Precious Metals
Precious Metals
The valuable, relatively rare, and highly corrosion resistant metals, which are found in the periodic table in the vertical groups VIIIB and IB and the horizontal periods 5 and 6, are called the precious metals. They include (with atomic numbers) ruthenium (44), rhodium (45), palladium (46), silver (47), osmium (76), iridium (77), platinum (78), and gold (79). The platinum group metals include (along with platinum): ruthenium, rhodium, palladium, osmium, and iridium. The three most popular precious metals are gold, silver, and platinum. They have historically been valued for their beauty and rarity, and are commonly referred to as the precious metals. Platinum usually costs slightly more than gold, and both metals are about 80 times more costly than silver. Precious metal weights are given in Troy ounces (named for Troyes, France, known for its fairs during the Middle Ages) a unit approximately 10% larger than 1 oz (28.35 g).
The ancients considered gold and silver to be of noble birth compared to the more abundant metals. Chemists have retained the term noble to indicate the resistance these metals have to corrosion, and their natural reluctance to combine with other elements.
History
The legends of King Midas and Jason’s search for the golden fleece hint at prehistoric humankind’s early fascination with precious metals. The proof comes in the gold and silver treasure found in ancient Egyptian tombs and even older Mesopotamian burial sites.
The course of recorded history also shows twists and turns influenced to a large degree by precious metals. It was Greek silver that gave Athens its Golden Age, Spanish gold and silver that powered the Roman empire’s expansion, and the desire for gold that motivated Christopher Columbus to sail west across the Atlantic. The exploration of Latin America was driven in large part by the search for gold, and the Jamestown settlers in North America had barely gotten their “land legs” before they began searching for gold.
Small amounts of gold found in North Carolina, Georgia, and Alabama played a role in the 1838 decision to remove the Cherokee Indians to Oklahoma. The California gold rush of 1849 made California a state in 1850, and California gold fueled northern industry and backed up union currency, two major factors in the outcome of the Civil War (1861–1865).
Gold
Since ancient times, gold has been associated with the sun. Its name is believed derived from a Sanskrit word meaning to shine, and its chemical symbol (Au) comes from aurum, Latin for glowing dawn. Pure gold has an exceedingly attractive, deep yellow color and a specific gravity of 19.3. Gold is soft enough to scratch with a fingernail, and the most malleable of metals. A block of gold about the size of a sugar cube can be beaten into a translucent film some 27 ft (8 m) on a side. Gold’s purity is expressed either as fineness (parts per 1,000) or in karats (parts per 24). An alloy containing 50% gold is 500 fine or 12 karat gold. Gold resists corrosion by air and most chemicals but can be dissolved in a mixture of nitric and hydrochloric acids, a solution called aqua regia because it dissolves the king of metals.
Occurrence
Gold is so rare that one ton of average rock contains only about eight pennies worth of gold. Gold ore occurs where geologic processes have concentrated gold to at least 250 times the value found in average rock. At that concentration there is still one million times more rock than gold and the gold is rarely seen. Ore with visible gold is fabulously rich.
Gold most commonly occurs as a pure metal called native gold or as a natural alloy with silver called electrum. Gold and silver combined with tellurium are of local importance. Gold and silver tellurides are found, for example, in the mountains around the old mining boomtown of Telluride, Colorado. Gold is found in a wide variety of geologic settings, but placer gold and gold veins are the most economically important.
Placer gold
Placer gold is derived from gold-bearing rock from which the metal has been freed by weathering. Gravity and running water then combine to separate the dense grains of gold from the much lighter rock fragments. Rich concentrations of gold can develop above deeply weathered gold veins as the lighter rock is washed away. The large Welcome Stranger gold nugget from the gold fields of Victoria, Australia, is a spectacular 71.3 lb (32.4 kg) example of this type of occurrence. (It was found by John Deason and Richard Oates on February 5, 1869.)
Gold washed into mountain streams also forms placer deposits where the stream’s velocity diminishes enough to deposit gold. Stream placers form behind boulders and other obstructions in the streambed, and where tributary streams merge with more slowly moving rivers. Placer gold is also found in gravel bars where it is deposited along with much larger rocky fragments.
The discovery of place gold set off the California gold rush of 1849 and the rush to the Klondike (of the Yukon territory in northwestern Canada) in 1897. The largest river placers known are in Siberia, Russia. Gold-rich sands there are removed with jets of water, a process known as hydraulic mining. A fascinating byproduct of Russia’s hydraulic mining is the unearthing of thousands of woolly mammoths, many with flesh intact, locked since the Ice Age in frozen tundra gravel.
