Platinum group metals

Although platinum group metals are among the rarest elements known, their chemical inertness, high melting points, and extraordinary catalytic properties make them an indispensable resource for modern industrial society.

Early Discovery of Platinum

The average crustal abundance of the platinum group metals is not known exactly but is comparable to that of gold. Platinum, like gold, can be found as a pure metal in stream placer deposits, where its density and resistance to corrosion have resulted in concentration of the metal during stream transport. Platinum appears to have been recovered from placer deposits since earliest times. A hieroglyphic character forged from a grain of platinum has been dated from the seventh century BCE, and ancient South American metalsmiths used platinum as an alloy to improve the hardness of gold as early as the first millennium BCE.

The name “platinum” is derived from the Spanish word for silver, to which it was originally considered to be inferior. Spanish conquistadors called the metal platina del Pinto (little silver of the Pinto) after its discovery in placer gold deposits of the Rio Pinto. Although the metal looked like silver, it proved to be more difficult to shape. Also, with a density similar to that of gold, platinum in small quantities mixed with gold was difficult to detect. Fear of its being used to degrade gold and silver led to a temporary ban against the importation of platinum to Europe. This was an ironic reversal from today’s perspective because platinum is frequently more costly than gold nowadays. The metal was eventually brought to Europe and was described by Sir William Watson in 1750.

Later Discovery of Other Group Metals

The platinum group metals consist of six elements so named because they occur with and have similar properties to platinum. In addition to platinum, they include osmium, iridium, rhodium, palladium, and ruthenium. These five metals were discovered in the first half of the nineteenth century by scientists who examined the residue left when crude platinum was dissolved in aqua regia, a mixture of hydrochloric and nitric acids. Because of their resistance to corrosion, all six platinum group metals, along with gold, are referred to by chemists as the noble metals. Economic geologists classify these metals together with silver as the precious metals. The weight of precious metals, like that of gems, is given in troy ounces.

Four of the platinum group metals were discovered by British scientists in 1803. Smithson Tennant discovered osmium and iridium. The name “osmium” was taken from the Greek word osme (meaning “smell”), because the metal exudes a distinctive odor, actually toxic osmium tetroxide, when it is powdered. Osmium is bluish white and extremely hard. Its melting point of about 3,045 degrees Celsius is the highest of the platinum group. The specific gravity of osmium has been measured at 22.57, making it the heaviest known element.

Iridium was named by Tennant for the Latin word iris, a rainbow, because of the variety of colors produced when iridium is dissolved in hydrochloric acid. Iridium is white with a slight yellowish cast. Like osmium, it is very hard, brittle, and dense. It has a melting point of 2,410 degrees Celsius and is the most corrosion-resistant metal known, rivaling osmium as the densest metal.

Rhodium and palladium were first discovered by William Hyde Wollaston, who named rhodium for the Greek word rhodon (a rose) because of the rose color produced by dilute solutions of rhodium salts. The metal is actually silvery white, has a melting point of about 1,966 degrees Celsius, exhibits a low electrical resistance, and is highly resistant to corrosion. Wollaston named palladium for Pallas, a recently discovered asteroid named for the ancient Greek goddess of wisdom. Palladium is steel-white, does not tarnish in air, and has the lowest specific gravity (12.02) and melting point (1,554 degrees Celsius) of the platinum group metals. Like platinum, it has the unusual property of absorbing enormous volumes of hydrogen.

The existence of ruthenium was proposed in 1828 but not established until 1844 by Russian chemist Karl Karlovich Klaus. He retained the previously suggested name ruthenium in honor of Ruthenia, the Latinized name for his adopted country of Russia. Ruthenium is a hard, white, nonreactive metal with a specific gravity slightly greater than that of palladium (12.41) and a melting point of 2,310 degrees Celsius.

Bushveld Complex

Important deposits of platinum group metals are found in the Bushveld complex of South Africa, the Stillwater complex of Montana, and at Sudbury, Ontario. A large deposit similar to that at Sudbury exists at Norilsk in Siberia.

