Where Found (Encyclopedia of Global Resources)
Although widely distributed in nature, gold is a rare element. It has been estimated that all of the Earth’s gold could be gathered into a single cube measuring only 20 meters on each side. Because of its rarity, gold is considered a precious metal. The largest deposits of gold have been found in South Africa and the former Soviet Union (in the Urals and Siberia). Other large deposits have been found in the western United States and in Canada, Mexico, and Colombia.
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Primary Uses (Encyclopedia of Global Resources)
Gold is used in jewelry, decorations, electroplating, and dental materials. Other uses include medicinal compounds for the treatment of arthritis and the use of the Au198 isotope, with a half-life of 2.7 days, for treating some cancers. Since gold is an excellent heat and electrical conductor, and remains inert when exposed to air or moisture, it has also been used in precision scientific and electrical instruments. Specifically, gold has been used to coat space satellites, to transmit infrared signals, and to serve as the contact point for triggering the inflation of protective air bags in some automobiles. Few countries today use gold coinage systems; an exception is the Krugerrand coin of South Africa. Most nations use gold symbolically as a standard of their monetary systems rather than as actual coinage. Similarly, international monetary exchanges remain based on the world market value of gold, but actual exchanges of gold are uncommon.
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Technical Definition (Encyclopedia of Global Resources)
Gold is represented by the chemical symbol Au, derived from the Latin word aurum, meaning “shining dawn.” The weighted mass average of these isotopes gives gold an atomic mass of 196.9665 atomic mass units. Pure gold is a soft, shiny, and ductile metal with a brilliant yellow luster. Changing from solid to liquid at 1,064° Celsius, gold has a high melting point. To vaporize gold requires an even higher temperature (2,808° Celsius). Highly purified gold has a specific gravity of 19.3 (at 20° Celsius).
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Description, Distribution, and Forms (Encyclopedia of Global Resources)
On the periodic table, gold (atomic number 79) is a member of Group IB of transition metals. This group, also known as the coinage metals, includes copper, silver, and gold. Chemically, gold behaves similarly to platinum, although the arrangement of its chemically reactive electrons is similar to that of copper and silver. Both gold and platinum are largely nonreactive metals. Elemental gold exists in eighteen isotopic forms in nature.
Gold is a rare and precious metal. As such, pure gold has been highly valued and coveted by societies over millennia. Because of its nonreactive nature, elemental gold maintains its brilliant yellow luster. Because of this luster, gold is widely considered the most beautiful and unique of all the metals, which typically display colors of gray, red, or white-silver. Gold does not air-oxidize (tarnish) or corrode upon exposure to moisture. Similarly, it does not readily react to common acids or bases. Nonetheless, gold does dissolve in a reagent known as aqua regia, which is a mixture of nitric acid and hydrochloric acid; alone, neither acid acts upon gold. Aqua regia is a Latin term meaning the “liquid” (aqua) that dissolves the “king” (regia) of all metals. This reagent is used to separate gold from its ores.
Although predominantly inert, gold can be oxidized to form compounds. When it oxidizes, gold atoms may lose either one, two, or three...
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History (Encyclopedia of Global Resources)
The Group IB metals, or coinage metals, were the first metals used in primitive cultures. It is believed that elemental metals were easy to find in nature because their bright lusters shone in natural light. Precious metals have been in use since at least seven thousand years ago by civilizations of the Middle East and Afghanistan. Wealthy members of these groups possessed decorative jewels fashioned from gold. The metalworkers of these ancient societies manipulated the gold physically using hammers or other tools to carve or cut the soft metal.
Exploration of the tomb of King Tutankhamen, from the fourteenth century b.c.e., revealed an entry guarded by gold funerary masks inlaid with colorful glass. A gold sarcophagus and gold panel behind the king’s throne were also found. Between 4000 and 3500 b.c.e., the Egyptians and Sumerians learned to smelt silver and gold. They were able to generate fires in furnaces that could achieve the extreme temperatures required to melt metals, to cast molten metal into molds, to forge metal, and to make alloys (by blending molten metals). The use of gold for dental fillings among wealthy Egyptians dates back to between 2680 and 2160 b.c.e.
