Gold is a chemical element, a dense, lustrous, yellow precious metal of Group Ib, Period 6, of the periodic table. Gold has several qualities that have made it exceptionally valuable throughout history. It is attractive in color and brightness, durable to the point of virtual indestructibility, highly malleable, and usually found in nature in a comparatively pure form. The history of gold is unequaled by that of any other metal because of its value in the minds of men from earliest times.
Gold is widespread in low concentrations in all igneous rocks. Its abundance in the Earth's crust is estimated at about 0.005 parts per million. It occurs mostly in the native state, remaining chemically uncombined except with tellurium, selenium, and possibly bismuth. The element's only naturally occurring isotope is gold-197. Gold often occurs in association with copper and lead deposits, and, though the quantity present is often extremely small, it is readily recovered as a byproduct in the refining of those base metals. Large masses of gold-bearing rock rich enough to be called ores are unusual. Two types of deposits containing significant amounts of gold are known: hydrothermal veins, where it is associated with quartz and pyrite (fool's gold); and placer deposits, both consolidated and unconsolidated, that are derived from the weathering of gold-bearing rocks.The gold in rocks usually occurs as invisible disseminated grains, more rarely as flakes large enough to be seen, and even more rarely as masses or veinlets. Crystals about 2.5 cm (1 inch) or more across have been found in California. Masses, some on the order of 90 kg (200 pounds), have been reported from Australia.Alluvial deposits of gold found in or along streams were the principal sources of the metal for ancient Egypt and Mesopotamia. Other deposits were found in Lydia (now in Turkey) and the lands of the Aegean and in Persia (now Iran), India, China, and other lands. During the Middle Ages the chief sources of gold in Europe were the mines of Saxony and Austria. The era of gold production that followed the Spanish discovery of the Americas in the 1490s was probably the greatest the world had witnessed to that time. The exploitation of mines by slave labor and the looting of Indian palaces, temples, and graves in Central and South America resulted in an unprecedented influx of gold that literally unbalanced the economic structure of Europe. From Christopher Columbus' discovery of the New World in 1492 to 1600, more than 225,000 kg (8,000,000 ounces) of gold, or 35 percent of world production, came from South America. The New World's mines--especially those in Colombia--continued into the 17th and 18th centuries to account for 61 and 80 percent, respectively, of world production; 1,350,000 kg (48,000,000 ounces) were mined in the 18th century.
Because pure gold is too soft to resist prolonged handling, it is usually alloyed with other metals to increase its hardness for use in jewelry, goldware, or coinage. Most gold used in jewelry is alloyed with silver, copper, and a little zinc to produce various shades of yellow gold or with nickel, copper, and zinc to produce white gold. The color of these gold alloys goes from yellow to white as the proportion of silver in them increases; more than 70 percent silver results in alloys that are white. Alloys of gold with silver or copper are used to make gold coins and goldware, and alloys with platinum or palladium are also used in jewelry. The content of gold alloys is expressed in 24ths, called karats; a 12-karat gold alloy is 50 percent gold, and 24-karat gold is pure.
Because of its high electrical conductivity (71 percent that of copper) and inertness, the largest industrial use of gold is in the electric and electronics industry for plating contacts, terminals, printed circuits, and semiconductor systems. Thin films of gold that reflect up to 98 percent of incident infrared radiation have been employed on satellites to control temperature and on space-suit visors to afford protection. Used in a similar way on the windows of large office buildings, gold reduces the air-conditioning requirement and adds to the beauty. Gold has also long been used for fillings and other repairs to teeth.
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