Encyclopedia of Global Resources part 103 pdf

Plate tectonics Category: Geological processes and formations The theory of plate tectonics provides an explanation for the present-day structure of the outer part of the Earth. It provides a framework for understanding the global distribution of mountain building, earthquake activity, and volcanism; the geology of ocean basins; various associations of igneous, metamorphic, and sedimentary rocks; and the formation and location of mineral resources. Background Plate tectonic theory is based on a concept of the Earth in which a rigid, outer shell, the lithosphere, lies abovea hotter, weaker, partially moltenpart ofthe mantle known asthe asthenosphere. Thethickness of the lithosphere variesbetween 50and 150kilometers, and itconsists ofcrust andthe underlyingupper mantle. The asthenosphere extends from the base of the lithosphere to a depth of about 700 kilometers. The brittle lithosphere is broken into a pattern of inter- nally rigid plates that move horizontally across the Earth’s surface relative to each other. Seven major plates and a number of smaller ones have been distinguished, and they grind and scrape against one another as they move independently, similar to chunks of ice on water. Most of the Earth’s dynamic activity, including earthquakes and volcanism, occurs along plate boundaries, and the global distribution of these tectonic phenomena delineates the boundaries ofthe plates. Plate Boundaries and Motion Geophysical data, geological observations, and theo- retical deductions support the existence of three ba- sic types of plate boundaries: divergent boundaries, whereadjacent plates move apart(diverge) from each other; convergent boundaries, where adjacent plates move toward each other; and transform boundaries, where plates slip past one anotherin a direction paral- lel to their common boundary. The velocity with which plates move varies from plate to plate and within portions of the same plate, rangingfrom two to twenty centimeters per year. This rate is determined from radioactive dating estimates of the age of the seafloor as a function of distance from mid-oceanic ridge crests (seafloor spreading ridges). Divergent Plate Boundaries At mid-oceanic ridges, or divergent plate boundaries, new seafloor is created from molten basalt (magma) rising from the asthenosphere. A great deal of volcanic activity thus occurs at divergent boundaries. Be- cause of the pulling apart (rifting) of the plates of lithosphere, earthquake activity will also occur along divergent boundaries, and since the rift is caused by magma rising from the mantle, the earthquakes will be frequent, shallow, and mild. 948 • Plate tectonics Global Resources Plate Plate Asthenosphere Divergent Boundary Plate Plate Asthenosphere Transform Fault Boundary Plate Plate Asthenosphere Convergent Boundary Plate Boundaries An example of continental rifting (divergence) in its embryonic stage is seen in the Red Sea, where the Arabian plate has separated from the African plate, creating a new oceanic ridge. Another modern-day example is the East African Rift system, which is the site of active rifting. If it continues, it will eventually fragment Africa, and an ocean will separate the resulting pieces. Through divergence, or rifting, large plates are broken up into smaller ones. Convergent Plate Boundaries Because the Earth is neither expanding nor contract- ing, the increase in lithosphere created along divergent boundaries must be compensated for by the de- struction of lithosphere elsewhere. Otherwise the radius of the Earth would change.At convergent plate boundaries, plates are moving together, and three scenarios are possible depending on whether the crust of the lithosphere is oceanic or continental. If both converging plates are made of oceanic crust, one will inevitablybe older, andthus cooler and denser than the other plate. The denser plate will plunge (sub- duct) below the less-dense plate and descend down into the asthenosphere. This typeof plateboundary is called a subduction zone, and theboundary along the two interacting plates forms a trench. The subducted plate is heated by the hot asthenosphere and, in time, becomes hot enough to melt. Some of the melted material rises buoyantly through fissures and cracks to form volcanoes on the overlying plate, whereas other parts of the melted material will eventually migrate to and rise again at a divergent boundary (spreading ridge). Thus the oceanic lithosphere is constantly being recycled. The volcanoes