In Mesopotamia, a region that is now part of Iraq, an ornate headdress of Queen Puabi, dated to 2700 b.c.e., was fashioned with gold-carved leaves to adorn her face. Trading and business deals of Mesopotamia involved the exchange of precious metals,...
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Obtaining Gold (Encyclopedia of Global Resources)
Gold is separated from rocks, minerals, and alluvial deposits by panning or sluicing methods. The extraction of gold from telluride ores (tetradymite, nagyagite, and sylvanite) requires chemical reactions. The use of cyanide compounds, formation of amalgams, or smelting gold may be necessary to extract the gold from ores. The extracted gold is frequently refined by electrolysis (the use of an electric current). Electrolysis is particularly useful in separating mercury-gold amalgams back into their separate and purified metallic state.
In telluride ores (minerals), gold is not in the free, elemental state; rather, it is in a cationic form. As a metallic cation, each atom of gold carries a positive charge of either +1 or +3. A chemical reaction involving the addition of potassium cyanide to the crushed rocks (covered with water) makes a new compound of gold that dissolves in the water. This layer of water can be collected off the crushed rock, and through electrolysis the gold cations can be converted into gold crystals.
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Uses of Gold (Encyclopedia of Global Resources)
As described previously, jewelry and decorative ornaments fashioned from gold are marketed using carats to describe the quantity of gold present. Compounds of gold are used for decorating china or glass items. Gold chloro compounds, containing gold cations having a +3 charge, are mixed with sulfurized terpenes or resins to form a mixture known as “liquid gold,” which can be applied directly to glass or china.
Compounds of gold with +1 cations are used in rheumatology as an anti-inflammatory agent for the treatment of active, refractory forms of juvenile and adult rheumatoid arthritis. These biologically active compounds are sodium gold thiomalate and sodium gold (or auro) thioglucose; aurothioglucose seems to be less painful when injected into a muscle near the joint. The gold therapy must be started before permanent changes have occurred in the afflicted joints if it is to benefit the patient. Some of the side effects of these therapies include skin, liver, and kidney changes or damage. Approximately 20 percent of patients who try gold therapy have to discontinue treatment because of these adverse reactions. However, newer anti-inflammatory agents have limited the number of patients who need to try gold therapy for relief.
Finally, gold has been used in an abstract manner as the basis or standard of valuation for currencies and monetary systems throughout the world. The origins of this ancient practice lie in...
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Further Reading (Encyclopedia of Global Resources)
Bernstein, Peter L. The Power of Gold: The History of an Obsession. New ed. New York: Wiley, 2004.
Boyle, Robert W. Gold: History and Genesis of Deposits. New York: Van Nostrand Reinhold, 1987.
Green, Timothy. The New World of Gold: The Inside Story of the Mines, the Markets, the Politics, the Investors. New York: Walker, 1981.
Greenwood, N. N., and A. Earnshaw. “Copper, Silver, and Gold.” In Chemistry of the Elements. 2d ed. Boston: Butterworth-Heinemann, 1997.
Macdonald, Eoin H. Handbook of Gold Exploration and Evaluation. Boca Raton, Fla.: CRC Press, 2007.
Marx, Jenifer. The Magic of Gold. Garden City, N.Y.: Doubleday, 1978.
Pellant, Chris. Rocks and Minerals. 2d American ed. New York: Dorling Kindersley, 2002.
Schumann, Walter. Handbook of Rocks, Minerals, and Gemstones. Translated by R. Bradshaw and K. A. G. Mills. Boston: Houghton Mifflin, 1993.
Natural Resources Canada. Canadian Minerals Yearbook, Mineral and Metal Commodity Reviews. http://www.nrcan-rncan.gc.ca/mms-smm/busi-indu/cmy-amc/com-eng.htm
U.S. Geological Survey. Gold: Statistics and Information. http://minerals.usgs.gov/minerals/pubs/commodity/gold/index.html#mcs
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Gold (Chemical Elements)
Gold has been called the most beautiful of all chemical elements. Its beauty has made it desirable for use in jewelry, coins, and artwork for thousands of years. It was one of the first pure metals to be used by humans.