Stream placer deposits have their giant ancient counterparts in paleoplacers, and the Witwatersrand district in South Africa outproduces all others combined. Gold was reported from the Witwatersrand (White Waters Ridge) as early as 1834, but it was not until 1886 that the main deposit was discovered. From that time until today, it has occupied the paramount position in gold mining history. Witwatersrand gold was deposited between 2.9 and 2.6 billion years ago in six major fields, each produced by an ancient river system.
Placer and paleoplacers are actually secondary gold deposits, their gold having been derived from older deposits in the mountains above. The California 49ers (people who went to California in and about the year 1849 to search for gold) looked upstream hoping to find the mother lode, and that is exactly what they called the system of gold veins they discovered.
Gold veins
Vein gold is deposited by hot subterranean water known as a hydrothermal fluid. Hydrothermal fluids circulate through rock to leach small amounts of gold from large volumes of rock and then deposit it in fractures to form veins. Major U.S. gold vein deposits have been discovered at Lead, South Dakota, in the Black Hills, and at Cripple Creek, Colorado, on the slopes of Pike’s Peak. Important vein deposits are also found in Canada and Australia. All these important deposits where located following the discovery of placer gold in nearby streams.
Production and uses
Gold’s virtual indestructibility means that almost all the gold ever mined is still in use today. It is entirely possible that some gold atoms that once graced the head of Cleopatra now reside in today’s jewelry, stereo, or teeth. Today, gold is being mined in ever-increasing amounts from increasingly lower-grade deposits. It is estimated that 70% of all gold recovered has been mined in this century. Each year nearly 2,000 tons are added to the total. Nevada currently leads the United States in gold production, and the Republic of South Africa is the world’s leading gold-producing nation.
Gold has traditionally been used for coinage, bullion, jewelry, and other decorative uses. Gold’s chemical inertness means that gold jewelry is nonallergenic and remains tarnish-free indefinitely. For much the same reasons gold has long been used in dentistry. Modern industry is consuming increasing quantities of gold, mostly as electrical contacts in micro-circuitry.
Silver
Silver is a brilliant white metal and the best metal in terms of thermal and electrical conductivity. Its chemical symbol, Ag, is derived from its Latin name, argentum, meaning white and shining. Silver is not nearly as precious, dense, or noble as gold or platinum. The ease with which old silverware tarnishes is an example of its chemical reactivity. Although native silver is found in nature, it most commonly occurs as compounds with other elements, especially sulfur.
Hydrothermal veins constitute the most important source of silver. The Comstock Lode, a silver bonanza 15 mi (24 km) southeast of Reno, Nevada, is a well-known example. Hydrothermal silver veins are formed in the same manner as gold veins, and the two metals commonly occur together. Silver, however, being more reactive than gold, can be leached from surface rocks and carried downward in solution. This process, called supergene enrichment, can concentrate silver into exceedingly rich deposits at depth.
Mexico has traditionally been the world’s leading silver producing country, but the United States, Canada, and Peru each contribute significant amounts. Although silver has historically been considered a precious metal, industrial uses now predominate. Significant quantities are still used in jewelry, silver ware, and coinage; but even larger amounts are consumed by the photographic and electronics industries.
Platinum
Platinum, like silver, is a beautiful silver-white metal. Its chemical symbol is Pt and its name comes from the Spanish world for silver (plata ), with which it was originally confused. Its specific gravity of 21.45 exceeds that of gold, and, like gold, it is found in pure metallic chunks in stream placers. The average crustal abundance of platinum is comparable to that of gold. The melting point of platinum is 3,219°F (1,769°C), unusually high for a metal, and platinum is chemically inert even at high temperature. In addition, platinum is a catalyst for chemical reactions that produce a wide range of important commodities.
Platinum commonly occurs with five similar metals known as the platinum group metals. The group includes osmium, iridium, rhodium, palladium, and ruthenium. All were discovered in the residue left when platinum ore was dissolved in aqua regia. All are rare, expensive, and classified chemically as noble metals.
Platinum is found as native metal, natural alloys, and as compounds with sulfur and arsenic. Platinum ore deposits are rare, highly scattered, and one deposit dominates all others much as South Africa’s Witwatersrand dominates world gold production. That platinum deposit is also in the Republic of South Africa.
KEY TERMS
Catalyst— Any agent that accelerates a chemical reaction without entering the reaction or being changed by it.
Electrum— A natural alloy of gold and silver.
Hydrothermal fluid— Hot water-rich fluid capable of transporting metals in solution.
Malleable— the ability of a substance to be pounded into thin sheets or otherwise worked, for example during the making of jewelry.
Placer— A mineral deposit formed by the concentration of heavy mineral grains such as gold or platinum.
Specific gravity— The weight of a substance relative to the weight of an equivalent volume of water; for example, basalt weighs 2.9 times as much as water, so basalt has a specific gravity of 2.9.
Troy ounce— The Troy ounce, derived from the fourteenth-century system of weights used in the French town of Troyes, is still the basic unit of weight used for precious metals.
