

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Tin is a chemical element with the symbol Sn (for Latin: stannum) and atomic number 50. It is a main group metal in group 14 of the periodic table. Tin shows chemical similarity to both neighboring group 14 elements, germanium and lead and has two possible oxidation states, +2 and the slightly more stable +4Sony VPCEH3T9E battery. Tin is the 49th most abundant element and has, with 10 stable isotopes, the largest number of stable isotopes in the periodic table. Tin is obtained chiefly from the mineral cassiterite, where it occurs as tin dioxide, SnO2.

This silvery, malleable post-transition metal is not easily oxidized in air and is used to coat other metals to prevent corrosion. The first alloy, used in large scale since 3000 BCSony VPCEH3N6E battery, was bronze, an alloy of tin and copper. After 600 BC pure metallic tin was produced. Pewter, which is an alloy of 85–90% tin with the remainder commonly consisting of copper, antimony and lead, was used for flatware from the Bronze Age until the 20th century. In modern times tin is used in many alloys, most notably tin/lead soft solders, typically containing 60% or more of tinSony VPCEH3N1E battery. Another large application for tin is corrosion-resistant tin plating of steel. Because of its low toxicity, tin-plated metal is also used for food packaging, giving the name to tin cans, which are made mostly of steel.

Characteristics

Physical properties

Droplet of molten tin

Tin is a malleable, ductile and highly crystalline silvery-white metal. When a bar of tin is bent, a crackling sound known as the tin cry can be heard due to the twinning of the crystals.[2] Tin melts at a low temperature of about 232 °C, which is further reduced to 177.3 °C for 11-nm particles. Sony VPCEH3D0E battery

β-tin (the metallic form, or white tin), which is stable at and above room temperature, is malleable. In contrast, α-tin (nonmetallic form, or gray tin), which is stable below 13.2 °C, is brittle. α-tin has a diamond cubic crystal structure, similar to diamond, silicon or germanium. α-tin has no metallic properties at all because its atoms form a covalent structure where electrons cannot move freelySony VPCEH3B1E battery. It is a dull-gray powdery material with no common uses, other than a few specialized semiconductor applications.[2] These two allotropes, α-tin and β-tin, are more commonly known as gray tin and white tin, respectively. Two more allotropes, γ and σ, exist at temperatures above 161 °C and pressures above several GPa.[4] Although the α-β transformation temperature is nominally 13.2 °CSony VPCEH2Z1E battery, impurities (e.g. Al, Zn, etc.) lower the transition temperature well below 0 °C, and upon addition of Sb or Bi the transformation may not occur at all, increasing the durability of the tin.[5][6]

Commercial grades of tin (99.8%) resist transformation because of the inhibiting effect of the small amounts of bismuth, antimony, lead and silver present as impurities. Alloying elements such as copper, antimony, bismuth, cadmium and silver increase its hardnessSony VPCEH2S9E battery. Tin tends rather easily to form hard, brittle intermetallic phases, which are often undesirable. It does not form wide solid solution ranges in other metals in general, and there are few elements that have appreciable solid solubility in tin. Simple eutectic systems, however, occur with bismuth, gallium, lead, thallium and zinc. Sony VPCEH2Q1E battery

Tin becomes a superconductor below 3.72 K.[7] In fact, tin was one of the first superconductors to be studied; the Meissner effect, one of the characteristic features of superconductors, was first discovered in superconducting tin crystals.[8]

[edit]Chemical properties

Tin resists corrosion from water but can be attacked by acids and alkalis. Tin can be highly polished and is used as a protective coat for other metals.[2] In this case the formation of a protective oxide layer is used to prevent further oxidationSony VPCEH2P0E battery. This oxide layer forms on pewter and other tin alloys.[9] Tin acts as a catalyst when oxygen is in solution and helps accelerate chemical attack.[2]

[edit]Isotopes

Main article: Isotopes of tin

Tin is the element with the greatest number of stable isotopes, ten; these include all those with atomic masses between 112 and 124, with the exception of 113, 121 and 123. Of these, the most abundant ones are 120Sn (at almost a third of all tin), 118Sn, and 116Sn, while the least abundant one is 115SnSony VPCEH2N1E battery. The isotopes possessing even mass numbers have no nuclear spin while the odd ones have a spin of +1/2. Tin, with its three common isotopes 115Sn, 117Sn and 119Sn, is among the easiest elements to detect and analyze by NMR spectroscopy, and its chemical shifts are referenced against SnMe4.[note 1][10]

This large number of stable isotopes is thought to be a direct result of tin possessing an atomic number of 50, which is a "magic number" in nuclear physicsSony VPCEH2M9E battery. There are 28 additional unstable isotopes that are known, encompassing all the remaining ones with atomic masses between 99 and 137. Aside from 126Sn, which has a half-life of 230,000 years, all the radioactive isotopes have a half-life of less than a year. The radioactive 100Sn is one of the few nuclides possessing a "doubly magic" nucleus and was discovered relatively recentlySony VPCEH2M1E battery, in 1994.[11] Another 30 metastable isomers have been characterized for isotopes between 111 and 131, the most stable of which being 121mSn, with a half-life of 43.9 years.

Etymology

The English word 'tin' is Germanic; related words are found in the other Germanic languages—German zinn, Swedish tenn, Dutch tin, etc.—but not in other branches of Indo-European except by borrowing (e.g. Irish tinne). Its origin is unknown. Sony VPCEH2L9E battery

The Latin name stannum originally meant an alloy of silver and lead, and came to mean 'tin' in the 4th century BCE[13]—the earlier Latin word for it was plumbum candidum 'white lead'. Stannum apparently came from an earlier stāgnum (meaning the same thing),[12] the origin of the Romance and Celtic terms for 'tin'.[12][14] The origin of stannum/stāgnum is unknown; it may be pre-Indo-European. Sony VPCEH2J1E battery The Meyers Konversationslexikon speculates on the contrary that stannum is derived from Cornish stean, and is proof that Cornwall in the first centuries AD was the main source of tin.

[edit]History

Main article: Tin sources and trade in ancient times

Ceremonial giant bronze dirk of the Plougrescant-Ommerschans type, Plougrescant, France, 1500–1300 BC.

Tin extraction and use can be dated to the beginnings of the Bronze Age around 3000 BC, when it was observed that copper objects formed of polymetallic ores with different metal contents had different physical properties. Sony VPCEH2H1E batteryThe earliest bronze objects had tin or arsenic content of less than 2% and are therefore believed to be the result of unintentional alloying due to trace metal content in the copper ore.[17] The addition of a second metal to copper increases its hardness, lowers the melting temperature, and improves the casting process by producing a more fluid melt that cools to a denser, less spongy metal.[17] This was an important innovation that allowed for the much more complex shapes cast in closed moulds of the Bronze AgeSony VPCEH2F1E battery. Arsenical bronze objects appear first in the Near East where arsenic is commonly found in association with copper ore, but the health risks were quickly realized and the quest for sources of the much less hazardous tin ores began early in the Bronze Age.[18] This created the demand for rare tin metal and formed a trade network that linked the distant sources of tin to the markets of Bronze Age cultures. Sony VPCEH2E0E battery

Cassiterite (SnO2), the tin oxide form of tin, was most likely the original source of tin in ancient times. Other forms of tin ores are less abundant sulfides such as stannite that require a more involved smelting process. Cassiterite often accumulates in alluvial channels as placer deposits due to the fact that it is harder, heavier, and more chemically resistant than the granite in which it typically forms. Sony VPCEH2D0E batteryThese deposits can be easily seen in river banks as cassiterite is usually black, purple or otherwise dark in colour, a feature exploited by early Bronze Age prospectors. It is likely that the earliest deposits were alluvial in nature, and perhaps exploited by the same methods used for panning gold in placer deposits.[citation needed]

[edit]Compounds and chemistry

See also Category: Tin compounds

In the great majority of its compounds, tin has the oxidation state II or IVSony VPCEH2C0E battery.

