History Of 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).
New York Herald, 7 April 1849: “Hurrah! Here we are at last! The Land of Promise—El Dorado of the West! Our own bright, beautiful, bountiful California lies before us—her lap full of riches…. Any strong, able bodied man who is willing to labor five or six hours a day in the broiling sun, can make from $10 to $20 per day for three or four months in the year.”
History of The Precious Metals
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 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).
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.
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 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 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.
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 USE
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 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, 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.
Palladium was discovered in 1803, in London, by English chemist William H. Wollaston (who also discovered rhodium in the same year).
He examined the residues left from platinum after dissolving it in aqua regia, a concentrated solution of hydrochloric and nitric acids.
He then isolated palladium in a series of chemical reactions, finally heating palladium cyanide to extract palladium metal.
Wollaston shared his discovery in an unconventional manner. He left a quantity of the metal for sale with a mineral dealer in Gerrard Street, London, and anonymously posted handbills throughout the city, describing the new metal’s properties.
Suspicions over this uncommon method of announcement arose and Richard Chenevix, a celebrated chemist at the time, declared palladium to be a platinum-mercury alloy.
In response, Wollaston anonymously offered a reward of twenty guineas to anyone who could artificially produce palladium. Nobody ever claimed this money.
In 1805, Wollaston spoke before the Royal Society of London about palladium’s properties and how it could be isolated from platinum. He finished his talk by revealing himself to be the discoverer of palladium.
He explained that he had remained anonymous so that he would have the time to understand and explain all of the metal’s properties before putting his name to an account.
The element is named after the then-recently discovered asteroid, Pallas. The asteroid’s name refers to the ancient Greek goddess of wisdom.