The Bushveld complex holds a special place as history’s greatest source of platinum, and it still contains the world’s largest reserves of platinum group elements. A large layered igneous complex, Bushveld is located north of the town of Pretoria in the northeast corner of South Africa and covers an area roughly the size of the state of Maine. It formed 1.95 billion years ago when an enormous intrusion of mafic magma, the largest known mafic igneous intrusion, was injected and slowly cooled in the earth’s crust. As cooling and solidification occurred, the denser, mafic minerals became concentrated downward in the magma chamber, and the igneous rock became stratified with ultramafic layers at greater depth and layers of increasingly less mafic rocks upward.

Placer platinum was discovered in South Africa in 1924 and was subsequently traced by Hans Merensky to its source, a distinctive igneous layer that became known as the Merensky Reef. The reef is located in the lower part of the Bushveld complex, about one-third of the distance from the base to the top. Although commonly less than 1 meter in thickness, it has been traced for 250 kilometers around the circumference of the complex, and nearly half of the world’s historic production of platinum group metals has come from this remarkable layer.

The average metal content in the layer is about one-third of a troy ounce per ton of rock, or about 1 part platinum group metals in 100,000 parts rock, or 10 parts per million—thousands of times the normal crustal abundance of these metals. Platinum is the most abundant metal extracted from the reef. Other platinum group minerals are, in order of abundance, palladium (27 percent), ruthenium (5 percent), rhodium (2.7 percent), iridium (0.7 percent), and osmium (0.6 percent). Significant quantities of gold, nickel, and copper are present as well. The mining of such a narrow layer is so labor-intensive that each South African miner produces only about 30 ounces of platinum group metals per year.

Stillwater Complex

The Stillwater complex is a large-layered, mafic to ultramafic igneous complex remarkably similar to Bushveld. It is exposed for about 45 kilometers along the north side of the Beartooth Mountains in southwest Montana. The Stillwater area has long been famous for its large but low-grade chromium-rich layers, and platinum was discovered there in the 1920s. Serious exploration for economic concentrations of platinum, however, was initiated in 1967 by the Johns-Manville Corporation. This led, in the 1970s, to identification of the J-M Reef, a palladium- and platinum-rich horizon between 1 and 3 meters thick, which, like the Merensky Reef, can be traced through most of the complex.

The Stillwater complex formed 2.7 billion years ago. Like Bushveld, the complex is layered with ultramafic igneous rocks at the base and mafic rocks higher up. The J-M Reef lies slightly above the ultramafic zone. It has an average ore grade of 0.8 ounce of platinum group metals per ton of rock with a 3:1 ratio of palladium to platinum. Mining of the J-M Reef commenced in 1987. The ore is concentrated at the mine site and shipped to Antwerp, Belgium, for refining. The J-M Reef is the only significant source of platinum in the United States.

Sudbury Complex

The Sudbury complex, just north of Lake Huron in southeast Ontario, Canada, is similar in many ways to Bushveld and Stillwater, but it is not conspicuously layered. Nickel and copper are the main products, with platinum group metals produced as a by-product. Nickel was discovered in the Sudbury area in 1856. At that time, the region was largely wilderness, and government survey parties were engaged in running base, meridian, and range lines in preparation for a general survey and subdivision of northeastern Ontario. Considerable local magnetic attraction and the presence of iron were noted during the survey. An analysis of the rock showed that it contained copper and nickel as well. The Sudbury magma formed 1.85 billion years ago as the result of a large meteorite impact and now appears as an elliptical ring of mafic igneous rock 60 kilometers long by 27 kilometers wide. Some fifty ore deposits are found along and just outside its outer edge. Platinum group elements are extracted at Sudbury from the residues left over after smelting of nickel and copper.