along the overriding plate may form a string of islands called island arcs. Ja- pan, the Aleutians, and the Marianas are good exam - ples of island arcs resulting from subduction of two plates consisting of oceanic lithosphere. Global Resources Plate tectonics • 949 Types of Boundaries: Divergent Convergent Transform ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ Eurasian Plate Eurasian Plate Pacific Plate North American Plate South American Plate Indo-Australian Plate Antarctic Plate African Plate Major Tectonic Plates and Mid-Ocean Ridges If the leading edge of one of the two convergent plates is oceanic crust but the other leading edge is continental crust, the subduction differs from the case above. Since continental crust is less dense than oceanic, the oceanic plate is always the one subducted. A classical example of this case is the western boundary of South America. On the oceanic side of the boundary, a trench is formed, where the oceanic plate plunges underneath the continental plate. On the continental side, a fold mountain belt (the An- des) is formed as the oceanic lithosphere pushes against the continental lithosphere. As the oceanic plate descends into the mantle, some of the material melts and works its way up through the fold mountain belt to form violent volcanoes. The boundary be- tween the plates is a region of earthquakeactivity, with the earthquakes ranging from shallow to relatively deep, and some are quite severe. The last type of convergent plate boundary involves the collision of two continental masses of lithosphere. When the plates collide, neither is dense enough to be forced into the asthenosphere. Thus the collision com- presses and thickens the continental edges, twisting and deforming the rocks and uplifting the land toform unusually high fold mountain belts. The prototype example is the collision of India with Asia that resulted in the formation of the Himalayas. In this case the earthquakes aretypically shallow,but frequent and severe. Transform Plate Boundaries The actual structure of a seafloor spreading ridge is more complex than a single straight crack. Rather, ridges consist of many short segments slightly offset from one another. The offsets are a special kind of fault, or break in the lithosphere, known as a transform fault, and their function is to connect segments of a spreading ridge. The opposite sides of a transform fault belong to two different plates, and these are moving apart in opposite directions. The transform faults are just boundaries along which the plates move past one another. The classic transform boundary is the San Andreas fault that slices off a sliver of western California that rides on the Pacific plate from the rest of the state, which is on the North American plate. As the two plates scrape past each other, stress builds up and is released in earthquakes. Why Plates Move One mechanism that creates energy tomove the huge plates is convection currents that are driven by heat from radioactive decay in the mantle. These convec - tion currents in the Earth’s mantle carry magma up from the asthenosphere. Some of this magma escapes to form new lithosphere, but the rest spreads out side- ways beneath the lithosphere, slowly cooling in the process. As it flows outward, it drags the overlying lithosphere outward with it, thus continuing to open the ridges. When it cools, the flowing material becomes dense enough to sink back deeper into the mantle at convergent boundaries. A second plate- driving mechanism is the pull of the dense, cold, downward-moving slab of lithosphere in a subduction zone on the rest of the trailing plate, opening up the spreading ridges so magma can move upward. Mineral Deposits The theory of plate tectonics has greatly enhanced understanding of why many mineral deposits form where they do and has thus mademineral exploration more efficient. During the evolution of new oceanic plates and mountain belts by plate tectonics, a large number of mineraldeposits form, particularly in asso- ciation with the plate boundaries. Hot fluids (hydrothermal fluids)circulate at spreading ridges (divergent boundaries) and deposit minerals. For example, niobium deposits are found in the intrusions in the East African Rift zone, and iron and manganese are found in the sediments of the Red Sea. Hydrothermal fluids also flow through the cracks and pores in rock along convergent boundaries and deposit metals along these boundaries as they cool. Good examples are the copper ore deposits associated with the collisional boundary of the Himalayas and tin ores in southwestern England. A general se- quence of minerals foundwhen passinginland from a trench associated with subduction is iron, gold, copper, molybdenum, gold, lead, zinc, tin, tungsten, antimony, and mercury. Alvin K. Benson Further Reading Brown, G. C., and A. E. Mussett. The Inaccessible Earth: An Integrated Approach to Geophysics and Geochemistry. 