[edit]Inorganic compounds

Halide compounds are known for both oxidation states. For Sn(IV), all four halides are well known: SnF4, SnCl4, SnBr4, and SnI4. The three heavier members are volatile molecular compounds, whereas the tetrafluoride is polymeric. All four halides are known for Sn(II) also: SnF2, SnCl2, SnBr2, and SnI2. All are polymeric solids. Of these eight compounds, only the iodides are coloured. Sony VPCEH29FJ/W battery

Tin(II) chloride (also known as stannous chloride) is the most important tin halide in a commercial sense. Illustrating the routes to such compounds, chlorine reacts with tin metal to give SnCl4 whereas the reaction of hydrochloric acid and tin gives SnCl2 and hydrogen gas. Alternatively SnCl4 and Sn combine to stannous chloride via a process called comproportionation: Sony VPCEH29FJ/P battery

SnCl4 + Sn → 2 SnCl2

Tin can form many oxides, sulfides, and other chalcogenide derivatives. The dioxide SnO2 (cassiterite) forms when tin is heated in the presence of air.[20] SnO2 is amphoteric, which means that it dissolves in both acidic and basic solutions.[22] There are also stannates with the structure [Sn(OH)6]2−, like K2[Sn(OH)6], although the free stannic acid H2[Sn(OH)6] is unknownSony VPCEH29FJ/B battery. The sulfides of tin exist in both the +2 and +4 oxidation states: tin(II) sulfide and tin(IV) sulfide (mosaic gold).

Ball-and-stick models of the structure of solid stannous chloride (SnCl2).

[edit]Hydrides

Stannane (SnH4), where tin is in the +4 oxidation state, is unstable. Organotin hydrides are however well known, e.g. tributyltin hydride (Sn(C4H9)3H).[2] These compound release transient tributyl tin radicals, rare examples of compounds of tin(III) Sony VPCEH28FN/L battery.

[edit]Organotin compounds

Organotin compounds, sometimes called stannanes, are chemical compounds with tin-carbon bonds.[25] Of the compounds of tin, the organic derivatives are the most useful commercially.[26] Some organotin compounds are highly toxic and have been used as biocides. The first organotin compound to be reported was diethyltin diiodide ((C2H5)2SnI2), reported by Edward Frankland in 1849. Sony VPCEH28FJ/W battery

Most organotin compounds are colorless liquids or solids that are stable to air and water. They adopt tetrahedral geometry. Tetraalkyl- and tetraaryltin compounds can be prepared using Grignard reagents:[26]

SnCl4 + 4 RMgBr → R4Sn + 4 MgBrCl

The mixed halide-alkyls, which are more common and more important commercially than the tetraorgano derivatives, are prepared by redistribution reactionsSony VPCEH28FJ/P battery:

SnCl4 + R4Sn → 2 SnCl2R2

Divalent organotin compounds are uncommon, although more common than related divalent organogermanium and organosilicon compounds. The greater stabilization enjoyed by Sn(II) is attributed to the "inert pair effect". Organotin(II) compounds include both stannylenes (formula: R2Sn, as seen for singlet carbenes) and distannylenes (R4Sn2), which are roughly equivalent to alkenes. Both classes exhibit unusual reactions. Sony VPCEH28FJ/B battery

Occurrence

See also Category: Tin minerals

Sample of cassiterite, the main ore of tin.

Map showing tin output in 2005.

Granular pieces of cassiterite, which are collected by placer mining

Tin is generated via the long S-process in low-medium mass stars (with masses of 0.6 to 10 times that of Sun). It arises via beta decay of heavy isotopes of indium.[29]

Tin is the 49th most abundant element in the Earth's crust, representing 2 ppm compared with 75 ppm for zinc, 50 ppm for copper, and 14 ppm for lead. Sony VPCEH28FH/B battery

Tin does not occur as the native element but must be extracted from various ores. Cassiterite (SnO2) is the only commercially important source of tin, although small quantities of tin are recovered from complex sulfides such as stannite, cylindrite, franckeite, canfieldite, and teallite. Minerals with tin are almost always associated with granite rock, usually at a level of 1% tin oxide content. Sony VPCEH28FG/P battery

Because of the higher specific gravity of tin dioxide, about 80% of mined tin is from secondary deposits found downstream from the primary lodes. Tin is often recovered from granules washed downstream in the past and deposited in valleys or under sea. The most economical ways of mining tin are through dredging, hydraulic methods or open cast miningSony VPCEH28FG/B battery. Most of the world's tin is produced from placer deposits, which may contain as little as 0.015% tin.[32]

It was estimated in January 2008 that there were 6.1 million tons of economically recoverable primary reserves, from a known base reserve of 11 million tons.[33] Below are listed the nations with the largest known reserves.

Estimates of tin production have historically varied with the dynamics of economic feasibility and the development of mining technologiesSony VPCEH28FF/B battery, but it is estimated that, at current consumption rates and technologies, the Earth will run out of tin that can be mined in 40 years.[34] However Lester Brown has suggested tin could run out within 20 years based on an extremely conservative extrapolation of 2% growth per year.[35]

Secondary, or scrap, tin is also an important source of the metal. The recovery of tin through secondary production, or recycling of scrap tin, is increasing rapidlySony VPCEH28FA/B battery. Whereas the United States has neither mined since 1993 nor smelted tin since 1989, it was the largest secondary producer, recycling nearly 14,000 tons in 2006.[33]

New deposits are reported to be in southern Mongolia,[37] and in 2009, new deposits of tin were discovered in Colombia, South America, by the Seminole Group Colombia CI, SAS.[38][39]

[edit]Production

Tin is produced by carbothermic reduction of the oxide ore with carbon or coke. Both reverberatory furnace and electric furnace can be used. Sony VPCEH27FG/W battery

[edit]Mining and smelting

Main article: Tin mining

[edit]Industry

The ten largest companies produced most of world's tin in 2007. It is not clear which of these companies include tin smelted from the mine at Bisie, Democratic Republic of the Congo, which is controlled by a renegade militia and produces 15,000 tons. Most of the world's tin is traded on the London Metal Exchange (LME), from 8 countries, under 17 brands. Sony VPCEH26FJ/W battery

Prices of tin were at US$11,900 per ton as of Nov 24, 2008. Prices reached an all-time high of nearly $25,000 per ton in May 2008, largely because of the effect of the decrease of tin production from Indonesia.

