Encyclopedia
Gold is a highly sought-after
precious metal that for many centuries has been used as
money, a store of value and in
jewelry. The metal occurs as
nuggets or grains in rocks and in
alluvial deposits and is one of the coinage metals. It is a soft, shiny, yellow, dense, malleable, and ductile
transition metal. Modern industrial uses include
dentistry and
electronics. Gold forms the basis for a
monetary standard used by the
International Monetary Fund and the
Bank for International Settlements . Its ISO currency code is
XAU.
Gold is a
chemical element in the periodic table with the symbol
Au and atomic number 79. The adjective auric refers to something made of gold. Gold does not react with most chemicals but is attacked by chlorine,
fluorine,
aqua regia and
cyanide. Gold dissolves in mercury. In particular, gold is insoluble in
nitric acid, which will dissolve most other metals. Nitric acid has long been used to confirm the presence of gold in items.
Notable characteristics
Gold is a metallic element with a characteristic
yellow color, but can also be black or
ruby when finely divided, while colloidal solutions are intensely colored and often purple. These colors are the result of gold's plasmon frequency lying in the visible range, which causes red and yellow light to be reflected, and blue light to be absorbed. Only silver colloids exhibit the same interactions with light, albeit at a shorter frequency, making silver colloids yellow in color.
It is the most malleable and ductile
metal known; a single
gram can be beaten into a sheet of one square meter, or an ounce into 300 square feet. Gold readily forms alloys with many other metals. These alloys can be produced to increase the hardness or to create exotic colors. Adding
copper yields a redder metal,
iron blue,
aluminium purple,
platinum metals white, and natural bismuth together with silver alloys produce black. Native gold contains usually eight to ten per cent
silver, but often much more — alloys with a silver content over 20% are called
electrum. As the amount of silver increases, the color becomes whiter and the specific gravity becomes lower.
Gold is a good conductor of
heat and
electricity, and is not affected by
air and most reagents. Heat, moisture,
oxygen, and most
corrosive agents have very little chemical effect on gold, making it well-suited for use in
coins and
jewelry; conversely,
halogens will chemically alter gold, and
aqua regia dissolves it.
Common
oxidation states of gold include +1 and +3 . Gold ions in solution are readily reduced and precipitated out as gold metal by the addition of virtually any other metal as the reducing agent. The added metal is
oxidized and dissolves allowing the gold to be displaced from solution and be recovered as a solid precipitate.
Recent research undertaken by Frank Reith of the Australian National University shows that microbes play an important role in the formation of gold deposits, transporting and precipitating gold to form grains and nuggets that collect in alluvial deposits.
The concentration of free electrons in gold metal is 5.90×10
22 cm
-3.
Applications
Pure gold is too soft for ordinary use and is hardened by
alloying with silver, copper, and other metals. Gold and its many alloys are most often used in
jewelry,
coinage and as a standard for
monetary exchange in various countries. When selling it in the form of jewelry, gold is measured in karats , with pure gold being 24k. However, it is more commonly sold in lower measurements of 22k, 18k, and 14k. A lower "k" indicates a higher percent of copper or silver mixed into the alloy, with copper being the more commonly used metal between the two. Fourteen karat gold-copper alloy will be nearly identical in color to certain
bronze alloys, and both may be used to produce police and other badges. Eighteen karat gold with a high copper content is found in some antique jewelry and will have a distinct, though not dominant copper cast, giving an attractively warm color. A similar karat weight when alloyed with silvery metals will appear less warm in color, and some low karat white metal alloys may be sold as "white gold", silvery in appearance with a sightly yellow cast but far more resistant to corrosion than
silver or
sterling silver. Karat weights of twenty and higher are more common in modern jewelry. Because of its high electrical conductivity and resistance to corrosion and other desirable combinations of physical and chemical properties, gold also emerged in the late
20th century as an essential industrial metal, particularly as a thin plating on electrical card contacts and connectors.
- Gold can be made into thread and used in embroidery.
- Gold performs critical functions in computers, communications equipment, spacecraft, jet aircraft engines, and a host of other products.
- The resistance to oxidation of gold has led to its widespread use as thin layers electroplated on the surface of electrical connectors to ensure a good connection.
- Gold is used in restorative dentistry especially in tooth restorations such as crowns and permanent bridges as its slight malleablity makes a superior molar mating surface to other teeth, unlike a harder ceramic crown. Use of gold crowns in more prominent teeth such as incisors is favored in some cultures and discouraged in others.
- Colloidal gold is an intensely colored solution that is currently studied in many labs for medical, biological and other applications. It is also the form used as gold paint on ceramics prior to firing.
- Chlorauric acid is used in photography for toning the silver image.
- Gold chloride is used as a catalyst in organic chemistry. It is also the usual starting point for making other gold compounds.
- Gold is used as a coating enabling biological material to be viewed under a scanning electron microscope.
- Many competitions and honors, such as the Olympics and the Nobel Prize, award a gold medal
...
to the winner
- Since it is a good reflector of both infrared and visible light, it is used for the protective coatings on many artificial satellites and on astronauts' helmets to prevent blindness from the sun.
- Disodium aurothiomalate is a treatment for rheumatoid arthritis . It inhibits lymphocyte proliferation, lysosomal enzyme release, the release of reactive oxygen species from macrophages, and IL-1 production. However, it can also cause photosensitive rashes, gastrointestinal disturbance, and kidney damage.
