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Gold

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Gold has been the most prized metal that is a chemical element with the symbol Au. It is opaque, has a highly attractive metallic golden yellow color, is extremely malleable, and is usually found in a relatively pure form. It is remarkably inert, so it resists tarnish. These qualities have made it exceptionally valuable. It usually occurs as treelike growths, grains, and scaly masses. It rarely occurs as well-formed crystals, but when found these are octahedral or dodecahedral. It is mostly found in hydrothermal veins with quartz and sulfides. Virtually all granitic igneous rocks in which it occurs as invisible, disseminated grains contain low concentrations of gold. Almost all of the gold recovered since antiquity has come from placer deposits weathered gold particles concentrated in river and stream gravel.

Chemically, it is a transition metal and a group 11 element. It is one of the least reactive chemical elements and is solid under standard conditions. It often occurs in free elemental (native) form, as nuggets or grains, in rocks, in veins, and in alluvial deposits.

A total of 186,700 tonnes of gold exists above ground, as of 2015. The world consumption of new gold produced is about

  • 50% in jewelry,
  • 40% in investments,
  • and 10% in industry.

Name: An Old English word for the metal; perhaps related to the Sanskrit jval; chemical symbol from the Latin aurum, shining dawn.

Association: Pyrite, chalcopyrite, arsenopyrite, pyrrhotite, sylvanite, krennerite, calaverite, altaite, tetradymite, scheelite, ankerite, tourmaline, quartz.

Polymorphism & Series: Forms a series with silver.

Cell Data: Space Group: Fm3m. a = 4.0786 Z = 4

Chemical Properties

Chemical Classification Native Element
Chemical Composition Au
Common Impurities: Ag,Cu,Pd,Hg,Bi

Gold Characteristics

It is the most malleable of all metals.  A single gram of gold can be beaten into a sheet of 1 square meter, and an avoirdupois ounce into 300 square feet. It leaf can be beaten thin enough to become semi-transparent. It is a good conductor of heat and electricity.

It has a density of 19.3 g/cm3, almost identical to that of tungsten at 19.25 g/cm3; as such, tungsten has been used in counterfeiting of gold bars, such as by plating a tungsten bar with gold, or taking an existing gold bar, drilling holes, and replacing the removed gold with tungsten rods.

Gold Physical Properties

Color Rich yellow, paling to whitish-yellow with increasing silver; blue & green in transmitted light (only thinnest folia [gold leaf])
Streak Shining yellow
Luster Metallic
Cleavage None Observed None
Diaphaneity Opaque
Mohs Hardness 2.5 to 3
Crystal System Isometric
Tenacity Malleable
Density 15 – 19.3

Gold Optical Properties

Type Isotropic
Color / Pleochroism Non-pleochroic
Internal Reflections: None
Colour in reflected light: Yellow to white with increasing silver, reddish with copper
Twinning Common on (111) to give herringbone twins. Repeated on (111) to give stacks of spinel twins that form hexagonal wires.

Origins

Gold is thought to be produced in the supernova nucleosynthesis and collision of neutron stars and present in the dust generated by the Solar System. Since the Earth was melted at the time of its formation, almost all of the gold found in the early days of the Earth probably sunk into the core of the planet. Therefore, most of the this alien mineral in the Earth’s crust and mantle is believed to have been delivered to Earth with asteroid effects during the Late Heavy Bombardment 4 billion years ago.

Occurrence

Widespread in very small quantities in rocks of many kinds throughout the world, and in sea water. In veins of epithermal origin, typically in quartz with pyrite and other sulfides, and with tellurides; in pegmatites; in contact metamorphic deposits. Common in placers.

Production

The World Gold Council states that as of the end of 2017, “there were 187,200 tonnes of stocks in existence above ground”. At $1,349 per troy ounce, 187,200 metric tonnes of gold would have a value of $8.9 trillion. According to the United States Geological Survey in 2016, about 5,726,000,000 troy ounces (178,100 t) of gold has been produced since the beginning of civilization, of which 85% remains in use.

In 2017, the world’s largest gold producer by far was China with 440 tonnes. The second-largest producer, Australia, mined 300 tonnes in the same year, followed by Russia with 255 tonnes.