Placer platinum was discovered in South Africa in 1924 and subsequently traced to a distinctively layered igneous rock known as the Bushveld Complex. Although the complex is enormous, the bulk of the platinum is found in a thin layer scarcely more than 3 ft (0.9 m) thick. Nearly half of the world’s historic production of platinum has come from this remarkable layer.
The Stillwater complex in the Beartooth mountains of southwestern Montana also contains a layer rich in platinum group metals. Palladium is the layer’s dominant metal, but platinum is also found. The layer was discovered during the 1970s, and production commenced in 1987.
Production and uses
Platinum is used mostly in catalytic converters for vehicular pollution control. Low-voltage electrical contracts form the second most common use for platinum, followed closely by dental and medical applications, including dental crowns, and a variety of pins and plates used internally to secure human bones. Platinum is also used as a catalyst in the manufacture of explosives, fertilizer, gasoline, insecticides, paint, plastic, and pharmaceuticals. Platinum crucibles are used to melt high-quality optical glass and to grow crystals for computer chips and lasers. Hot glass fibers for insulation and nylon fibers for textiles are extruded through platinum sieves.
Future outlook
Because of their rarity and unique properties, the demand for gold and platinum are expected to continue to increase. Silver is more closely tied to industry, and the demand for silver is expected to rise and fall with economic conditions.
See also Element, chemical; Mining.
Resources
BOOKS
Boyle, Robert. Gold History and Genesis of Deposits. New York: Van Nostrand Reinhold, 1987.
Cotton, Simon. Chemistry of Precious Metals. London, UK, and New York: Blackie Academic & Professional, 1997.
Gasparrini, Claudia. Gold and Other Precious Metals: From Ore to Market. Raleigh, NC: Division of Archives and History, North Carolina Department of Cultural Resources, 2001.
Kesler, Stephen. Mineral Resources, Economics and the Environment. New York: MacMillan, 1994.
Klein, C. The Manual of Mineral Science. 22nd ed. New York: John Wiley & Sons, Inc., 2002.
Eric R. Swanson
Precious Metals
Precious metals
Gold, silver, and platinum have historically been valued for their beauty and rarity. They are the precious metals. Platinum usually costs slightly more than gold, and both metals are about 80 times more costly than silver. Precious metal weights are given in Troy ounces (named for Troyes, France, known for its fairs during the Middle Ages) a unit approximately 10% larger than 1 oz (28.35 g).
The ancients considered gold and silver to be of noble birth compared to the more abundant metals. Chemists have retained the term noble to indicate the resistance these metals have to corrosion , and their natural reluctance to combine with other elements.
History
The legends of King Midas and Jason's search for the golden fleece hint at prehistoric mankind's early fascination with precious metals. The proof comes in the gold and silver treasure found in ancient Egyptian tombs and even older Mesopotamian burial sites.
The course of recorded history also shows twists and turns influenced to a large degree by precious metals. It was Greek silver that gave Athens its Golden Age, Spanish gold and silver that powered the Roman empire's expansion, and the desire for gold that motivated Columbus to sail west across the Atlantic. The exploration of Latin America was driven in large part by the search for gold, and the Jamestown settlers in North America had barely gotten their "land legs" before they began searching for gold. Small amounts of gold found in North Carolina, Georgia, and Alabama played a role in the 1838 decision to remove the Cherokee Indians to Oklahoma. The California gold rush of 1849 made California a state in 1850, and California gold fueled northern industry and backed up union currency, two major factors in the outcome of the Civil War.
Gold
Since ancient times, gold has been associated with the sun . Its name is believed derived from a Sanskrit word meaning "to shine," and its chemical symbol (Au) comes from aurum, Latin for "glowing dawn." Pure gold has an exceedingly attractive, deep yellow color and a specific gravity of 19.3. Gold is soft enough to scratch with a fingernail, and the most malleable of metals. A block of gold about the size of a sugar cube can be beaten into a translucent film some 27 ft (8 m) on a side. Gold's purity is expressed either as fineness (parts per 1,000) or in karats (parts per 24). An alloy containing 50% gold is 500 fine or 12 karat gold. Gold resists corrosion by air and most chemicals but can be dissolved in a mixture of nitric and hydrochloric acids, a solution called aqua regia because it dissolves the "king of metals."
Occurrence
Gold is so rare that one ton of average rock contains only about eight pennies worth of gold. Gold ore occurs where geologic processes have concentrated gold to at least 250 times the value found in average rock. At that concentration there is still one million times more rock than gold and the gold is rarely seen. Ore with visible gold is fabulously rich.
Gold most commonly occurs as a pure metal called native gold or as a natural alloy with silver called electrum. Gold and silver combined with tellurium are of local importance. Gold and silver tellurides are found, for example, in the mountains around the old mining boom-town of Telluride, Colorado. Gold is found in a wide variety of geologic settings, but placer gold and gold veins are the most economically important.