Exchanges

London Metal Exchange (LME), contract market

Kuala Lumpur Tin Market (KLTM), contract market

Indonesia Tin Exchange (INATIN), the solely physical market for tin will formally launchs at January 12, 2012[45] (postponed to February 1, 2012 due to 10 days holiday of Chinese Lunar New year) Sony VPCEH26EN/B battery

Applications

World consumption of refined tin by end use, 2006

In 2006, about half of tin produced was used in solder. The rest was divided between tin plating, tin chemicals, brass and bronze, and niche uses.[46]

Solder

A coil of lead-free solder wire

Tin has long been used as a solder in the form of an alloy with lead, tin accounting for 5 to 70% w/w. Tin forms a eutectic mixture with lead containing 63% tin and 37% lead. Such solders are primarily used for solders for joining pipes or electric circuitsSony VPCEH26EH/P battery. Since the European Union Waste Electrical and Electronic Equipment Directive (WEEE Directive) and Restriction of Hazardous Substances Directive (RoHS) came into effect on 1 July 2006, the use of lead in such alloys has decreased. Replacing lead has many problems, including a higher melting point, and the formation of tin whiskers causing electrical problems. Replacement alloys are rapidly being found, however. Sony VPCEH26EG/P battery

[edit]Tin plating

Tin bonds readily to iron and is used for coating lead or zinc and steel to prevent corrosion. Tin-plated steel containers are widely used for food preservation, and this forms a large part of the market for metallic tin. A tinplate canister for preserving food was first manufactured in London in 1812.[48] Speakers of British English call them "tins"Sony VPCEH26EF/B battery, while speakers of American English call them "cans" or "tin cans". One thus-derived use of the slang term "tinnie" or "tinny" means "can of beer". The tin whistle is so called because it was first mass-produced in tin-plated steel.

Specialized alloys

Pewter plate

Tin in combination with other elements forms a wide variety of useful alloys. Tin is most commonly alloyed with copper. Pewter is 85–99% tin;[51] Babbitt metal has a high percentage of tin as well.[52][53] Bronze is mostly copper (12% tin) Sony VPCEH26EA/W battery, while addition of phosphorus gives phosphor bronze. Bell metal is also a copper-tin alloy, containing 22% tin. Tin has also sometimes been used in coinage; for example, it once formed a single figure percentage of the American[54] and Canadian[55] pennies. Because copper is often the major metal in such coins, and zinc is sometimes present as well, these could technically be called bronze and/or brass alloysSony VPCEH25EN/W battery.

Tin plated metal from can

The niobium-tin compound Nb3Sn is commercially used as wires for superconducting magnets, due to the material's high critical temperature (18 K) and critical magnetic field (25 T). A superconducting magnet weighing only a couple of kilograms is capable of producing magnetic fields comparable to a conventional electromagnet weighing tons. Sony VPCEH25EG/B battery

A addition of a few percent tin is commonly used in zirconium alloys for the cladding of nuclear fuel.[57]

Most metal pipes in a pipe organ are made of varying amounts of a tin/lead alloy, with 50%/50% being the most common. The amount of tin in the pipe defines the pipe's tone, since tin is the most tonally resonant of all metals. When a tin/lead alloy cools, the lead cools slightly faster and produces a mottled or spotted effect. This metal alloy is referred to as spotted metal. Sony VPCEH24FX/W battery

Other applications

A 21st century reproduction barn lantern made of punched tin.

Punched tin, also called pierced tin, is an artisan technique originating in central Europe for creating housewares that are both functional and decorative. Decorative piercing designs exist in a wide variety, based on geography or the artisan's personal creations. Punched tin lanterns are the most common application of this artisan techniqueSony VPCEH24FX/P battery. The light of a candle shining through the pierced design creates a decorative light pattern in the room where it sits. Punched tin lanterns and other punched tin articles were created in the New World from the earliest European settlement. A well-known example is the Revere type lantern, named after Paul Revere. Sony VPCEH24FX/L battery

Before the modern era, in some areas of the Alps, a goat or sheep's horn would be sharpened and a tin panel would be punched out using the alphabet and numbers from one to nine. This learning tool was known appropriately as "the horn". Modern reproductions are decorated with such motifs as hearts and tulipsSony VPCEH24FX/B battery.

In America, pie safes and food safes came into use in the days before refrigeration. These were wooden cupboards of various styles and sizes - either floor standing or hanging cupboards meant to discourage vermin and insects and to keep dust from perishable foodstuffs. These cabinets had tinplate inserts in the doors and sometimes in the sides, punched out by the homeownerSony VPCEH1Z1E battery, cabinetmaker or a tinsmith in varying designs to allow for air circulation. Modern reproductions of these articles remain popular in North America.[61]

Window glass is most often made by floating molten glass on top of molten tin (creating float glass) in order to produce a flat surface. This is called the "Pilkington process".[62]

Tin is also used as a negative electrode in advanced Li-ion batteries. Its application is somewhat limited by the factSony VPCEH1S9E battery, that some tin surfaces catalyze decomposition of carbonate-based electrolytes used in Li-ion batteries.[63]

Tin(II) fluoride is added to some dental care products[64][65] as stannous fluoride (SnF2). Tin(II) fluoride can be mixed with calcium abrasives while the more common sodium fluoride gradually becomes biologically inactive combined with calcium compounds.[66] It has also been shown to be more effective than sodium fluoride in controlling gingivitis. Sony VPCEH1S8E battery

[edit]Organotin compounds

Main article: Organotin chemistry

Of all the chemical compounds of tin, the organotin compounds are most heavily used. Worldwide industrial production probably exceeds 50,000 tonnes.[68]

[edit]PVC stabilizers

The major commercial application of organotin compounds is in the stabilization of PVC plastics. In the absence of such stabilizers, PVC would otherwise rapidly degrade under heat, light, and atmospheric oxygen, to give discolored, brittle products. Tin scavenges labile chloride ions (Cl-), which would otherwise initiate loss of HCl from the plastic material. Sony VPCEH1S1E battery Typical tin compounds are carboxylic acid derivatives of dibutyltin dichloride, such as the dilaurate.[70]

[edit]Biocides

Organotin compounds can have a relatively high toxicity, which is both advantageous and problematic. They have been used for their biocidal effects in/as fungicides, pesticides, algaecides, wood preservatives, and antifouling agents.[69] Tributyltin oxide is used as a wood preservative. Sony VPCEH1S0E battery Tributyltin was used as additive for ship paint to prevent growth of marine organisms on ships, with use declining after organotin compounds were recognized as persistent organic pollutants with an extremely high toxicity for some marine organisms, for example the dog whelk.[72] The EU banned the use of organotin compounds in 2003,[73] while concerns over the toxicity of these compounds to marine life and their effects on the reproduction and growth of some marine species, Sony VPCEH1M9E battery (some reports describe biological effects to marine life at a concentration of 1 nanogram per liter) have led to a worldwide ban by the International Maritime Organization.[74] Many nations now restrict the use of organotin compounds to vessels over 25 meters long.[69]

[edit]Organic chemistry

Some tin reagents are useful in organic chemistry. In the largest application, stannous chloride is a common reducing agent for the conversion of nitro and oxime groups to amines. The Stille reaction couples organotin compounds with organic halides or pseudohalides. Sony VPCEH1M1E battery

Gold ( /ˈɡoʊld/) is a chemical element with the symbol Au (from Latin: aurum "gold") and an atomic number of 79. Gold is a dense, soft, shiny, malleable and ductile metal. Pure gold has a bright yellow color and luster traditionally considered attractive, which it maintains without oxidizing in air or water. Chemically, gold is a transition metal and a group 11 elementSony VPCEH1L9E battery. It is one of the least reactive chemical elements solid under standard conditions. The metal therefore occurs often in free elemental (native) form, as nuggets or grains in rocks, in veins and in alluvial deposits. Less commonly, it occurs in minerals as gold compounds, usually with tellurium.