2d ed. New York: Chapman & Hall, 1993. Cox, Allan, and Robert Brian Hart. Plate Tectonics: How It Works. Palo Alto, Calif.: Blackwell Scientific, 1986. Erickson, Jon. Plate Tectonics: Unraveling the Mysteries of the Earth. Rev. ed. New York: Facts On File, 2001. Hamblin, W. Kenneth, and Eric H. Christiansen. 950 • Plate tectonics Global Resources Earth’s Dynamic Systems. 10th ed. Upper Saddle River, N.J.: Pearson/Prentice Hall, 2004. Kearey, Philip, Keith A. Klepeis, and Frederick J. Vine. Global Tectonics. 3d ed. Hoboken, N.J.: Wiley- Blackwell, 2009. Keller, Edward A., and Nicholas Pinter. Active Tecton- ics: Earthquakes, Uplift, and Landscape. 2d ed. Upper Saddle River, N.J.: Prentice Hall, 2002. Kusky, Timothy. Earthquakes: Plate Tectonics and Earth- quake Hazards. New York: Facts On File, 2008. Montgomery, Carla W. Fundamentals of Geology. 3d ed. Dubuque, Iowa: Wm. C. Brown, 1997. Oreskes, Naomi, ed. Plate Tectonics: An Insider’s History of the Modern Theory of the Earth. Boulder, Colo.: Westview Press, 2001. Van der Pluijm, Ben A., and Stephen Marshak. Earth Structure: An Introduction to Structural Geology and Tectonics. 2d ed. New York: W. W. Norton, 2003. Web Site U.S. Geological Survey This Dynamic Earth: The Story of Plate Tectonics http://pubs.usgs.gov/gip/dynamic/dynamic.html See also: Earth’s crust; Earthquakes; Geology; Hy- drothermal solutions and mineralization; Litho- sphere; Seafloor spreading; Volcanoes. Platinum and the platinum group metals Category: Mineral and other nonliving resources Where Found The platinum metals are extremely rare in the Earth’s crust. All occur together, with platinum and palladium predominating. The mineral sperrylite (platinum arsenide) is a major source in Canada. Signifi- cant deposits are also located in South Africa and the former Soviet Union. Smaller deposits have been found in Colombia, Australia, and the United States, chiefly in Alaska and Montana. Primary Uses The most common application of the platinum metals is as catalysts for various industrial chemical reac - tions. They are also used to make a variety of alloys and are frequently used in jewelry. Technical Definition Chemists generally refer to the block of six transition metals—ruthenium (Ru), rhodium (Rh), palladium (Pd), osmium (Os), iridium (Ir), and platinum (Pt)— as the platinum metals. Their atomic numbers are, respectively, 44, 45, 46, 76, 77, and 78. Using the recom- mended group designations of the International Union of Pure and Applied Chemistry, ruthenium and osmium belong toGroup 8, rhodiumand iridium to Group 9, andpalladium andplatinum toGroup 10. In the older system of group numbering, the platinum metals were placed in Group VIII. Description, Distribution, and Forms Ruthenium has seven naturally occurring isotopes with an average atomic mass of 101.07. It has another thirteen artificial (radioactive) isotopes. Pure ruthenium is ahard metal and has a gray-white appearance. Rhodium has only one natural isotope, with an atomic mass of 102.906. More than thirty artificial isotopes are known. Rhodium has a silvery white metallic luster. Palladium has six natural isotopes with an average atomic mass of 106.42. It has eighteen artificial isotopes. Palladium is a steel-white metal thatdoes not tarnish in air. Osmium has seven natural isotopes and an average atomic mass of 190.2. Ithas nearly thirty artificial isotopes. The metal has a slight bluish color because of a thin surface film of the oxide. Iridium has only two natural isotopes, with an average atomic mass of 192.2. It has nearly forty artificial isotopes. The metal has a white appearance with a slight yellowish tinge and is hard and brittle. Platinum has six natural isotopes and an average atomic mass of 195.08. It has thirty artificial isotopes. It is a silvery-white metal with a lustrous appearance. Ruthenium, rhodium, and palladium all have densities of about 12 grams per cubic centimeter (12.45, 12.41, and 12.02, respectively), while osmium, iridium, and platinum are about twice as dense (22.61, 22.65, and 21.45 grams percubic centimeter, respectively). The melting points increase