Placer gold
Placer gold is derived from gold-bearing rock from which the metal has been freed by weathering . Gravity and running water then combine to separate the dense grains of golds from the much lighter rock fragments. Rich concentrations of gold can develop above deeply weathered gold veins as the lighter rock is washed away. The "Welcome Stranger" from the gold fields of Victoria, Australia , is a spectacular 71.3 lb (32.4 kg) example of this type of occurrence.
Gold washed into mountain streams also forms placer deposits where the stream's velocity diminishes enough to deposit gold. Stream placers form behind boulders and other obstructions in the streambed and where a tributary stream merges with a more slowly moving river. Placer gold is also found in gravel bars where it is deposited along with much larger rocky fragments.
The discovery of place gold set off the California gold rush of 1849 and the rush to the Klondike in 1897. The largest river placers known are in Siberia, Russia. Gold-rich sands there are removed with jets of water, a process known as hydraulic mining. A fascinating byproduct of Russia's hydraulic mining is the unearthing of thousands of woolly mammoths, many with flesh intact, locked since the Ice Age in frozen tundra gravel.
Stream placer deposits have their giant ancient counterparts in paleoplacers, and the Witwatersrand district in South Africa outproduces all others combined. Gold was reported from the Witwatersrand (White Waters Ridge) as early as 1834, but it was not until 1886 that the main deposit was discovered. From that time until today, it has occupied the paramount position in gold mining history. Witwatersrand gold was deposited between 2.9 and 2.6 billion years ago in six major fields, each produced by an ancient river system.
Placer and paleoplacers are actually secondary gold deposits, their gold having been derived from older deposits in the mountains above. The California 49ers looked upstream hoping to find the mother lode, and that's exactly what they called the system of gold veins they discovered.
Gold veins
Vein gold is deposited by hot subterranean water known as a hydrothermal fluid. Hydrothermal fluids circulate through rock to leach small amounts of gold from large volumes of rock and then deposit it in fractures to form veins. Major U.S. gold vein deposits have been discovered at Lead in the Black Hills of South Dakota and at Cripple Creek on the slopes of Pike's Peak, Colorado. Important vein deposits are also found in Canada and Australia. All these important deposits where located following the discovery of placer gold in nearby streams.
Production and uses
Gold's virtual indestructibility means that almost all the gold ever mined is still in use today. It is entirely possible that some gold atoms that once graced the head of Cleopatra now reside in your jewelry, stereo, or teeth. Today, gold is being mined in ever increasing amounts from increasingly lower-grade deposits. It is estimated that 70% of all gold recovered has been mined in this century. Each year nearly 2,000 tons are added to the total. Nevada currently leads the nation in gold production, and the Republic of South Africa is the world's leading gold-producing nation.
Gold has traditionally been used for coinage, bullion, jewelry, and other decorative uses. Gold's chemical inertness means that gold jewelry is nonallergenic and remains tarnish-free indefinitely. For much the same reasons gold has long been used in dentistry . Modern industry is consuming increasing quantities of gold, mostly as electrical contacts in micro circuitry.
Silver
Silver is a brilliant white metal and the best metal in terms of thermal and electrical conductivity . Its chemical symbol, Ag, is derived from its Latin name, argentum, meaning "white and shining." Silver is not nearly as precious, dense, or noble as gold or platinum. The ease with which old silverware tarnishes is an example of its chemical reactivity. Although native silver is found in nature, it most commonly occurs as compounds with other elements, especially sulfur .
Hydrothermal veins constitute the most important source of silver. The Comstock Lode, a silver bonanza 15 mi (24 km) southeast of Reno, Nevada, is a well-known example. Hydrothermal silver veins are formed in the same manner as gold veins, and the two metals commonly occur together. Silver, however, being more reactive than gold, can be leached from surface rocks and carried downward in solution. This process, called super-gene enrichment, can concentrate silver into exceedingly rich deposits at depth.
Mexico has traditionally been the world's leading silver producing country, but the United States, Canada, and Peru each contribute significant amounts. Although silver has historically been considered a precious metal, industrial uses now predominate. Significant quantities are still used in jewelry, silver ware, and coinage; but even larger amounts are consumed by the photographic and electronics industries.
Platinum
Platinum, like silver, is a beautiful silver-white metal. Its chemical symbol is Pt and its name comes from the Spanish world for silver (plata), with which it was originally confused. Its specific gravity of 21.45 exceeds that of gold, and, like gold, it is found in pure metallic chunks in stream placers. The average crustal abundance of platinum is comparable to that of gold. The melting point of platinum is 3,219°F (1,769°C), unusually high for a metal, and platinum is chemically inert even at high temperature . In addition, platinum is a catalyst for chemical reactions that produce a wide range of important commodities.