Gold resists attacks by individual acids, but it can be dissolved by the aqua regia (nitro-hydrochloric acid), so named because it dissolves gold. Gold also dissolves in alkaline solutions of cyanideSony VPCEH1L8E battery, which have been used in mining. Gold dissolves in mercury, forming amalgam alloys. Gold is insoluble in nitric acid, which dissolves silver and base metals, a property that has long been used to confirm the presence of gold in items, giving rise to the term the acid test.

Gold has been a valuable and highly sought-after precious metal for coinage, jewelry, and other arts since long before the beginning of recorded history. Gold standards have been the most common basis for monetary policies throughout human historySony VPCEH1L0E battery, being widely supplanted by fiat currency starting in the 1930's. Last US Gold Certificates and coined gold coins for circulating medium were issued in the USA in 1932. In Europe, most countries left the gold standard with the Start of WWI in 1914 and with huge War debts, failed to return to gold as a medium of exchangeSony VPCEH1J8E battery.

Gold has also been frequently linked to a wide variety of symbolisms and ideologies. A total of 165,000 tonnes of gold have been mined in human history, as of 2009.[2] This is roughly equivalent to 5.3 billion troy ounces or, in terms of volume, about 8500 m3, or a cube 20.4 m on a side. The world consumption of new gold produced is about 50% in jewelry, 40% in investments, and 10% in industry. Sony VPCEH1J1E battery

Besides its widespread monetary and symbolic functions, gold has many practical uses in dentistry, electronics, and other fields. Its high malleability, ductility, resistance to corrosion and most other chemical reactions, and conductivity of electricity led to many uses of gold, including electric wiring, colored-glass production and even gold leaf eatingSony VPCEH1E1E battery.

Characteristics

Gold is the most malleable and ductile of all metals; a single gram can be beaten into a sheet of 1 square meter, or an ounce into 300 square feet. Gold leaf can be beaten thin enough to become transparent. The transmitted light appears greenish blue, because gold strongly reflects yellow and red.[4] Such semi-transparent sheets also strongly reflect infrared lightSony VPCEH1AJ battery, making them useful as infrared (radiant heat) shields in visors of heat-resistant suits, and in sun-visors for spacesuits.[5]

Gold readily creates alloys with many other metals. These alloys can be produced to modify the hardness and other metallurgical properties, to control melting point or to create exotic colors (see below).[6] Gold is a good conductor of heat and electricity and reflects infrared radiation stronglySony VPCEH19FJ/W battery. Chemically, it is unaffected by air, moisture and most corrosive reagents, and is therefore well suited for use in coins and jewelry and as a protective coating on other, more reactive, metals. However, it is not chemically inert.

Common oxidation states of gold include +1 (gold(I) or aurous compounds) and +3 (gold(III) or auric compounds). Gold ions in solution are readily reduced and precipitated out as gold metal by adding any other metal as the reducing agentSony VPCEH19FJ/P battery. The added metal is oxidized and dissolves allowing the gold to be displaced from solution and be recovered as a solid precipitate.

High quality pure metallic gold is tasteless and scentless, in keeping with its resistance to corrosion (it is metal ions which confer taste to metals).[7]

In addition, gold is very dense, a cubic meter weighing 19,300 kg. By comparison, the density of lead is 11,340 kg/m3, and that of the densest element, osmium, is 22,610 kg/m3Sony VPCEH19FJ/B battery.

Color

Different colors of Ag-Au-Cu alloys

Whereas most other pure metals are gray or silvery white, gold is yellow. This color is determined by the density of loosely bound (valence) electrons; those electrons oscillate as a collective "plasma" medium described in terms of a quasiparticle called plasmon. The frequency of these oscillations lies in the ultraviolet range for most metals, but it falls into the visible range for gold due to subtle relativistic effects that affect the orbitals around gold atoms. Sony VPCEH18FJ/W battery Similar effects impart a golden hue to metallic caesium (see relativistic quantum chemistry).

Common colored gold alloys such as rose gold can be created by the addition of various amounts of copper and silver, as indicated in the triangular diagram to the left. Alloys containing palladium or nickel are also important in commercial jewelry as these produce white gold alloys. Less commonly, addition of manganese, aluminium, iron, indium and other elements can produce more unusual colors of gold for various applications. Sony VPCEH18FJ/P battery

[edit]Isotopes

Main article: Isotopes of gold

Gold has only one stable isotope, 197Au, which is also its only naturally occurring isotope. Thirty-six radioisotopes have been synthesized ranging in atomic mass from 169 to 205. The most stable of these is 195Au with a half-life of 186.1 days. The least stable is 171Au, which decays by proton emission with a half-life of 30 µsSony VPCEH18FJ/B battery. Most of gold's radioisotopes with atomic masses below 197 decay by some combination of proton emission, α decay, and β+ decay. The exceptions are 195Au, which decays by electron capture, and 196Au, which decays most often by electron capture (93%) with a minor β- decay path (7%).[10] All of gold's radioisotopes with atomic masses above 197 decay by β- decay. Sony VPCEH18FH/P battery

At least 32 nuclear isomers have also been characterized, ranging in atomic mass from 170 to 200. Within that range, only 178Au, 180Au, 181Au, 182Au, and 188Au do not have isomers. Gold's most stable isomer is 198m2Au with a half-life of 2.27 days. Gold's least stable isomer is 177 m2Au with a half-life of only 7 ns. 184 m1Au has three decay paths: β+ decay, isomeric transition, and alpha decay. No other isomer or isotope of gold has three decay paths. Sony VPCEH18FG/L battery

Use and applications

Monetary exchange

Gold has been widely used throughout the world as a vehicle for monetary exchange, either by issuance and recognition of gold coins or other bare metal quantities, or through gold-convertible paper instruments by establishing gold standards in which the total value of issued money is represented in a store of gold reservesSony VPCEH18FF/B battery.

However, production has not grown in relation to the world's economies. Today, gold mining output is declining.[12] With the sharp growth of economies in the 20th century, and increasing foreign exchange, the world's gold reserves and their trading market have become a small fraction of all markets and fixed exchange rates of currencies to gold were no longer sustainedSony VPCEH18FA/B battery.

At the beginning of World War I the warring nations moved to a fractional gold standard, inflating their currencies to finance the war effort. After World War II gold was replaced by a system of convertible currency following the Bretton Woods system. Gold standards and the direct convertibility of currencies to gold have been abandoned by world governmentsSony VPCEH17FJ/W battery, being replaced by fiat currency in their stead. Switzerland was the last country to tie its currency to gold; it backed 40% of its value until the Swiss joined the International Monetary Fund in 1999.[13]

Pure gold is too soft for day-to-day monetary use and is typically hardened by alloying with copper, silver or other base metals. The gold content of alloys is measured in carats (k). Pure gold is designated as 24k. English gold coins intended for circulation from 1526 into the 1930s were typically a standard 22k alloy called crown gold, for hardness Sony VPCEH17FG/W battery(American gold coins for circulation after 1837 contained the slightly lower amount of 0.900 fine gold, or 21.6 kt).

[edit]Investment

Main article: Gold as an investment

Gold Price per gram between Jan 1971 and Jan 2012. The graph shows nominal price in US Dollars, the price in 1971 and 2011 US Dollars

Many holders of gold store it in form of bullion coins or bars as a hedge against inflation or other economic disruptions. However, some economists do not believe gold serves as a hedge against inflation or currency depreciation. Sony VPCEH17FG/P battery

The ISO 4217 currency code of gold is XAU.