Research into Origin of Deposits

Recent industry demands for platinum have stimulated research into the origin of platinum group metal deposits. Much of this research has been directed toward understanding the world’s great mafic igneous complexes. It has long been recognized that the origin of the Merensky and J-M reefs is tied to the formation of the layering within these mafic igneous complexes. Geologists have firmly established that the mafic magmas originate in the earth’s mantle and that they derive trace amounts of the platinum group metals from their mantle source rocks. It is also well known that as these magmas crystallize, various minerals are precipitated from the magma in a fairly well-established sequence. Geologists have long believed that the layered mafic igneous complexes represent the settling into layers of precipitated mineral grains according to density. Repetitions and modifications in the layering are considered to be the result of currents churning within the hot magma, or pulses of new magma injected into the intrusion. As crystallization proceeds, volatile elements such as water, carbon dioxide, and sulfur gradually become concentrated in the remaining magma. Sulfur has the ability to scavenge many metals, including iron, copper, nickel, and platinum group metals. Laboratory studies have shown that if the sulfur concentration is high enough, metallic sulfide droplets can form a separate, immiscible liquid. Like water in oil, the denser sulfide magma droplets sink and accumulate toward the base of the intrusion. Many geologists believe the layers and masses of metallic sulfide ore found in the large mafic igneous complexes formed in this manner.

Detailed studies of the chemical composition of the Stillwater complex, however, suggest that the crystallization sequence was interrupted at about the level of the J-M Reef by an influx of new, somewhat different magma. The evidence suggests that the magma was sulfur-saturated, and its influx is believed to have triggered the precipitation of the platinum minerals. Research on the Bushveld complex has also suggested multiple episodes of magma injection, with the Merensky Reef forming at the base of a magma pulse. It should be emphasized, however, that even after a century of investigation, the origin of the ore at the Bushveld, Stillwater, and Sudbury complexes is still the subject of considerable debate.

Hypotheses for ore formation at Sudbury include the separation of droplets of immiscible sulfide liquid from a mafic magma, but a lively debate exists regarding the mechanism by which the mafic magma was produced. It has long been noted that a distinctive zone of broken and shattered rock many kilometers wide underlies and surrounds the Sudbury igneous complex. Overlying the complex is a thick sequence of fragmentary rocks, originally interpreted as being volcanic in origin. In 1964, Robert Dietz suggested that Sudbury was the site of a tremendous meteorite impact that formed a large crater and shattered the surrounding rock. It was later proposed that the impact caused the melting that produced the mafic igneous rock and that the supposed volcanic rock was actually material that had been ejected during the meteorite impact and had fallen back into the crater. In this view, the igneous rocks are not intrusions, as was long believed, but a great sheet of impact melt, or molten rock formed at the surface by the heat generated by impact. This theory continues to cause controversy. The evidence for a meteorite impact is strong, but some geologists consider any impact to be unrelated to the Sudbury deposit. Others not only believe the impact theory but also suggest that the Bushveld magma was triggered by a meteorite impact. The debate continues, and its outcome has implications for the presence or absence of metallic ore deposits beneath the large lunar craters.

While field and laboratory work on the great platinum deposits of the world continues, so does experimental work aimed at understanding the conditions under which these deposits formed. Laboratory scientists are duplicating conditions found in nature in order to increase their understanding of the behavior of platinum group elements during crystallization from magmas, and during the formation of immiscible liquids, as well as their mobility at submagmatic temperatures in water-rich solutions.

Industrial Value

The platinum group metals are used extensively in modern industrial society because of their chemical inertness, high melting points, and extraordinary catalytic properties. Platinum group metals are important parts of the automotive, chemical, petroleum, glass, and electrical industries. Other important uses are found in dentistry, medicine, pollution control, and jewelry. The automotive industry is the single largest consumer of platinum group metals. Since 1974, platinum-palladium catalysts have been used in the United States to reduce the emission of pollutants from automobiles and light-duty trucks. A typical catalytic converter contains 0.057 ounce of platinum, 0.015 ounce of palladium, and 0.006 ounce of rhodium. In the European Economic Community, all cars with engines larger than 2 liters produced after October, 1988, must have converters.