Platinum commonly occurs with five similar metals known as the platinum group metals. The group includes osmium, iridium, rhodium, palladium, and ruthenium. All were discovered in the residue left when platinum ore was dissolved in aqua regia. All are rare, expensive, and classified chemically as noble metals.
Platinum is found as native metal, natural alloys, and as compounds with sulfur and arsenic. Platinum ore deposits are rare, highly scattered, and one deposit dominates all others much as South Africa's Witwatersrand dominates world gold production. That platinum deposit is also in the Republic of South Africa.
Placer platinum was discovered in South Africa in 1924 and subsequently traced to a distinctively layered igneous rock known as the Bushveld Complex. Although the complex is enormous, the bulk of the platinum is found in a thin layer scarcely more than 3 ft (0.9 m) thick. Nearly half of the world's historic production of platinum has come from this remarkable layer.
The Stillwater complex in the Beartooth mountains of southwestern Montana also contains a layer rich in platinum group metals. Palladium is the layer's dominant metal, but platinum is also found. The layer was discovered during the 1970s, and production commenced in 1987.
Production and uses
Platinum is used mostly in catalytic converters for vehicular pollution control . Low-voltage electrical contracts form the second most common use for platinum, followed closely by dental and medical applications, including dental crowns, and a variety of pins and plates used internally to secure human bones. Platinum is also used as a catalyst in the manufacture of explosives , fertilizer, gasoline, insecticides , paint, plastic, and pharmaceuticals. Platinum crucibles are used to melt high-quality optical glass and to grow crystals for computer chips and lasers. Hot glass fibers for insulation and nylon fibers for textiles are extruded through platinum sieves.
Future outlook
Because of their rarity and unique properties, the demand for gold and platinum are expected to continue to increase. Silver is more closely tied to industry, and the demand for silver is expected to rise and fall with economic conditions.
See also Element, chemical; Mining.
Resources
books
Boyle, Robert. Gold History and Genesis of Deposits. New York: Van Nostrand Reinhold, 1987.
Kesler, Stephen. Mineral Resources, Economics and the Environment. New York: MacMillan, 1994.
Klein, C. The Manual of Mineral Science. 22nd ed. New York: John Wiley & Sons, Inc., 2002.
St. John, Jeffrey. Noble Metals. Alexandria, VA: Time-Life Books, 1984.
Eric R. Swanson
KEY TERMS
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .- Catalyst
—Any agent that accelerates a chemical reaction without entering the reaction or being changed by it.
- Electrum
—A natural alloy of gold and silver.
- Hydrothermal fluid
—Hot water-rich fluid capable of transporting metals in solution.
- Malleable
—the ability of a substance to be pounded into thin sheets or otherwise worked, for example during the making of jewelry.
- Placer
—A mineral deposit formed by the concentration of heavy mineral grains such as gold or platinum.
- Specific gravity
—The weight of a substance relative to the weight of an equivalent volume of water; for example, basalt weighs 2.9 times as much as water, so basalt has a specific gravity of 2.9.
- Troy ounce
—The Troy ounce, derived from the fourteenth-century system of weights used in the French town of Troyes, is still the basic unit of weight used for precious metals.
Precious Metals
Precious metals
Gold, silver, and platinum have historically been valued for their beauty and rarity. They are the precious metals . Platinum usually costs slightly more than gold, and both metals are about 80 times more costly than silver. Precious metal weights are given in Troy ounces (named for Troyes, France, known for its fairs during the Middle Ages) a unit approximately 10% larger than 1 oz (28.35 g).
The ancients considered gold and silver to be of noble birth compared to the more abundant metals. Chemists have retained the term noble to indicate the resistance these metals have to corrosion , and their natural reluctance to combine with other elements.
The legends of King Midas and Jason's search for the golden fleece hint at prehistoric mankind's early fascination with precious metals. The proof comes in the gold and silver treasure found in ancient Egyptian tombs and even older Mesopotamian burial sites.
The course of recorded history also shows twists and turns influenced to a large degree by precious metals. It was Greek silver that gave Athens its Golden Age, Spanish gold and silver that powered the Roman Empire's expansion, and the desire for gold that motivated Columbus to sail west across the Atlantic. The exploration of Latin America was driven in large part by the search for gold, and the Jamestown settlers in North America had barely gotten their "land legs" before they began searching for gold. Small amounts of gold found in North Carolina, Georgia, and Alabama played a role in the 1838 decision to remove the Cherokee Indians to Oklahoma. The California gold rush of 1849 made California a state in 1850, and California gold fueled northern industry and backed up union currency, two major factors in the outcome of the Civil War.