Modern bullion coins for investment or collector purposes do not require good mechanical wear properties; they are typically fine gold at 24k, although the American Gold Eagle, the British gold sovereign, and the South African Krugerrand continue to be minted in 22k metal in historical tradition. The special issue Canadian Gold Maple Leaf coin contains the highest purity gold of any bullion coin, at 99.999% or 0.99999Sony VPCEH17FG/L battery, while the popular issue Canadian Gold Maple Leaf coin has a purity of 99.99%.

Several other 99.99% pure gold coins are available. In 2006, the United States Mint began production of the American Buffalo gold bullion coin with a purity of 99.99%. The Australian Gold Kangaroos were first coined in 1986 as the Australian Gold Nugget but changed the reverse design in 1989. Other popular modern coins include the Austrian Vienna Philharmonic bullion coin and the Chinese Gold PandaSony VPCEH17FG/B battery.

[edit]Jewelry

Main article: Jewellery

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 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, or other base metals or silver or palladium in the alloy. Copper is the most commonly used base metal, yielding a redder color. Sony VPCEH16EN/B battery

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 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 aluminium, although rarely done except in specialized jewelrySony VPCEH16EH/W battery. Blue gold is more brittle and therefore more difficult to work with when making jewelry.[15]

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 silvery in appearance. Nickel is toxic, however, and its release from nickel white gold is controlled by legislation in Europe. Sony VPCEH16EG/W battery

Alternative white gold alloys are available based on palladium, silver and other white metals,[15] 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 effectsSony VPCEH16EF/B battery.

[edit]Medicine

In medieval times, gold was often seen as beneficial for the health, in the belief that something so rare and beautiful could not be anything but healthy. Even some modern esotericists and forms of alternative medicine assign metallic gold a healing power.[16] Some gold salts do have anti-inflammatory properties and are used as pharmaceuticals in the treatment of arthritis and other similar conditionsSony VPCEH16EA/P battery. Gold based injections have been explored as a means to help to reduce the pain and swelling of rheumatoid arthritis and tuberculosis.[17] 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 alloys are used in restorative dentistry, especially in tooth restorations, such as crowns and permanent bridgesSony VPCEH15EN/W battery. 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 othersSony VPCEH15EG/B battery.

Colloidal gold preparations (suspensions of gold nanoparticles) in water are intensely red-colored, and can be made with tightly controlled particle sizes up to a few tens of nanometers across by reduction of gold chloride with citrate or ascorbate ions. Colloidal gold is used in research applications in medicine, biology and materials scienceSony VPCEH13FX/W battery. 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.[18] In ultrathin sections of tissues viewed by electron microscopy, the immunogold labels appear as extremely dense round spots at the position of the antigen. Sony VPCEH13FX/P battery

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, has a triple role in this application. Gold's very high electrical conductivity drains electrical charge to earthSony VPCEH13FX/L battery, and its very high density provides stopping power for electrons in the 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 electrons when irradiated by an electron beamSony VPCEH13FX/B battery, and these low-energy electrons are the most commonly used signal source used in the scanning electron microscope.[20]

The isotope gold-198, (half-life 2.7 days) is used in some cancer treatments and for treating other diseases.[21][22]

[edit]Food and drink

Gold can be used in food and has the E number 175.[23]

Gold leaf, flake or dust is used on and in some gourmet foods, notably sweets and drinks as decorative ingredient.[24] Gold flake was used by the nobility in Medieval Europe as a decoration in food and drinksSony VPCCA3SFX/R battery, in the form of leaf, 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.

Danziger Goldwasser (German: Gold water of Danzig) or Goldwasser (English: Goldwater) is a traditional German herbal liqueur[25] produced in what is today Gdańsk, Poland, and Schwabach, Germany, and contains flakes of gold leaf. There are also some expensive (~$1000) cocktails which contain flakes of gold leaf.[26] However, since metallic gold is inert to all body chemistry, it has no taste, it provides no nutrition, and it leaves the body unaltered. Sony VPCCA3S1E battery

[edit]Industry

The 220 kg gold brick displayed in Jinguashi Gold Museum, Taiwan, Republic of China.

The world's largest gold bar has a mass of 250 kg. Toi museum, Japan.

A gold nugget of 5 mm in diameter (bottom) can be expanded through hammering into a gold foil of about 0.5 square meter. Toi museum, Japan.

Gold solder is used for joining the components of gold jewelry by high-temperature hard soldering or brazingSony VPCCA3E1E battery. If the work is to be of hallmarking 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 pointsSony VPCCA38EC/R battery, goldsmiths can assemble complex items with several separate soldered joints.

Gold can be made into thread and used in embroidery.

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 published formulas for several types of gold toners, which use gold as the chloride. Sony VPCCA38EC battery

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 satellites, in infrared protective faceplates in thermal protection suits and astronauts' helmets and in electronic warfare planes like the EA-6B ProwlerSony VPCCA37EC/B battery.

Gold is used as the reflective layer on some high-end CDs.

Automobiles may use gold for heat shielding. McLaren uses gold foil in the engine compartment of its F1 model.[29]

Gold can be manufactured so thin that it appears transparent. It is used in some aircraft cockpit windows for de-icing or anti-icing by passing electricity through it. The heat produced by the resistance of the gold is enough to deter ice from forming. Sony VPCCA37EC battery

[edit]Electronics

The concentration of free electrons in gold metal is 5.90×1022 cm−3. Gold is highly conductive to electricity, and has been used for electrical wiring in some high-energy applications (only silver and copper are 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 experimentsSony VPCCA36FW/W battery, 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 cablesSony VPCCA36FW/B battery. 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 Sony VPCCA36FH/W battery (certain computers, communications equipment, spacecraft, jet aircraft engines) remains very common.[31]

Besides sliding electrical contacts, gold is also used in electrical contacts because of its resistance to corrosion, electrical conductivity, ductility and lack of toxicity.[32] Switch contacts are generally subjected to more intense corrosion stress than are sliding contacts. Fine gold wires are used to connect semiconductor devices to their packages through a process known as wire bondingSony VPCCA36FG/B battery.

[edit]Commercial chemistry

Gold is attacked by and dissolves in alkaline solutions of potassium or sodium cyanide, to form the salt gold cyanide—a technique that has been used in extracting metallic gold from ores in the cyanide process. Gold cyanide is the electrolyte used in commercial electroplating of gold onto base metals and electroformingSony VPCCA36FA/B battery.

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 colloidal gold suspensions, contains evenly sized spherical gold nanoparticles. Sony VPCCA36EC/W battery

[edit]History

The Turin Papyrus Map

Funerary mask of Tutankhamun

Jason returns with the golden fleece on an Apulian red-figure calyx krater, ca. 340–330 BC.

Gold artifacts found at the Nahal Kana cave cemetery dated during the 1980's, showed these to be from within the Chalcolithic, and considered the earliest find from the Levant (Gopher et al. 1990). [34] Gold artifacts in the Balkans also appear from the 4th millennium BCSony VPCCA36EC battery, such as those found in the Varna Necropolis near Lake Varna in Bulgaria, thought by one source (La Niece 2009) to be the earliest "well-dated" find of gold artifacts. [35] Gold artifacts such as the golden hats and the Nebra disk appeared in Central Europe from the 2nd millennium BC Bronze Age.