in the order: palladium, platinum, rhodium, ruthenium, iridium, and osmium—1,554°, 1,772°, 1,966°, 2,310°, 2,410°, and 3,054° Celsius, respectively. The boiling points increase in the order: palladium, rhodium, platinum, ruthenium, iridium, and osmium— 3,140°, 3,727°, 3,827°, 3,900°, 4,130°, and 5,027° Cel- sius, respectively. The platinum metals are among the rarest of all nonradioactive elements in the Earth’s crust. As a group, they are strong siderophiles (they tend to be Global Resources Platinum and the platinum group metals • 951 concentrated in the Earth’s metallic core). Conse - quently, they are generally found in areas rich in other transition metals such as nickel and copper. In these concentrated regions, the abundance of theplatinum metals can be more than a million times that of the crustal average. Most ofthe world’s platinum resources come from sulfide ores of magmatic origin found in large stratiform bodies of basaltic rocks. The annual world production of all the platinum metals totals only about 500 metric tons. By contrast, millions of metric tons of copper are produced worldwide annually. Since the quantity of platinum metals mined is compara- tively small, the environmental impact of these metals is minimal. None of the six metals has any significant biological role in the plant or animal kingdoms. As a group, the platinum metals have the lowest abundances of nearly all nonradioactive elements in the Earth’s crust; only gold, rhenium, and bismuth are metals with comparable low abundances. Values range from 0.0001 part per million (ppm) for ruthenium to about 0.015 ppm for palladium. In regions where the platinum metals are concentrated (Can- ada, South Africa, and the former Soviet Union), levels of platinum reach 0.5 part to 20 parts per million. However, the platinum metals frequently occur in ores that contain large quantities of other metals, such as nickel, making recovery of the platinum metals commercially feasible. The low crustal abundances of the platinum metals and the factthat their minerals usually occur as small inclusions (less than 1 millime- ter) in other minerals hindered the development of platinum metals mineralogy. With the development of the electron microprobe in the 1960’s and itsability to analyze mineral particles as small as 10 microme- ters, the mineralogy of the platinum metals was greatly enhanced. More than eighty clearly defined minerals containing the platinum metals have been identified, most of which contain palladium and platinum. These minerals are generally compounds with other elements such as sulfur, selenium, tellurium, ar- senic, and antimony, or alloys with metals such as tin, lead, and bismuth. Several hundred less clearly defined minerals have also been detected. In the Canadian deposits, platinum occurs in copper-nickel sulfide ores that are associated with the igneous rock norite. The South African ores are pre- dominantly pyroene as well as chromite and sulfides of iron, copper, and nickel. Platinum is also found in native metallic form alloyed with iron or in mineral form as the sulfide or arsenide. Iridium, osmium, ru - thenium, and rhodium generally occur uncombined in nature; they can also be considered by-products of the transition metals mining industry. Osmium and iridium occur alloyed as iridosmine (also known as os- miridium). This alloy also contains varying amounts of platinum, ruthenium, and rhodium,depending on the location. Palladium is the most reactive of the platinum metals, and it readily dissolves in acids. At room tempera- ture it has the unusual property of absorbing up to nine hundred times its own volume of hydrogen. Pal- ladium is highly malleable and can be beaten into sheets as thin as 0.000002 centimeter thick. Iridium has the greatest resistance to corrosion of any metal. It was used to make the old standard meter bar in Paris, which was an alloy of platinum (90 percent) and iridium (10 percent). Iridium levels in certain regions have been related to meteor impacts on Earth and have been used tostudy geological andbiological processes such as extinction. Levels of iridium in meteors are generally higher than levels found on Earth. High terrestrialiridium levels in rocks from the Cretaceous- Tertiary boundary have providedevidence that extensive meteor impacts could have played a role in the Earth’s geologic history. History Hundreds of years before Europeans explored the Americas, the Indians of Colombia and Ecuador used platinum-gold alloys to make small artifacts by