Gold is one of the few elements that can affect politics and economics. Wars have been fought over access to gold. Cities and towns have sprung up and died out as gold was discovered and then mined out. Many nations still count their wealth according to the amount of gold they keep in storage.
Gold lies in the middle of the periodic table. The periodic table is a chart that shows how elements are related to one another. Gold is a heavy metal in a group known as the transition metals. Gold is also known as a precious metal (as are platinum and silver).
Large amounts of gold are still used in the manufacture of coins, medals, jewelry, and art. Gold also has a number of uses in industry, medicine, and other applications. For example, one radioactive isotope of gold is commonly used to treat cancer.
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Gold (How Products are Made)
Gold, recognizable by its yellowish cast, is one of the oldest metals used by humans. As far back as the Neolithic period, humans have collected gold from stream beds, and the actual mining of gold can be traced as far back as 3500 B.C., when early Egyptians (the Sumerian culture of Mesopotamia) used mined gold to craft elaborate jewelry, religious artifacts, and utensils such as goblets.
Gold's aesthetic properties combined with its physical properties have long made it a valuable metal. Throughout history, gold has often been the cause of both conflict and adventure: the destruction of both the Aztec and Inca civilizations, for instance, and the early American gold rushes to Georgia, California, and Alaska.
The largest deposit of gold can be found in South Africa in the Precambrian Witwatersrand Conglomerate. This deposit of gold ore is hundreds of miles across and more than two miles deep. It is estimated that two-thirds of the gold mined comes from South Africa. Other major producers of gold include Australia, the former Soviet Union, and the United States (Arizona, Colorado, California, Montana, Nevada, South Dakota, and Washington).
About 65 percent of processed gold is used in the arts industry, mainly to make jewelry. Besides jewelry, gold is also used in the electrical, electronic, and ceramics industries. These industrial applications have grown in recent years and now occupy an estimated 25 percent of the gold market. The remaining percentage of mined gold is used to make a type of ruby colored glass called purple of Cassius, which is applied to office building windows to reduce the heat in the summer, and to mirrors used in space and in electroscopy so that they reflect the infrared spectrum.
Gold, whose chemical symbol is Au, is malleable, ductile, and sectile, and its high thermal and electrical conductivity as well as its resistance to oxidation make its uses innumerable. Malleability is the ability of gold and other metals to be pressed or hammered into thin sheets, 10 times as thin as a sheet of paper. These sheets are sometimes evaporated onto glass for infrared reflectivity, molded as fillings for teeth, or used as a coating or plating for parts. Gold's ability to be drawn into thin wire (ductility) enables it to be deposited onto circuits such as transistors and to be used as an industrial solder and brazing alloy. For example, gold wire is often used for integrated circuit electrical connections, for orthodontic and prosthetic appliances, and in jet engine fabrication.
Gold's one drawback for use in industry is that it is a relatively soft metal (sectile). To combat this weakness, gold is usually alloyed with another member of the metal family such as silver, copper, platinum, or nickel. Gold alloys are measured by karats (carats). A karat is a unit equal to 1/24 part of pure gold in an alloy. Thus, 24 karat (24K) gold is pure gold, while 18 karat gold is 18 parts pure gold to 6 parts other metal.
Extraction and Refining
Gold is usually found in a pure state; however, it can also be extracted from silver, copper, lead and zinc. Seawater can also contain gold, but in insufficient quantities to
- 1 In lode or vein deposits, the gold is mixed with another mineral, often quartz, in a vein that has filled a split in the surrounding rocks. Gold is obtained from lode deposits by drilling, blasting, or shoveling the surrounding rock.
Lode deposits often run deep underground. To mine underground, miners dig shafts into the ground along the vein. Using picks and small explosives, they then remove the gold ore from the surrounding rock. The gold ore is then gathered up and taken to a mill for refinement.
- 2 Placer deposits contain large pieces of gold ore (nuggets) and grains of gold that have been washed downstream from a lode deposit and that are usually mixed with sand or gravel. The three main methods used to mine placer deposits are hydraulic mining, dredging, and power shoveling. All methods of placer deposit mining use gravity as the basic sorting force.