Since ancient times, gold has been associated with the Sun . Its name is believed to be derived from a Sanskrit word meaning "to shine," and its chemical symbol (Au) comes from aurum, Latin for "glowing dawn." Pure gold has an attractive, deep yellow color and a specific gravity of 19.3. Gold is soft enough to scratch with a fingernail, and the most malleable of metals. A block of gold about the size of a sugar cube can be beaten into a translucent film some 27 ft (8 m) on a side. Gold's purity is expressed either as fineness (parts per 1,000) or in karats (parts per 24). An alloy containing 50% gold is 500 fine or 12 karat gold. Gold resists corrosion by air and most chemicals but can be dissolved in a mixture of nitric and hydrochloric acids, a solution called aqua regia because it dissolves the "king of metals".
Gold is so rare that one ton of average rock contains only about eight pennies worth of gold. Gold ore occurs where geologic processes have concentrated gold to at least 250 times the value found in average rock. At that concentration, there is still one million times more rock than gold and the gold is rarely seen. Ore with visible gold is fabulously rich.
Gold most commonly occurs as a pure metal called native gold or as a natural alloy with silver called electrum. Gold and silver combined with tellurium are of local importance. Gold and silver tellurides are found, for example, in the mountains around the old mining boom-town of Telluride, Colorado. Gold is found in a wide variety of geologic settings, but placer gold and gold veins are the most economically important.
Placer gold is derived from gold-bearing rock from which the metal has been freed by weathering . Gravity and running water then combine to separate the dense grains of gold from the much lighter rock fragments. Rich concentrations of gold can develop above deeply weathered gold veins as the lighter rock is washed away. The "Welcome Stranger" from the gold fields of Victoria, Australia , is a spectacular 158–16 (71.5-kg) example of this type of occurrence.
Gold washed into mountain streams also forms placer deposits where the stream's velocity diminishes enough to deposit gold. Stream placers form behind boulders and other obstructions in the streambed, and where a tributary stream merges with a more slowly moving river. Placer gold is also found in gravel bars where it is deposited along with much larger rocky fragments.
The discovery of placer gold set off the California gold rush of 1849 and the rush to the Klondike in 1897. The largest river placers known are in Siberia, Russia. Gold-rich sands there are removed with jets of water, a process known as hydraulic mining. A fascinating byproduct of Russia's hydraulic mining is the unearthing of thousands of woolly mammoths, many with flesh intact, locked since the Ice Age in frozen tundra gravel.
Stream placer deposits have their giant ancient counterparts in paleoplacers, and the Witwatersrand district in South Africa outproduces all others combined. Gold was reported from the Witwatersrand (White Waters Ridge) as early as 1834, but it was not until 1886 that the main deposit was discovered. From that time until today, it has occupied the paramount position in gold mining history. Witwatersrand gold was deposited between 2.9 and 2.6 billion years ago in six major fields, each produced by an ancient river system.
Placer and paleoplacers are actually secondary gold deposits, their gold having been derived from older deposits in the mountains above. The California 49ers looked upstream hoping to find the mother lode, and that's exactly what they called the system of gold veins they discovered.
Vein gold is deposited by hot subterranean water known as a hydrothermal fluid. Hydrothermal fluids circulate through rock to leach small amounts of gold from large volumes of rock and then deposit it in fractures to form veins. Major U.S. gold vein deposits have been discovered at Lead in the Black Hills of South Dakota and at Cripple Creek on the slopes of Pike's Peak, Colorado. Important vein deposits are also found in Canada and Australia. All these important deposits were located following the discovery of placer gold in nearby streams.
Gold's virtual indestructibility means that almost all gold ever mined could still be in use today. Today, gold is being mined in ever-increasing amounts from increasingly lower-grade deposits. It is estimated that 70% of all gold recovered has been mined in this century. Each year nearly 2,000 tons are added to the total. Nevada currently leads the nation in gold production, and the Republic of South Africa is the world's leading gold-producing nation.
Gold has traditionally been used for coinage, bullion, jewelry, and other decorative uses. Gold's chemical inertness means that gold jewelry is hypoallergenic and remains tarnish-free indefinitely.
Silver is a brilliant white metal and the best metal in terms of thermal and electrical conductivity. Its chemical symbol, Ag, is derived from its Latin name, argentum, meaning "shining white." Silver is not nearly as precious, dense, or noble as gold or platinum. The ease with which silverware tarnishes is an example of its chemical reactivity. Although native silver is found in nature, it most commonly occurs as compounds with other elements, especially sulfur.
Hydrothermal veins constitute the most important source of silver. The Comstock Lode, a silver bonanza 15 mi (24 km) southeast of Reno, Nevada, is a well-known example. Hydrothermal silver veins are formed in the same manner as gold veins, and the two metals commonly occur together. Silver, however, being more reactive than gold, can be leached from surface rocks and carried downward in solution. This process, called supergene enrichment, can concentrate silver into exceedingly rich deposits at depth.