heat - 952 • Platinum and the platinum group metals Global Resources Platinum and Paladium: World Mine Production, 2008 Kilograms Nation Platinum Palladium Canada 7,200 12,500 Colombia 1,700 — Russia 25,000 88,000 South Africa 153,000 80,000 United States 3,700 12,400 Zimbabwe 5,600 4,400 Other countries 3,500 8,300 Source: Data from the U.S. Geological Survey, Mineral Commodity Summaries, 2009. U.S. Government Printing Office, 2009. ing and hammering the alloy. Because of platinum’s high melting point, it was not possible for these peo- ple to melt and work pure platinum. In their relent- less search for gold in the late seventeenth century, the invading Spanish conquistadores discovered the Indians’ platinum. Being a white-colored metal it was called platina, derived from the Spanish word for silver. Initially it was considered a rather annoying con- taminant rather than a precious metal. By the mid- eighteenth century, samplesof platinum had reached Europe. In 1803, William Wollaston produced the first pure samples of the metal after dissolving crude platinum in aqua regia (a mixture of hydrochloric and nitric acids). In the same year, Wollaston’s studies with platinum ores led to his discovery of two new metals, palladium and rhodium, in the samples of crude platinum. After dissolving the ore in aqua regia and neutralizing it with sodium hydroxide, he added ammonium chlo- ride to remove the platinum as ammonium chloro- platinate. By then adding mercuric cyanide, he removed the palladium as palladium cyanide. Metallic palladium was recovered by reduction of the palladium cyanide compound. After the palladium cyanide was extracted, the residue was washed and dried; it yielded a red compound of rhodium, which was reduced to the metal itself. Because many of the rhodium compounds Wollaston prepared were pink to red in color, he named the new metal from the Greek word rhodon, meaning rose. Palladium was named after the asteroid Pallas. Both osmium and iridium were also discovered in 1803. In London, Smithson Tennant showed that the black metallic substance remaining after reacting platinum ores with aqua regia was actually a mixture of two new metals. He named one iridium (from the Latin iris, meaning rainbow) because it formed many colored compounds. The other new metal he called osmium (from the Latin osme, meaning odor) because of its unpleasant smell. Ruthenium was the last platinum metal to be discovered. In 1808, the Polish chemist J ò drzej Kniadecki claimed to have discovered and extracted a new metal from platinum ores. Since others were unable to re- produce Kniadecki’s work, his discovery was soon dis- missed. In the mid-1820’s, extensive alluvial deposits of platinum were discovered in the Russian Ural Moun- tains. Soon after, platinum coins were minted and is - sued by the Russian government. As a result of the new mining industry, scientists began to examine the insoluble residues that were produced from the plati - num refining. In 1828, Gottfried Osann claimed to have discovered three new metals in these residues. However, it was not until 1844, when Karl Ernst Klaus showed that there was only one new metal in the residues, that ruthenium was actually isolated and shown to be a new element. Its name was taken from Ruthenia, the Latin name for Russia. Obtaining Platinum Metals Because of platinum metals’ low natural abundances and the difficultyin extracting them, commercial production of the platinum metals is often viewed as a by- product of the mining of other metals such as nickel, copper, and silver. For example, if not for the huge tonnage of nickel ore processed annually, the extrac- tion of the rarer platinum metals would not be economically feasible. In general, the platinum metals are obtained by subjecting the ores to a series of com- plicated and costly chemical reactions. Not surpris- ingly, the platinum metals are among the most expen- sive of all elements to produce. Prices can fluctuate enormously depending on economic and environmental conditions. The high cost and rarity of the platinum metals are also responsible for their extensive recycling. Platinum is obtained from crude ores by a process which eliminates other impurities: Magnetic metals such as iron and nickel are removed with powerful electromagnets; less dense impurities are removed by flotation methods in aqueous solution; volatile impurities are baked off at high temperatures; various acids dissolve away other metals. Pure platinum is obtained through additional chemical