Gold (Au)
Gold (Au) is a rare and precious metal, valued for its beauty and unique properties, including its brilliant yellow luster and resistance to tarnishing. It is found in small amounts across the globe, with significant deposits located in South Africa, Russia, the United States, Canada, and several South American countries. Gold has been used for thousands of years in jewelry, decorations, and as an important element in various ceremonial and medicinal applications. Its chemical stability makes it suitable for use in electronics and precision instruments, while its ductility allows it to be fashioned into very thin sheets or drawn into fine wire.
Historically, gold has played a crucial role in various cultures, symbolizing wealth and power, and it was among the first metals used by human societies. Despite its many uses, there are environmental concerns related to gold mining, particularly due to the harmful practices involving mercury. Today, gold continues to serve as a standard for monetary systems, though it is rarely used as actual currency. The pursuit of gold remains a significant cultural and economic activity, shaping both historical events and modern practices worldwide.
Gold (Au)
Where Found
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.
Primary Uses
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.
Technical Definition
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).
Description, Distribution, and Forms
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 outer electrons to generate a +1, +2, or +3 charged metal cation, respectively. The most common oxidation state of gold is the +3 form.
Gold is the softest of all metals; thus, it is also the most ductile (capable of being drawn into thin wire) and most malleable (capable of being hammered into thin sheets, or foil). Gold can be hammered into foil sheets so thin that it would take 300,000 sheets, stacked on top of one another, to make a pile 2.5 centimeters high. It has been estimated that one gram of gold could be drawn into a wire that would span about 2.5 kilometers.
Jewelry and coins are rarely made of pure gold because the very soft nature of pure gold makes these items susceptible to loss of gold mass as well as loss of the intended artistic form. To prevent this problem, gold is alloyed with metals such as copper (into materials called red, pink, or yellow gold), palladium, nickel, or zinc (called white gold), and silver or platinum. The purity of gold that is “diluted” by another metal in an alloy is expressed in carats. Pure gold is 24 carats, meaning that 24 out of 24 parts are made of gold. In 18-carat gold, 18 out of 24 parts of the alloy are gold, and the other 6 parts are some other metal. Similarly, 10-carat gold means 10 of 24 parts are gold.
Gold is widely distributed across the world’s continents. Approximately half of the world’s gold has come from South Africa, including the region near Johannesburg. Other major gold deposits have been found in regions of the Urals and Siberia (Russia), Canada, the western United States, Mexico, and Colombia. Less significant deposits are found in Egypt, Australia, Asia, and Europe.
Two-thirds of all the gold produced in the United States originates in regions of South Dakota and Nevada. Locations of other important U.S. gold finds include California, made famous by the California gold rush of 1849; Alaska, popularized by the Klondike gold rush of 1896; and Colorado, with a ski resort town named Telluride because the gold-containing ore telluride is found in the region.
Through geological activity, the genesis of elemental gold is favored by postmagmatic processes occurring in the presence of medium-intensity hydrothermal energy. Such activity upon gold-bearing lavas produces primary deposits of gold, in which elemental gold remains in the site where it was formed. Postmagmatic processes also favor the formation of quartz, copper and iron pyrites, and other minerals containing the metals copper, gold, cobalt, and silver. As could be expected, these minerals and metals often occur together. Because copper and iron pyrites have a golden luster, although less brilliant than that of gold, their presence in primary gold deposits posed problems for miners. These pyrites are responsible for the term “fool’s gold,” and many a miner was betrayed by partners, bankers, or himself when mistaking chunks of cheap copper and lead pyrites for real gold.