Egyptian hieroglyphs from as early as 2600 BC describe gold, which king Tushratta of the Mitanni claimed was "more plentiful than dirt" in Egypt. Sony VPCCA35FW/W battery 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 Map and shows the plan of a gold mine in Nubia together with indications of the local geology. The primitive working methods are described by both Strabo and Diodorus Siculus, and included fire-setting. Large mines were also present across the Red Sea in what is now Saudi ArabiaSony VPCCA35FW/B battery.

The legend of the golden fleece may refer to the use of fleeces to trap gold dust from placer 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 the first chapters of Matthew New TestamentSony VPCCA35FN/R battery. 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 around 610 BC.[37] From the 6th or 5th century BC, the Chu (state) circulated the Ying Yuan, one kind of square gold coinSony VPCCA35FH/D battery.

In Roman metallurgy, new methods for extracting gold on a large scale were developed by introducing hydraulic mining methods, especially in Hispania from 25 BC onwards and in Dacia from 106 AD onwards. One of their largest mines was at Las Medulas in León (Spain), where seven long aqueducts enabled them to sluice most of a large alluvial depositSony VPCCA35FG/PI battery. 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 Britain, 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 ADSony VPCCA35FF/P battery.

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 1324, he was reportedly accompanied by a camel train that included thousands of people and nearly a hundred camelsSony VPCCA35FA/PI battery. He gave away so much gold that it depressed the price in Egypt for over a decade.[38] A contemporary Arab historian remarked:

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 lessSony VPCCA2Z0E battery. 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[39]

The European exploration of the Americas was fueled in no small part by reports of the gold ornaments displayed in great profusion by Native American peoples, especially in Central America, Peru, Ecuador and Colombia. The Aztecs regarded gold as literally the product of the gods, calling it "god excrement" (teocuitlatl in Nahuatl). Sony VPCCA2SFX/R battery However, for the indigenous peoples of North America, gold was considered useless, and they saw much greater value in other minerals, which were directly related to their utility, such as obsidian, flint, and slate.[41]

Although the price of some platinum group metals can be much higher, gold has long been consider

ed the most desirable of precious metals, and its value has been used as the standard for many currencies (known as the gold standard) in historySony VPCCA2S1E battery. 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 ridiculed by Thomas More in his treatise 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 badgesSony VPCCA2S0E battery, 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 to test its authenticity. Although this is certainly not a professional way of examining gold, the bite test was not to check if the coin was gold (90% gold coins are fairly strong) but to see if the coin was gold plated LEAD (Specific Gravity of 14 g/cc with gold 19.3) A Lead coin would be very soft and thus teeth marks would resultSony VPCCA2AJ battery. Fake gold coins were a common problem before 1932 so weighing a coin and also sliding a coin through a "counterfeit detector" slot was common (making a lead coin thicker would add weight thus why slide it through a measured slot). Most establishments (especially USA Western saloons) would never accept a gold (or silver) coin of high value before weighing such an itemSony VPCCA1S3C/CN1 battery.

Gold in antiquity was relatively easy to obtain geologically; however, 75% of all gold ever produced has been extracted since 1910.[42] It has been estimated that all gold ever refined would form a single cube 20 m (66 ft) on a side (equivalent to 8,000 m3).[42]

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 the philosopher's stoneSony VPCCA1S2C/CN1 battery. 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 center (☉), which was also the astrological symbol and the ancient Chinese character for the Sun. For modern creation of artificial gold by neutron capture, see gold synthesisSony VPCCA1S1E/W battery.

During the 19th century, gold rushes occurred whenever large gold deposits were discovered. The first documented discovery of gold in the United States was at the Reed Gold Mine near Georgeville, North Carolina in 1803.[43] The first major gold strike in the United States occurred in a small north Georgia town called Dahlonega.[44] Further gold rushes occurred in CaliforniaSony VPCCA1S1E/P battery, Colorado, the Black Hills, Otago in New Zealand, Australia, Witwatersrand in South Africa, and the Klondike in Canada.

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

[edit]Occurrence

This 156-troy-ounce (4.9 kg) nugget, known as the Mojave Nugget, was found by an individual prospector in the Southern California Desert using a metal detectorSony VPCCA1S1E/G battery.

Gold's atomic number of 79 makes it one of the higher atomic number elements which occur naturally. Like all elements with atomic numbers larger than iron, gold is thought to have been formed from a supernova nucleosynthesis process. Their explosions scattered metal-containing dusts (including heavy elements like gold) into the region of space in which they later condensed into our solar system and the Earth. Sony VPCCA1S1E/D battery Because the Earth was molten when it was just formed, almost all of the gold present on Earth sank into the core. Most of the gold that is present today in the Earth's crust and mantle was delivered to Earth by asteroid impacts during the late heavy bombardment.[46]

On Earth, whenever elemental gold occurs, it appears most often as a metal solid solution of gold with silver, i.e. a gold silver alloySony VPCCA1S1E/B battery. Such alloys usually have a silver content of 8–10%. Electrum is elemental gold with more than 20% silver. Electrum's color runs from golden-silvery to silvery, dependent upon the silver content. The more silver, the lower the specific gravity.

Relative sizes of an 860 kg block of gold ore, and the 30 g of gold that can be extracted from it. Toi gold mine, JapanSony VPCCA1S1E battery.

Gold left behind after a pyrite cube was oxidized to hematite. Note cubic shape of cavity.

Gold is found in ores in rock formed from the Precambrian time onward. [35] Sometimes occurring as very small to microscopic particles, this ore is often found together with quartz or sulfide minerals such as Fool's Gold, which is a pyriteSony VPCCA1S1C/CN1 battery.[47] These are called lode deposits. The metal in a native state is also found in the form of free flakes, grains or larger nuggets [35] that have been eroded from rocks and end up in alluvial deposits (called placer deposits). 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 nuggetsSony VPCCA1C5E battery.

Gold sometimes occurs combined with tellurium as the minerals calaverite, krennerite, nagyagite, petzite and sylvanite, and as the rare bismuthide maldonite (Au2Bi) and antimonide aurostibite (AuSb2). Gold also occurs in rare alloys with copper, lead, and mercury: the minerals auricupride (Cu3Au), novodneprite (AuPb3) and weishanite ((Au, Ag)3Hg2) Sony VPCCA1AHJ battery.

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 collect in alluvial deposits.[48]

The world's oceans contain gold. Measured concentrations of gold in the Atlantic and Northeast Pacific are 50–150 fmol/L or 10–30 parts per quadrillion (about 10–30 g/km3) Sony VPCCA1AGJ battery. In general, Au concentrations for Atlantic and Pacific samples are the same (~50 fmol/L) but less certain. Mediterranean deep waters contain higher concentrations of Au (100–150 fmol/L) attributed to wind-blown dust and/or rivers. At 10 parts per quadrillion the Earth's oceans would hold 15,000 tons of gold.[49] These figures are three orders of magnitude less than reported in the literature prior to 1988, indicating contamination problems with the earlier dataSony VPCCA1AFJ battery.

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 acted in an intentional deception. A so-called reverend, Prescott Jernegan ran a gold-from-seawater swindle in the United States in the 1890s. A British fraudster ran the same scam in England in the early 1900sSony VPCCA190 battery.[50] Fritz Haber (the German inventor of the Haber process) did research on the extraction of gold from sea water in an effort to help pay Germany's reparations following World War I.[51] Based on the published values of 2 to 64 ppb of gold in seawater a commercially successful extraction seemed possible. After analysis of 4,000 water samples yielding an average of 0.004 ppb it became clear that the extraction would not be possible and he stopped the projectSony VPCCA18EC/D battery.[52] 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.