processes. The method for separating palladium from platinum is often determined by the type of ore being refined, but in general also involves a series of chemical processes to obtain the metal. Like platinum, iridium is separated by treating the other accompanying platinum metals as impurities and removing them stepwise. Treatment with molten lead, followed by aqua regia, and then baking at 2,000° Celsius concentrates iridium. Ruthe- nium and osmium are converted into highly volatile (and toxic) tetroxide compounds that can easily be collected by distillation. Reaction with base converts them to safer substances, such as sodium osmate, which are then reduced to the metal. Rhodium is obtained from the residue remaining after the removal of platinum, and the total world production of this rare metal is only a few metric tons annually. Global Resources Platinum and the platinum group metals • 953 Uses of Platinum Metals Of the six platinum metals, palladium and platinum have the greatest economic importance. Both are fairly soft metals having a brilliant silvery appearance and are therefore widely used in the jewelry trade. When alloyed with palladium, gold takes on a silvery appearance (white gold) but will not tarnish as jewelry made from pure silver does. Palladium is also used in dentistry, surgical instruments, and electrical contacts. The mainsprings of many older wristwatches were fashioned from palladium. Powdered palladium is a good catalyst and is used for hydrogenation and dehydrogenation reactions. Platinum is used to make wires and vessels for laboratory use and as a coating on missile nose cones and jets, which are subject to very high temperatures. It is also used to make medical and dental alloys and electrical contacts. Finely di- vided platinum powder is an excellent catalyst that is used in the production of sulfuric acid and in petro- leum refining. The uses of theother fourplatinum metalsare very limited. Their major use is in alloys, and most have some catalytic activity. All are fairly brittle metals and therefore difficult to machine into shapes when pure. Ruthenium, rhodium, and iridium are all used as hardening agents for softer platinum and palladium. Osmium is used to strengthen alloys where frictional wear must be minimized as in electrical switch contacts, ballpoint pen tips, phonograph needles, and in- strument pivots. Rhodium lends itself readily to elec- troplating and has been used to protect silver objects from tarnishing, on optical instruments, and on high- grade reflectors for searchlights. Because of its resistance, iridium has been used for spark-plug elec- trodes in aircraft engines. When alloyed with other metals, such as titanium, the presence of platinum metals can enhance corrosion resistance. Literally thousands of chemical compounds that contain the platinum metals have been prepared, and many play important roles as industrial catalysts. Considerable research has revealed that some platinum compounds can inhibit the growth of certain tumors and therefore have applications in chemotherapy. Nicholas C. Thomas Further Reading Greenwood, N. N., and A. Earnshaw. “Nickel, Palla- dium, and Platinum.” In Chemistry of the Elements.2d ed. Boston: Butterworth-Heinemann, 1997. Heiserman, David L. Exploring Chemical Elements and Their Compounds. Blue Ridge Summit, Pa.: Tab Books, 1992. Lide, David R., ed. CRC Handbook of Chemistry and Phys- ics: A Ready-Reference Book of Chemical and Physical Data. 85th ed. Boca Raton, Fla.: CRC Press, 2004. McDonald, Donald, and Leslie B. Hunt. A History of Platinum and Its Allied Metals. London: Johnson Matthey, 1982. Weeks, Mary Elvira. Discovery of the Elements. 7th ed. New material added by HenryM. Leicester.Easton, Pa.: Journal of Chemical Education, 1968. Web Sites Natural Resources Canada Canadian Minerals Yearbook, Mineral and Metal Commodity Reviews http://www.nrcan-rncan.gc.ca/mms-smm/busi- indu/cmy-amc/com-eng.htm U.S. Geological Survey Platinum-Group Metals: Statistics and Information http://minerals.usgs.gov/minerals/pubs/ commodity/platinum See also: Alloys; Canada; Metals and metallurgy; Na- tive elements; Nickel; Russia. Plutonic rocks and mineral deposits Categories: Geological processes and formations; mineral and other nonliving resources Plutonic rocks are formed by slow magma crystallization below the Earth’s surface. Erosion can expose plutonic formations containing valuable mineral deposits, including metallic resources vital to industry. Background Plutonic rocks (from Pluto, Roman