Mexico has traditionally been the world's leading silver producing country, but the United States, Canada, and Peru each contribute significant amounts. Although silver has historically been considered a precious metal, industrial uses now predominate. Significant quantities are still used in jewelry, silver ware, and coinage; but even larger amounts are consumed by the photographic and electronics industries.
Platinum, like silver, is a silver-white metal. Its chemical symbol is Pt and its name comes from the Spanish world for silver (plata ), with which it was originally confused. Its specific gravity of 21.45 exceeds that of gold, and, like gold, it is found in pure metallic chunks in stream placers. The average crustal abundance of platinum is comparable to that of gold. The melting point of platinum is 3,219°F (1,769°C), unusually high for a metal, and platinum is chemically inert even at high temperature . In addition, platinum is a catalyst for chemical reactions that produce a wide range of important commodities.
Platinum commonly occurs with five similar metals known as the platinum group metals. The group includes osmium, iridium, rhodium, palladium, and ruthenium. All were discovered in the residue left when platinum ore was dissolved in aqua regia. All are rare, expensive, and classified chemically as noble metals.
Platinum is found as native metal, in natural alloys, and in compounds with sulfur and arsenic. Platinum ore deposits are rare, highly scattered, and one deposit dominates all others much as South Africa's Witwatersrand dominates world gold production. That platinum deposit is also in the Republic of South Africa.
Placer platinum was discovered in South Africa in 1924 and subsequently traced to a distinctively layered igneous rock known as the Bushveld Complex. Although the complex is enormous, the bulk of the platinum is found in a thin layer scarcely more than three feet thick. Nearly half of the world's historic production of platinum has come from this remarkable layer.
The Stillwater complex in the Beartooth mountains of southwestern Montana also contains a layer rich in platinum group metals. Palladium is the layer's dominant metal, but platinum is also found. The layer was discovered during the 1970s, and production commenced in 1987.
Platinum is used mostly in catalytic converters for vehicular pollution control. Low-voltage electrical contracts form the second most common use for platinum, followed closely by dental and medical applications, including dental crowns, and a variety of pins and plates used internally to secure human bones. Platinum is also used as a catalyst in the manufacture of explosives, fertilizer, gasoline, insecticides, paint, plastics, and pharmaceuticals. Platinum crucibles are used to melt high-quality optical glass and to grow crystals for computer chips and lasers. Hot glass fibers for insulation and nylon fibers for textiles are extruded through platinum sieves.
Because of their rarity and unique properties, the demand for gold and platinum are expected to continue to increase. Silver is more closely tied to industry, and the demand for silver is expected to rise and fall with economic conditions.
Precious Metals
Precious metals
The precious metals—gold, silver, and platinum—have historically been valued for their beauty and rarity. Ancient people considered gold and silver to be of noble birth compared to the more abundant metals. Chemists have retained the term noble to indicate the resistance these metals have to corrosion and their natural reluctance to combine with other elements.
The course of recorded human history shows twists and turns influenced to a large degree by precious metals. It was Greek silver that gave Athens its Golden Age, Spanish gold and silver that powered the Roman Empire's expansion, and the desire for gold that motivated Columbus to sail west across the Atlantic. The Spanish exploration of Latin America was also driven in large part by the search for gold. Small amounts of gold found in North Carolina, Georgia, and Alabama played a role in the 1838 decision to remove the Cherokee nation to Oklahoma. The California gold rush of 1849 made California a state the following year, and California gold fueled northern industry and backed up Union currency, two major factors in the outcome of the American Civil War (1861–65).
Gold
Since ancient times, gold has been associated with the Sun. Its name is believed to be derived from a Sanskrit word meaning "to shine," and its chemical symbol (Au) comes from aurum, Latin for "glowing dawn." Pure gold has an exceedingly attractive, deep yellow color. It is soft enough to scratch with a fingernail, and it is the most malleable of metals. A block of gold about the size of a sugar cube can be beaten into a translucent film some 27 feet (8 meters) on a side. Gold's purity is expressed either as fineness (parts per 1,000) or in karats (parts per 24). An alloy (mixture of two or more metals with properties different from those of the metals of which it is made) containing 50 percent gold is 500 fine or 12 karat. Gold resists corrosion by air and most chemicals, but it can be dissolved in a mixture of nitric and hydrochloric acids, a solution called aqua regia because it dissolves the "king of metals."
Gold is so rare that one ton of average rock contains only about eight pennies worth of gold. Gold ore (an ore is a mineral compound that is mined for one of the elements it contains, usually a metal element) occurs where geologic processes have concentrated gold to at least 250 times the value found in average rock. At that concentration, there is still one million times more rock than gold and the gold is rarely seen. Ore with visible gold is incredibly valuable. Gold most commonly occurs as a pure metal called native gold or as a natural alloy with silver called electrum. Gold is found in a wide variety of geologic settings, but placer gold and gold veins are the most economically important.