[edit]Gallery of specimens of crystalline native gold

Native gold nuggets

"Rope gold" from Lena River, Sakha Republic, Russia. Size: 2.5×1.2×0.7 cm.

Crystalline gold from Mina Zapata, Santa Elena de Uairen, Venezuela. Size: 3.7×1.1×0.4 cm.

Gold leaf from Harvard Mine, Jamestown, California, USA. Size 9.3×3.2× >0.1 cm.

[edit]Production

Main articles: Gold prospecting, Gold mining, Gold extraction, and List of countries by gold productionSony VPCCA18EC battery

Global gold output in 2005.

The entrance to an underground gold mine in Victoria, Australia

Pure gold precipitate produced by the aqua regia refining process

Gold extraction is most economical in large, easily mined deposits. Ore grades as little as 0.5 mg/kg (0.5 parts per million, ppm) can be economical. Typical ore grades in open-pit mines are 1–5 mg/kg (1–5 ppm); ore grades in underground or hard rock mines are usually at least 3 mg/kg (3 ppm). Because ore grades of 30 mg/kg (30 ppm) are usually needed before gold is visible to the naked eye, in most gold mines the gold is invisibleSony VPCCA17FX/G battery.

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 1,480 tonnes. 2008 production was 2,260 tonnes. 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. Sony VPCCA17FX/D battery

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 mSony VPCCA17EC/W battery. 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 the United States, Australia, 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 ArgentinaSony VPCCA17EC battery. Today about one-quarter of the world gold output is estimated to originate from artisanal or small scale mining.[54]

After initial production, gold is often subsequently refined industrially by the Wohlwill process which is based on electrolysis or by the Miller process, that is chlorination in the melt. The Wohlwill process results in higher purity, but is more complex and is only applied in small-scale installations. Sony VPCCA16FW/W batteryOther methods of assaying and purifying smaller amounts of gold include parting and inquartation as well as cupellation, or refining methods based on the dissolution of gold in aqua regia.[57]

At the end of 2009, it was estimated that all the gold ever mined totaled 165,000 tonnes.[2] This can be represented by a cube with an edge length of about 20.28 meters. At $1,600 per ounce, 165,000 tons of gold would have a value of $8.8 trillionSony VPCCA16FH/B battery.

The average gold mining and extraction costs were about US$317/oz in 2007, but these can vary widely depending on mining type and ore quality; global mine production amounted to 2,471.1 tonnes.[58]

Most of the gold used in manufactured goods, jewelry, and works of art is eventually recovered and recycled. Some gold used in spacecraft and electronic equipment cannot be profitably recoveredSony VPCCA16FG/W battery, but it is generally used in these applications in the form of extremely thin layers or extremely fine wires so that the total quantity used (and lost) is small compared to the total amount of gold produced and stockpiled. Thus there is little true consumption of new gold in the economic sense; the stock of gold remains essentially constant (at least in the modern world) while ownership shifts from one party to another. Sony VPCCA16FG/B battery One estimate is that 85% of all the gold ever mined is still available in the world's easily recoverable stocks, with 15% having been lost, or used in non-recyclable industrial uses.[60]

[edit]Consumption

The consumption of gold produced in the world is about 50% in jewelry, 40% in investments, and 10% in industry.

India is the world's largest single consumer of gold, as Indians buy about 25% of the world's gold,[61] purchasing approximately 800 tonnes of gold every year, mostly for jewelry. India is also the largest importer of goldSony VPCCA16EC/P battery; in 2008, India imported around 400 tonnes of gold.[62] Indian households hold 18,000 tonnes of gold which represents 11% of the global stock and worth more than $950 billion.[63]

Pollution

Gold production is associated with contribution to hazardous pollution.[65] The ore generally containing less than one ppm gold metal, is ground and mixed with sodium cyanide or mercury to react with gold in the ore for gold separationSony VPCCA16EC battery. Cyanide is highly poisonous chemical, which can kill living creatures when exposed in minute quantities. Many cyanide spills[66] from gold mines have taken place worldwide both in developed and poor countries which killed the marine life in long stretch of affected rivers. Environmentalists consider these pollution disasters as major environmental disastersSony VPCCA15FX/W battery. When mercury is used in gold production, minute quantity of mercury compounds enter in to water bodies causing heavy metal contamination of water. Mercury enters in to human food chain in the form of methyl mercury through fish, etc. Mercury poisoning in humans causes incurable severe retardation of brain functionsSony VPCCA15FW/W battery.

Thirty tonnes of used ore is dumped as waste for producing one finger ring of gold.[69] Gold ore dumps are the source of many heavy elements such as cadmium, lead, zinc, copper, arsenic, selenium and mercury. Water is unsuitable for human consumption if these Heavy metal (chemistry) are found in more than one ppm concentration. When sulfide bearing minerals in these ore dumps are exposed to air and waterSony VPCCA15FW/P battery, the sulfide transforms in to sulfuric acid which in turn dissolves the heavy metals facilitating their passage in to surface water and ground water. This process is called acid mine drainage. The gold ore dumps are considered as long term man made hazardous waste next only to nuclear waste dumps.[69] Billions of dollars need to be spent to mitigate the heavy metals pollution from worldwide gold ore dumps which are increasing every yearSony VPCCA15FW/D battery.

Gold extraction is also highly energy intensive industry (25 kWh of electricity per one gram of gold production) to extract ore from deep mines and to grind the large quantity of ore for further chemical extraction.[70]

Chemistry

Gold (III) chloride solution in water

Although gold is a noble metal, it forms many diverse compounds. The oxidation state of gold in its compounds ranges from −1 to +5, but Au(I) and Au(III) dominate its chemistry. Au(I), referred to as the aurous ionSony VPCCA15FW/B battery, is the most common oxidation state with soft ligands such as thioethers, thiolates, and tertiary phosphines. Au(I) compounds are typically linear. A good example is Au(CN)2−, which is the soluble form of gold encountered in mining. Curiously, aurous complexes of water are rare. The binary gold halides, such as AuCl, form zigzag polymeric chains, again featuring linear coordination at Au. Most drugs based on gold are Au(I) derivatives. Sony VPCCA15FH/W battery

Au(III) (auric) is a common oxidation state, and is illustrated by gold(III) chloride, Au2Cl6. The gold atom centers in Au(III) complexes, like other d8 compounds, are typically square planar, with chemical bonds that have both covalent and ionic character.

Aqua regia, a 1:3 mixture of nitric acid and hydrochloric acid, dissolves gold. Nitric acid oxidizes the metal to +3 ions, but only in minute amounts, typically undetectable in the pure acid because of the chemical equilibrium of the reactionSony VPCCA15FH/P battery. However, the ions are removed from the equilibrium by hydrochloric acid, forming AuCl4− ions, or chloroauric acid, thereby enabling further oxidation.

Some free halogens react with gold.[72] Gold also reacts in alkaline solutions of potassium cyanide. With mercury, it forms an amalgam.

[edit]Less common oxidation states

Less common oxidation states of gold include −1, +2, and +5.

The −1 oxidation state occurs in compounds containing the Au− anion, called aurides. Caesium auride (CsAu), for example, crystallizes in the caesium chloride motifSony VPCCA15FH/L battery.[73] Other aurides include those of Rb+, K+, and tetramethylammonium (CH3)4N+.[74] Gold has the highest Pauling electronegativity of any metal, with a value of 2.54, making the auride anion relatively stable.