In the first method, a machine called a "hydraulic giant" uses a high pressure stream of water to knock the gold ore off of banks containing the ore. The gold ore is then washed down into sluices or troughs that have grooves to catch the gold.
Dredging and power shoveling involve the same techniques but work with different size buckets or shovels. In dredging, buckets on a conveyor line scoop sand, gravel, and gold ore from the bottom of streams. In power shoveling, huge machines act like shovels and scoop up large quantities of gold-bearing sand and gravel from stream beds.
Hydraulic mining and dredging are outlawed in many countries because they are environmentally destructive to both land and streams.
- 3 Once the gold ore has been mined, it usually is washed and filtered at the mine as a preliminary refinement technique. It is then shipped to mills, where it is first combined with water and ground into smaller chunks. The resulting mixture is then further ground in a ball mill rotating cylindrical vessel that uses steel balls to pulverize the ore.
Separating the gold from the ore
- 4 The gold is then separated from the ore using one of several methods. Floatation involves the separation of gold from its ore by using certain chemicals and air. The finely ground ore is dumped into a solution
Image Pop-UpFloatation, cyanidation, and the carbon-in-pulp method are 3 processes used to refine gold. They can be used alone or in combination with one another.
Cyanidation also involves using chemicals to separate the gold from its contaminants. In this process, the ground ore is placed in a tank containing a weak solution of cyanide. Next, zinc is added to the tank, causing a chemical reaction in which the end result is the precipitation (separation) of the gold from its ore. The gold precipitate is then separated from the cyanide solution in a filter press. A similar method is amalgamation, which uses the same process with different chemicals. First, a solution carries the ground ore over plates covered with mercury. The mercury attracts the gold, forming an alloy called an amalgam. The amalgam is then heated, causing the mercury to boil off as a gas and leaving behind the gold. The mercury is collected, recycled and used again in the same process.
The carbon-in-pulp method also uses cyanide, but utilizes carbon instead of zinc to precipitate the gold. The first step is to mix the ground ore with water to form a pulp. Next, cyanide is added to dissolve the gold, and then carbon is added to bond with the gold. After the carbon particles are removed from the pulp, they are placed in a hot caustic (corrosive) carbon solution, which separates the gold from the carbon.
- 5 If the gold is still not pure enough, it can be smelted. Smelting involves heating the gold with a chemical substance called flux. The flux bonds with the contaminants and floats on top of the melted gold. The gold is then cooled and allowed to harden in molds, and the flux-contaminant mixture (slag) is hauled away as a solid waste.
Because gold is a finite resource, its long-term future is limited. In the short term, however, it will continue to find widespread use in jewelry and in industrial applications, especially in the electronics field.
In the last few years, several companies have focused on extracting gold from sulphide ore rather than oxide ore. Previous techniques made such extraction difficult and expensive, but a newer technique called bioleaching has made extraction more feasible. The process involves combining the sulphide ore with special bacteria that "eat" the ore or break it down into a more manageable form.
Where To Learn More
Coombs, Charles. Gold and Other Precious Metals. Morrow Publishing, 1981.
Gasparrini, Claudia. Gold & Other Precious Metals: From Ore to Market. Springer-Verlag, 1993.
Green, Timothy. The World of Gold. Walker Publishing, 1968.
Hawkins, Clint. Gold & Lead. HarperCollins, 1993.
Lye, Keith. Spotlight on Gold. Rourke Enterprises, 1988.
McCracken, Dave. Gold Mining in the Nineteen Nineties: The Complete Book of Modern Gold Mining Procedure. New Era Publications, 1993.
Wise, Edmund, ed. Gold: Recovery, Properties, and Applications. Van Nostrand, 1964.
Abelson, Philip H. "Gold." Science. July 11, 1986, p. 141.
Dworetzky, Tom. "Gold Bugs." Discover, March, 1988, p. 32.
"Some Like It Hot." Economist. June 25, 1988, p.88.
"Mining with Microbes: A Labor of Bug." Science News. April 14, 1990, p. 236.
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