Words to Know
Alloy: A mixture of two or more metals with properties different from those of the metals of which it is made.
Catalyst: A compound that speeds up the rate of a chemical reaction without undergoing any change in its own composition.
Compound: A substance consisting of two or more chemical elements in specific proportions.
Hydrothermal fluid: Underground hot water-rich fluid capable of transporting metals in solution.
Malleable: Capable of being rolled or hammered into thin sheets.
Ore: Mineral compound that is mined for one of the elements it contains, usually a metal element.
Placer: A gravel or sand deposit left by a river containing a concentration of heavy mineral grains such as gold or platinum.
Placer gold is derived from gold-bearing rock from which the metal has been freed by weathering. Gravity and running water then combine to separate the dense grains of gold from the much lighter rock fragments. Rich concentrations of gold can develop above deeply weathered gold veins as the lighter rock is washed away. Gold washed into mountain streams forms placer deposits where the stream slows enough to deposit the gold. Placer gold is also found in gravel bars where it is deposited along with much larger rocky fragments. The discovery of placer gold set off the California gold rush of 1849 and the rush to the Klondike in 1897. The largest river placers known are in Siberia, Russia. Gold-rich sands there are removed with jets of water, a process known as hydraulic mining.
Vein gold is deposited by hot subterranean water known as a hydrothermal fluid. Hydrothermal fluids circulate through rock to leach (dissolve out) small amounts of gold from large volumes of rock and then deposit it in fractures to form veins. Major United States gold vein deposits have been discovered at Lead in the Black Hills of South Dakota and at Cripple Creek on the slopes of Pikes Peak, Colorado. Important vein deposits are also found in Canada and Australia.
Gold's virtual indestructibility means that almost all the gold ever mined is still in use. It is entirely possible that some gold atoms that once graced the head of Egyptian queen Cleopatra (69–30 b.c.) now reside in someone's jewelry, stereo, or teeth today. Gold is being mined in everincreasing amounts from increasingly lower-grade deposits. It is estimated
that 70 percent of all gold recovered has been mined since the beginning of the twentieth century. Each year, nearly 2,000 tons (1,800 metric tons) are added to the total. Nevada currently leads the United States in gold production, while the Republic of South Africa is the world's leading gold-producing nation.
Gold has traditionally been used for coins, bullion, jewelry, and other decorative items. Because of its chemical properties, gold is nonallergenic and remains tarnish-free indefinitely. For much the same reasons, gold has long been used in dentistry. Modern industry employs increasing quantities of gold, mostly for use as electrical contacts in microcircuits.
Silver
Silver is a brilliant white metal and the best metal in terms of thermal (heat) and electrical conductivity. Its chemical symbol, Ag, is derived from its Latin name, argentum, meaning "white and shining." Silver is not nearly as precious, dense, or noble as gold or platinum. The ease with which old silverware tarnishes is an example of its chemical reactivity. Although native silver is found in nature, it most commonly occurs in compounds with other elements, especially sulfur.
Hydrothermal veins constitute the most important source of silver. The Comstock Lode, located 15 miles (24 kilometers) southeast of Reno, Nevada, is a well-known example. Hydrothermal silver veins are formed in the same manner as gold veins, and the two metals commonly occur together. Silver, however, being more reactive than gold, can be leached from surface rocks and carried downward in solution. This process can concentrate silver into exceedingly rich deposits deep underground.
Mexico has traditionally been the world's leading silver producing country, but the United States, Canada, and Peru each contribute significant amounts. Vast quantities of silver are used in jewelry, silverware, and coins, but even larger amounts are used in products of the photographic and electronics industries.
Platinum
Platinum, like silver, is a beautiful silver-white metal. Its chemical symbol is Pt, and its name comes from the Spanish world for silver (plata ), with which it was originally confused. Like gold, it can be found in pure metallic chunks in stream placers.
Platinum commonly occurs with five similar metals known as the platinum group metals. The group includes osmium, iridium, rhodium, palladium, and ruthenium. All are rare, expensive, and classified chemically as noble metals. Platinum is found as native metal, natural alloys, and as compounds with sulfur and arsenic. Platinum ore deposits are rare and highly scattered. Nearly half the world's historic production of platinum comes from the Republic of South Africa.
Platinum is used mostly in catalytic converters for pollution control on automobiles. Low-voltage electrical contacts form the second most common use for platinum, followed closely by dental and medical applications, including dental crowns and a variety of pins and plates used internally to secure human bones. Platinum is also used as a catalyst in the manufacture of explosives, fertilizer, gasoline, insecticides, paint, plastic, and pharmaceuticals.
[See also Minerals; Mining ]