Gold(II) compounds are usually diamagnetic with Au–Au bonds such as [Au(CH2)2P(C6H5)2]2Cl2. The evaporation of a solution of Au(OH)3 in concentrated H2SO4 produces red crystals of gold(II) sulfate, Au2(SO4)2. Originally thought to be a mixed-valence compound, it has been shown to contain Au4+2 cationsSony VPCCA15FH/G battery. A noteworthy, legitimate gold(II) complex is the tetraxenonogold(II) cation, which contains xenon as a ligand, found in [AuXe4](Sb2F11)2.

Gold pentafluoride, along with its derivative anion, AuF−

6, and its difluorine complex, gold heptafluoride, is the sole example of gold(V), the highest verified oxidation state.[78]

Some gold compounds exhibit 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 than van der Waals bonding. The interaction is estimated to be comparable in strength to that of a hydrogen bondSony VPCCA15FH/B battery.

[edit]Mixed valence compounds

Well-defined cluster compounds are numerous.[74] In such cases, gold has a fractional oxidation state. A representative example is the octahedral species {Au(P(C6H5)3)}62+. Gold chalcogenides, such as gold sulfide, feature equal amounts of Au(I) and Au(III).

Toxicity

Pure metallic (elemental) gold is non-toxic and non-irritating when ingested[79] and is sometimes used as a food decoration in the form of gold leafSony VPCCA15FG/W battery. Metallic gold is also a component of the alcoholic drinks Goldschläger, Gold Strike, and Goldwasser. Metallic gold is approved as a food additive in the EU (E175 in the Codex Alimentarius). Although gold ion is toxic, the acceptance of metallic gold as a food additive is due to its relative chemical inertness, and resistance to being corroded or transformed into soluble salts (gold compounds) by any known chemical process which would be encountered in the human bodySony VPCCA15FG/R battery.

Soluble compounds (gold salts) such as gold chloride are toxic to the liver and kidneys. Common cyanide salts of gold such as potassium gold cyanide, used in gold electroplating, are toxic by virtue of both their cyanide and gold content. There are rare cases of lethal gold poisoning from potassium gold cyanide. Gold toxicity can be ameliorated with chelation therapy with an agent such as DimercaprolSony VPCCA15FG/P battery.

Gold metal was voted Allergen of the Year in 2001 by the American Contact Dermatitis Society. Gold contact allergies affect mostly women.[82] Despite this, gold is a relatively non-potent contact allergen, in comparison with metals like nickel.[83]

Price

Gold price history in 1960-2011

Like other precious metals, gold is measured by troy weight and by grams. When it is alloyed with other metals the term carat or karat is used to indicate the purity of gold present, with 24 carats being pure gold and lower ratings proportionally lessSony VPCCA15FG/G battery. The purity of a gold bar or coin 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 through trading in the gold and derivatives markets, but a procedure known as the Gold Fixing in London, originating in September 1919, provides a daily benchmark price to the industry. The afternoon fixing was introduced in 1968 to provide a price when US markets are openSony VPCCA15FG/D battery.

Historically gold coinage was widely used as currency; when paper money was introduced, it typically was a receipt redeemable for gold coin or bullion. In a monetary 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 ounce was equal to $20.67 ($664.56/kg) Sony VPCCA15FG/B battery, but in 1934 the dollar was devalued 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.

A Swiss-cast 1 kg gold bar.

On March 17, 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) Sony VPCCA15FG battery 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 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 YorkSony VPCCA15FF/W battery, which holds about 3%[84] 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.[85]

Since 1968 the price of gold 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 June 21, 1999 (London Gold Fixing). Sony VPCCA15FF/P battery The period from 1999 to 2001 marked the "Brown Bottom" after a 20-year bear market.[87] Prices increased rapidly from 1991, but the 1980 high was not exceeded until January 3, 2008 when a new maximum of $865.35 per troy ounce was set.[88] Another record price was set on March 17, 2008 at $1023.50/oz ($32,900/kg). Sony VPCCA15FF/L battery

In late 2009, gold markets experienced renewed momentum upwards due to increased demand and a weakening US dollar. On December 2, 2009, Gold passed the important barrier of US$1200 per ounce to close at $1215.[89] Gold further rallied hitting new highs in May 2010 after the European Union debt crisis prompted further purchase of gold as a safe asset. Sony VPCCA15FF/G battery On March 1, 2011, gold hit a new all-time high of $1432.57, based on investor concerns regarding ongoing unrest in North Africa as well as in the Middle East.[92]

Since April 2001 the gold price has more than quintupled in value against the US dollar, hitting a new all-time high of $1,913.50 on August 23, 2011,[93] prompting speculation that this long secular bear market has ended and a bull market has returned.Sony VPCCA15FF/B battery

Symbolism

Gold bars at the Emperor Casino in Macau

Gold has been highly valued in many societies throughout the ages. In keeping with this it has often had a strongly positive symbolic meaning closely connected to the values held in the highest esteem in the society in question. Gold may symbolize power, strength, wealth, warmth, happiness, love, hope, optimism, intelligence, justice, balance, perfection, summer, harvest and the sunSony VPCCA15FA/W battery.

Great human achievements are frequently rewarded with gold, in the form of gold medals, golden trophies and other decorations. Winners of athletic events and other graded competitions are usually awarded a gold medal (e.g., the Olympic Games). Many awards such as the Nobel Prize are made from gold as well. Other award statues and prizes are depicted in gold or are gold plated Sony VPCCA15FA/P battery (such as the Academy Awards, the Golden Globe Awards, the Emmy Awards, the Palme d'Or, and the British Academy Film Awards).

Aristotle in his ethics used gold symbolism when referring to what is now commonly known as the golden mean. Similarly, gold is associated with perfect or divine principles, such as in the case of the golden ratio and the golden rule.

Gold is further associated with the wisdom of aging and fruition. The fiftieth wedding anniversary is golden. Our precious latter years are sometimes considered "golden years"Sony VPCCA15FA/L battery. The height of a civilization is referred to as a "golden age".

In Christianity gold has sometimes been associated with the extremities of utmost evil and the greatest sanctity. In the Book of Exodus, the Golden Calf is a symbol of idolatry. In the 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. In Christian art the halos of Christ, Mary and the Christian saints are goldenSony VPCCA15FA/G battery.

Medieval kings were inaugurated under the signs of sacred oil and a golden crown, the latter symbolizing the eternal shining light of heaven and thus a Christian king's divinely inspired authority.

Those who had something of gold, were in possession of something of great value on earth and a substance to even help souls to paradiseSony VPCCA15FA/B battery

—according to Christopher Columbus[95]

Wedding rings have long been made of gold. It is long lasting and unaffected by the passage of time and may aid in the ring symbolism of eternal vows before God and/or the sun and moon and the perfection the marriage signifies. In Orthodox Christianity, the wedded couple is adorned with a golden crown during the ceremony, an amalgamation of symbolic ritesSony VAIO VPCF135Z1E/B battery.

In popular culture gold holds many connotations but is most generally connected to terms such as good or great, such as in the phrases: "has a heart of gold", "that's golden!", "golden moment", "then you're golden!" and "golden boy". Gold also still holds its place as a symbol of wealth and through that, in many societies, successSony VAIO VPCF137HG/BI battery.

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