Gold Uses Area

  • It has been widely used throughout the world as money, for efficient indirect exchange (versus barter), and to store wealth in hoards. For exchange purposes, mints produce standardized gold bullion coins, bars and other units of fixed weight and purity.
  • After World War II it was replaced by a system of nominally convertible currencies related by fixed exchange rates following the Bretton Woods system. Gold standards and the direct convertibility of currencies to gold have been abandoned by world governments, led in 1971 by the United States’ refusal to redeem its dollars in gold. Fiat currency now fills most monetary roles. 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.
  • Central banks continue to keep a portion of their liquid reserves as gold in some form, and metals exchanges such as the London Bullion Market Association still clear transactions denominated in gold, including future delivery contracts.
  • It has been used as a symbol for purity, value, royalty, and particularly roles that combine these properties.
  • The ISO 4217 currency code of gold is XAU. Many holders of gold store it in form of bullion coins or bars as a hedge against inflation or other economic disruptions, though its efficacy as such has been questioned; historically, it has not proven itself reliable as a hedging instrument.

Jewelry

Because of the softness of pure (24k), it is usually alloyed with base metals for use in jewelry, altering its hardness and ductility, melting point, color and other properties.

Electronics

  • Only 10% of the world consumption of new gold produced goes to industry,[8] but by far the most important industrial use for new gold is in fabrication of corrosion-free electrical connectors in computers and other electrical devices.
  • Though it 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 on electrical connectors, thereby ensuring good connection. For example, it is used in the connectors of the more expensive electronics cables, such as audio, video and USB cables.
  • Besides sliding electrical contacts, it is also used in electrical contacts because of its resistance to corrosion, electrical conductivity, ductility and lack of toxicity.

Medicine

  • Metallic and compounds have long been used for medicinal purposes. It, usually as the metal, is perhaps the most anciently administered medicine (apparently by shamanic practitioners) and known to Dioscorides.
  • In the 19th century gold had a reputation as a “nervine”, a therapy for nervous disorders. Depression, epilepsy, migraine, and glandular problems such as amenorrhea and impotence were treated, and most notably alcoholism (Keeley, 1897, Wikipedia).
  • It alloys are used in restorative dentistry, especially in tooth restorations, such as crowns and permanent bridges. The gold alloys’ slight malleability facilitates the creation of a superior molar mating surface with other teeth and produces results that are generally more satisfactory than those produced by the creation of porcelain crowns. The use of gold crowns in more prominent teeth such as incisors is favored in some cultures and discouraged in others.
  • Gold, 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.

Distribution

Many localities for fine specimens.

  • In Russia, in Siberia, along the eastern slope of the Ural Mountains; important localities near Yekaterinburg (Sverdlovsk), as at Beresovsk; in the Miass district; large crystal groups from along the Lena River, Sakha.
  • Sharply crystallized from Romania, at Ro¸sia Montan˘a (Verespatak) and Sacarımb (Nagyag).
  • In Australia, many occurrences, as at Bendigo, Ballarat, and Matlock, Victoria; along the Palmer River and at Gympie, Queensland; from Kalgoorlie, Western Australia, with gold telluride ores, also very large alluvial nuggets.
  • At the Porgera mine, Mt. Kare, Papua New Guinea.
  • The world’s most important gold district is the Witwatersrand, Transvaal, South Africa, which, however, only rarely produces crystalline material.
  • In Canada, especially in Ontario, in the Porcupine and Hemlo districts.
  • In the USA, in California, in the Mother Lode belt of the Sierra Nevada, with fine examples from both lode and placer deposits.
  • In South Dakota, from the Homestake mine at Lead, Lawrence Co.; in Colorado, wire and leaf gold from Breckenridge, Summit Co.; in Lake Co., at Leadville; in Alaska, in lode mines in the Juneau district and placers along the Yukon River.
  • Near Santa Elena, in the Grand Savannah River region, Venezuela, a placer producing exceptional skeletal crystals.
  • A bonanza gold rush occurred at Serra Pelada, Para, Brazil.

References

  • Bonewitz, R. (2012). Rocks and minerals. 2nd ed. London: DK Publishing.
  • Handbookofmineralogy.org. (2019). Handbook of Mineralogy. [online] Available at: http://www.handbookofmineralogy.org [Accessed 4 Mar. 2019].
  • Mindat.org. (2019). Orpiment: Mineral information, data and localities.. [online] Available at: https://www.mindat.org/ [Accessed. 2019].
  • Smith.edu. (2019). Geosciences | Smith College. [online] Available at: https://www.smith.edu/academics/geosciences [Accessed 15 Mar. 2019].
  • Wikipedia contributors. (2019, May 19). Gold. In Wikipedia, The Free Encyclopedia. Retrieved 00:26, June 10, 2019, from https://en.wikipedia.org/w/index.php?title=Gold&oldid=897845459
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