god of the under- world) form by slow crystallization of molten silicate magma intruded below the Earth’s surface. Subse- quent erosion may expose “plutons” that have the area of a large house or huge granitic “batholiths” that encompass thousands of square kilometers. Many plutons are sources of valuable industrialand metallic mineral deposits, including granite building stone, gold, copper, molybdenum, chromium, and other metals. Plutonic bodies occur on all the major conti - 954 • Plutonic rocks and mineral deposits Global Resources nents of the Earth. They are common in Precambrian shield areas, the ancient cores of continents com- posed of rocks formed billions of years ago, such as the Canadian Shield of North America. They also occur in younger mountain ranges such as the Rockies and the Appalachian Mountains of North America. Most of the major metallic resources consumed by modern industries—and thus many commercial products—have their origin in plutonic rocks. The rocks themselves may be cut as building stones or used for monuments and art objects. Plutons are also an important source of metal ores, including gold, silver, platinum, chromium, copper, molybdenum, lithium, beryllium, and nickel. Plutonic rocks crystallize from igneous intrusions of molten magma that cool slowly beneath the Earth’s surface. Plutonic intrusions underlie many volcanic areas where magma has made its way to the surface. Later erosion by water or glacial scouring removes most or all traces of the overlying volcanic material and other overburden to expose thepluton at the surface. Plutonic igneous rocks can be distinguished from volcanic rocks by their grain size: The slowly cooled plutonic rocks allow for relatively large grain sizes (from a few millimeters to several centimeters in diameter) compared with very fine-grained, quickly cooled volcanic rocks. Types of Plutons and Plutonic Rocks Major plutonic rock bodies in the world can be di- vided into two major compositions based mainly on silica (SiO 2 ) content: “felsic-intermediate” rocks, in which silica content ranges from about 52 to more than 70 percent (examples include granite and diorite), and “mafic-ultramafic” rocks, in which silica is much lower, 52 down to about 45 percent SiO 2 (examples are gabbro and peridotite). Light-colored felsic-intermediate, or “granitic,” plutons are the most common type and include relatively small “stocks” (surface exposures of tens of square kilometers) up to immense “batholiths” that cover hundreds or thousands of square kilometers. The largest of these in North America are the Idaho, Boulder (Montana), Sierra Nevada (California), and Southern California batholiths. Many large stocks and batholiths also occur in theCanadian Shieldof Canada and the northern states of Minnesota, Wisconsin, and New York. Granitic plutons also occur in every conti - nental shield area (ancient core) of every continent and in most of the world’s major mountain ranges. Less common are the iron-rich, dark-colored mafic- ultramafic plutons. Most of these bodies consist of relatively iron-rich basaltic magma (like the dark lava flows in Hawaii) that, upon intrusion, crystallize dense minerals that settle to the bottom of the magma cham- ber to form mineralogically distinct layers. These layered rocks may contain economically important ore minerals. Prominent examples are the Muskox intrusion (Canada), the Skaergaard complex (Greenland), the Stillwater complex (Montana), and the metallic ore-rich intrusions in South Africa (Bushveld, Great Dyke). Ore Deposits in Felsic-Intermediate Plutons Granitic plutons are the source of many valuable mineral commodities, either directly from the rock itself or indirectly as placer minerals (for example, gold or cassiterite in stream sediments). Some important sedimentary ore deposits form by deposition of minerals leached from granitic plutons by groundwater (for example, many uranium ores). Ore minerals may also occur in hydrothermal (deposited by hot water) quartz veins as in the case of native gold. These veins may occur within the plutons themselves or penetrate adjacent “country rocks.” Most North American copper mines (principally in New Mexico, Arizona, and Utah) obtain their ore from so-called porphyry copper deposits. These are low-grade deposits (about 0.65 percent copper) of widely disseminated copper sulfide grains within the graniticpluton or inadjacent rocks mined from large open-pit mines. Typical of these mines is the Kennecott Utah Copper mine at Bingham, Utah, thelargestcopper