- The isotope gold-198, is used in some cancer treatments and for treating other diseases.
- Gold flake is used on and in some gourmet sweets and drinks. Called varak or in India. Having no reactivity it adds no taste but is taken as a delicacy. Some use it as an excuse to create super-expensive delicacies . For similar reasons, it is also used as the basis for some superstitious, over the top, health claims. Only the salts and radioisotopes have any evidence of medicinal value.
- White gold serves as a substitute for platinum.
- Green gold is used in specialized jewelry while gold alloys with copper are more widely used for that purpose .
History
Gold has been known and highly valued since prehistoric times. It may have been the first metal used by
humans and was valued for ornamentation and rituals.
Egyptian hieroglyphs from as early as 2600 BCE describe gold, which king Tushratta of the Mitanni claimed was as "common as dust" in Egypt. Egypt and
Nubia had the resources to make them major gold-producing areas for much of history. Gold is also mentioned several times in the Old Testament. The south-east corner of the
Black Sea was famed for its gold. Exploitation is said to date from the time of Midas, and this gold was important in the establishment of what is probably the world's earliest coinage in
Lydia between 643 and 630 BCE.
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, and
Colombia.
Gold has long been considered one of the most
precious metals, and its value has been used as the standard for many currencies in history. Gold has been used as a symbol for purity, value, royalty, and particularly roles that combine these properties . Gold as a sign of wealth and prestige was made fun of by
Thomas More in his treatise
Utopia. On that imaginary island, gold is so abundant that it is used to make chains for slaves, tableware and lavatory-seats. When ambassadors from other countries arrive, dressed in ostentatious gold jewels and badges, the Utopians mistake them for menial servants, paying homage instead to the most modestly-dressed of their party.
There is an age-old tradition of biting gold in order to test its authenticity. Although this is certainly not a professional way of examining gold, the
bite test should score the gold because gold is considered a soft metal according to the Mohs' scale of mineral hardness. The purer the gold the easier it should be to mark it. Painted lead can cheat this test because lead is softer than gold.
Gold in antiquity was relatively easy to obtain geologically; however, 75% of all gold ever produced has been extracted since 1910. It has been estimated that all the gold in the world that has ever been refined would form a single cube 20 m a side .
The primary 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 stone. Although they never succeeded in this attempt, the alchemists promoted an interest in what can be done with substances, and this laid a foundation for today's
chemistry. Their symbol for gold was the
circle with a point at its center , which was also the
astrological symbol, the
Egyptian
hieroglyph and the ancient
Chinese character for the
Sun . For modern attempts to produce artificial gold, see gold synthesis.
During the
19th century,
gold rushes occurred whenever large gold deposits were discovered, including the
California,
Colorado,
Otago,
Australian, Witwatersrand,
Black Hills, and
Klondike gold rushes.
Because of its historically high value, much of the gold mined throughout history is still in circulation in one form or another.
Occurrence
Due to its relative chemical inertness, gold is usually found as the native metal or alloy. Occasionally large accumulations of native gold occur but usually gold occurs as minute grains. These grains occur between mineral grain boundaries or as inclusions within minerals. Common gold associations are
quartz often as
veins and sulfide minerals. The most common sulfide associations are
pyrite,
chalcopyrite,
galena,
sphalerite,
arsenopyrite,
stibnite and
pyrrhotite. Rarer mineral associations are petzite, calaverite, sylvanite, muthmannite, nagyagite and krennerite.
Gold is widely distributed in the Earth's crust at a background level of 0.03
g/1000
kg . Hydrothermal
ore deposits of gold occur in
metamorphic rocks and
igneous rocks;
alluvial deposits and
placer deposits originate from these sources.
The primary source of gold is usually igneous rocks or surface concentrations. A deposit usually needs some form of secondary enrichment to form an economically viable ore deposit: either chemical or physical processes like
erosion or solution or more generally metamorphism, which concentrates the gold in sulfide minerals or quartz. There are several primary deposit types, common ones are termed reef or vein. Primary deposits can be weathered and eroded, with most of the gold being transported into stream beds where it congregates with other heavy minerals to form placer deposits. In all these deposits the gold is in its native form. Another important ore type is in
sedimentary black
shale and
limestone deposits containing finely disseminated gold and other platinum group metals.
Gold occurs in sea water at 0.1 to 2
mg/t depending on sample location.
Production
Economic gold extraction can be achieved from ore grades as little as 0.5 g/1000 kg on average in large easily mined deposits, typical ore grades in
open-pit mines are 1–5 g/1000 kg , ore grades in underground or
hard rock mines are usually at least 3 g/1000 kg on average. Ore grades of 30 g/1000 kg are usually needed before gold is visible to the naked eye, therefore in most gold mines you will not see any gold.
Since the 1880s,
South Africa has been the source for a large proportion of the world’s gold supply. Production in 1970 accounted for 79% of the world supply, producing about 1,000 tonnes. However, production in 2004 was 342 tonnes. This decline was due to the increasing difficulty of extraction and changing economic factors affecting the industry in South Africa.
The city of
Johannesburg was built atop the world's greatest gold finds. Gold fields in the
Free State and
Gauteng provinces are deep and require the world's deepest mines. 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
Canada,
United States and
Australia 