The electrical industry is the second largest consumer of platinum group metals. Palladium is used in low-voltage electrical contacts, and platinum electrical contacts protect ships’ hulls from the corrosive activity of seawater. The dental and medical professions utilize nearly as much platinum group metals as the electrical industry. Palladium is alloyed with silver, gold, and copper to produce hard, tarnish-resistant dental crowns and bridges. Other medical uses include treatment for arthritis and some forms of cancer. Platinum group metals are also used internally in cardiac pacemakers and in a variety of pin, plate, and hinge devices used for securing human bones.

The chemical industry uses platinum and palladium as catalysts for a variety of reactions involving hydrogen and oxygen. Molecules of either of these gases are readily adsorbed onto the surface of the metals, where they dissociate into a layer of reactive atoms. Oxygen atoms on platinum, for example, increase the rate at which sulfur dioxide, a common industrial pollutant, is converted into sulfur trioxide, a component of sulfuric acid, the most widely used industrial chemical. Other pollution-control devices are aimed at the control of ozone levels in the cabins of commercial jet airplanes and the oxidation of noxious organic fumes from factories and sewage treatment plants. Platinum group catalysts are also used in the production of insecticides, some plastics, paint, adhesives, polyester and nylon fibers, pharmaceuticals, fertilizers, and explosives. In the petroleum industry, platinum group metals are used by refineries both to increase the gasoline yield from crude oil and to upgrade its octane level. Because material used as a catalyst is not consumed in the chemical reactions (although small amounts are lost), the many important chemical uses actually consume only a small amount of the platinum group metals.

Platinum group metals’ ability to withstand high temperatures and corrosive environments has led to their use in the ceramics and glass industry. Thin strands of glass are extruded through platinum sieves to make glass fibers for insulation, textiles, and fiber-reinforced plastics. High-quality optical glass for television picture tubes and eyeglasses is also melted in pots lined with nonreactive platinum alloys. Crystals for computer memory devices and solid-state lasers are grown in platinum and iridium crucibles. As ingots and bars, platinum group metals are sold to investors, and platinum and palladium alloys are commonly used for jewelry. Brilliant rhodium is electroplated on silver or white gold to increase whiteness, wear, and resistance to tarnishing.

Supply Versus Demand

The world’s reserves of platinum group metals are large, but distribution is concentrated in a relatively few locations. South Africa is the largest producer of platinum. Japan is the largest consumer nation, and the United States is second.

Historically, US production has been extremely small and has consisted almost entirely of platinum and palladium extracted during the refining of copper. Stillwater’s J-M Reef is a significant discovery, but it is expected to supply only about 7 percent of the nation’s projected needs. Therefore, the US State Department has added the platinum group metals to a list of strategic materials that are considered essential for the economy and for the defense of the United States and are unavailable in adequate quantities from reliable and secure suppliers.

Consequently, the search continues for new sources of platinum group metals. A potential source may be in the incrustations of iron and manganese found on the submerged slopes of islands and seamounts throughout the world’s oceans. These metallic crusts and nodules are believed to have formed by extremely slow precipitation from seawater. Although they are composed mostly of iron and manganese, they contain many metals, including those of the platinum group, and the volume of these deposits is staggering. While commercial development is unlikely before the early part of the twenty-first century, these ferromanganese crusts are considered to be an attractive, long-term resource.

Principal Terms

catalyst: a substance that facilitates a chemical reaction but is not consumed in that reaction

immiscible liquids: liquids not capable of being mixed or mingled

layered igneous complex: a large and diverse body of igneous rock formed by intrusion of magma into the crust; it consists of layers of different mineral compositions

mafic/ultramafic: compositional terms referring to igneous rocks rich (mafic) and very rich (ultramafic) in magnesium- and iron-bearing minerals

magma: molten rock material that solidifies to produce igneous rocks

placer: a mineral deposit formed by the concentration of heavy mineral grains such as gold or platinum during stream transport

reef: a provincial ore deposit term referring to a metalliferous mineral deposit, commonly of gold or platinum, which is usually in the form of a layer

specific gravity: the ratio of the weight of any volume of a substance to the weight of an equal volume of water

troy ounce: a unit of weight equal to 31.1 grams; used in the United States for precious metals and gems

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