Gold can also be found in areas where mechanical processes acted upon sedimentaryrock to yield secondary deposits of gold. Wind and water act to pulverize rock into sand and gravel. Through erosion, clastic and placer deposits of gold and platinum form. Since gold and platinum are inert, they remain unaltered by erosive forces. As rock erosion continues, the movement and accumulation of these metals along rivers occur. Since these metals are seven times denser than sand and gravel, they migrate downstream at a more sluggish rate. This sluggish movement, plus the heavy density of gold and platinum, encourages the metals to settle in riverbeds. Conglomerates, or large nuggets, of gold and platinum, can be found only in placer deposits formed in this manner. Among the more famous nuggets found are a 93-kilogram nugget found in Hill End, Australia, and a 153-kilogram nugget found in Chile. The spectacular classic placer deposits found in the Klondike, in the Yukon (Canada), and near Sacramento, California, explain the subsequent gold bonanzas and migration of prospectors, then settlers, into the American West. Secondary deposits have also yielded the abundant alluvial gold deposits found near Johannesburg. By far, most gold is found in placers of sedimentary origin. In areas of recent erosion, gold is usually found in small, shapeless grains, in small sheets, or as flakes. When fine-grain gold is found in alluvial deposits, “panning for gold” is performed to separate the precious metal from the sand.
Formed in primary deposits, crystals of elemental gold may occur as veins or as dendritic (arborescent) aggregates in association with quartz crystals. Dendritic aggregates look as though the metallic crystal developed with a fernlike growth on large, colorful and translucent quartz crystals. Gold veins are often natural alloys of gold and silver rather than pure gold. These naturally occurring gold-silver alloys are known as electrum, in which the silver content may range from 15 percent to 50 percent. Other natural alloys, as of gold and palladium (porpezite) or gold and rhodium (rhodite), are less frequently found. Gold also occurs in telluride ores, such as tetradymite, nagyagite, and sylvanite. These ores are primarily sulfide compounds of tellurium. In addition to tellurium (Te) and sulfur (S) atoms, tetradymite contains gold and lead. Similarly, sylvanite and nagyagite (black tellurium) contain gold and silver, but in different arrangements and ratios. Elemental gold can be extracted from these minerals via chemical reactions.
Gold is not an essential element for life, although trace amounts are found in humans and some plants concentrate the element. Trace amounts in humans may arise from ingestion of gold from certain alcoholic beverages, from gold dental amalgams, or from exposure to gold therapy for arthritis. Because gold is minimally absorbed by the digestive system, these trace amounts pose no toxic concern.
Most of the world’s gold deposits have been well exploited and are therefore nearly devoid of the precious metal. In the South American continent, which was the least mined of all continents up to the 1980s, the environment has begun to suffer from the hunt for gold. Past methods of obtaining gold have yielded to the more dangerous practice of using liquid mercury to form a mercury-gold amalgam in the panning process. To recover even the tiniest amount of gold, a large quantity of mercury must be used. In South American rivers, gold occurs in brown, iron-stained sand. Some deposits have been profitable even though only a few dollars’ worth of gold may be gathered per metric ton of sand panned and amalgamated. Whether or not gold is actually found, the leftover mercury is dumped directly into the rivers. Mercury, a neurotoxin, is lethal in high amounts. Reportedly, the dumping of untreated mercury has reduced populations of fish, has caused high levels of mercury in fish eaten by people, and is likely to have health impacts on children, pregnant women, and future generations. Some researchers suspect that neurological symptoms that suggest mercury poisoning can already be seen in some South American population areas.
History
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, although there was no system of standardized coins. Archaeological studies have also shown that the Incan civilization of pre-Columbian South America possessed considerable gold-working skill and achieved mastery of soldering and welding techniques.
The alchemists of the medieval period believed that gold was one of the most important keys to immortality. They also believed that base metals, which were abundant and cheap, could be converted into gold, which was rare and expensive. It was assumed that by simple manipulation in the presence of a spiritual agent—such as the Philosopher’s Stone—an elixir could be formed that possessed all the ingredients required for immortality. Because of its inert behavior and timeless beauty, gold was believed to impart some qualities required to achieve worldly immortality. During medieval times it was widely thought that the emperors and kings who had the most gold would have the longest lives. If a king ruled for many years there could be long periods of economic stability, access to food, security of family, and safety from conquerors. Thus the pursuit of gold was serious business, and the king’s magician, who was usually an alchemist, was highly regarded in the king’s court. As a final historical note, gold amalgams, mixtures of mercury and gold, were described in the year 27 b.c.e. by a Roman architect, Vitruvius. Mercury and gold amalgamation is still in use today as a means of collecting gold from sand deposits of riverbeds.
Obtaining Gold
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.
Uses of Gold
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 Mesopotamian, Assyrian, and Lydian societies.
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