mine in the world. Perhaps the richest source of valuable minerals in granitic plutons is pegmatite deposits, generally small exposures of large crystals, the largest measuring many meters in diameter. Pegmatites are the source of many rare metals (beryllium, lithium, zirconium, bo- ron, tantalum, and niobium) and some valuable gem- stones. Ore Deposits in Mafic-Ultramafic Plutons Because the magma associated with these plutons is low in viscosity (“thin”) compared with felsic-intermediate bodies, many important ore deposits in mafic- ultramafic intrusions arise by gravitysettling ofminer- als. South African chromite deposits, for example, form as thick-layered accumulations of crystallized chromite grains that have settled on the floor of the plutonlike sand falling through thick motor oil. Such Global Resources Plutonic rocks and mineral deposits • 955 ore-rich layers are called “cumulates” (from “accumu - lation”), and include platinum-palladium deposits commonly associated with chromite layers. Similar layered ore bodies involve the formation of large blobs of dense sulfide liquids that separate from the silicate liquidand accumulate on the pluton floor. These “segregation” deposits include some of the richest nickel and copper mines in the world, including the nickel mine at Sudbury, Ontario, the Duluth complex in Minnesota, and the Bushveld of South Af- rica. Gold, silver, and other valuable metals are also mined from these rich deposits. Some iron and titanium deposits were created this wayas well butinvolve the segregation of titanium or iron-rich fluids in mafic plutons, which eventually crystallize the mineral mag- netite (Fe 3 O 4 ; iron ore) or ilmenite (FeTiO 3 ; titanium ore). An example of this type of iron deposit is the Kiruna district in Sweden. Allard Lake, Quebec, is a good example of a segregation titanium mine. John L. Berkley Further Reading Best, Myron G. Igneous and Metamorphic Petrology.2d ed. Malden, Mass.: Blackwell, 2003. Best, Myron G., and Eric H. Christiansen. Igneous Pe- trology. Malden, Mass.: Blackwell Science, 2001. Jensen, Mead L., and Alan M. Bateman. Economic Min- eral Deposits. 3d ed. New York: Wiley, 1979. McBirney, Alexander R. Igneous Petrology. 3d ed. Bos- ton: Jones and Bartlett, 2007. Philpotts, Anthony R., and Jay J. Ague. Principles of Ig- neous and Metamorphic Petrology. 2d ed. New York: Cambridge University Press, 2009. Young, Davis A. Mind over Magma: The Story of Igneous Petrology. Princeton, N.J.: Princeton University Press, 2003. Web Site U.S. Geological Survey Volcanic Rocks http://volcanoes.usgs.gov/images/pglossary/ VolRocks.php See also: Chromium; Copper; Earth’s crust; Gold; Granite; Hydrothermal solutions and mineralization; Igneous processes, rocks, and mineral deposits; Lith- ium; Magma crystallization; Molybdenum; Nickel; Pegmatites; Placer deposits; Platinum and the plati - num group metals; Rare earth elements; Tantalum; Tin; Titanium; Tungsten; Zirconium. Plutonium Categories: Energy resources; mineral and other nonliving resources Plutonium is both very useful and very dangerous. It contributes a significant percentage of the power produced in nuclear reactors, but it is also a radiological poison and a nuclear explosive. Background Plutonium (abbreviated Pu), element number 94 in the periodic table,is a silvery-white metalthat oxidizes readily. Normally hard and brittle, it can be molded and machined if it is alloyed with gallium (0.9 percent by weight). Plutonium has fifteen known isotopes, ranging from plutonium 232 to plutonium 246; all of them are radioactive. Half-lives range from twenty- one minutes for plutonium 233 to eighty-one million years for plutonium 244. The most abundant isotope is plutonium 239, which has a half-life of 24,390 years. Minuscule amounts of plutonium 244 occur naturally in uranium ore, but the only way to obtain usable amounts is to make plutonium in a nuclear reactor. Plutonium is radiotoxic: It harms by radiation. Plu- tonium primarily decays byemission ofan alpha particle (a helium 4 nucleus, a grouping of two neutrons and two protons). Nonetheless, a grape-sized plutonium sample could be safely held in the hand, even though it would feel warm because of its radioactivity. Plutonium’s alpha particle radiation is easily blocked by the outermost layers of a person’s skin. Further- more, plutonium is not easily absorbed by the body. If plutonium were ingested with food or water, almost all of it would be excreted. However, 4 parts per 10,000 might be absorbed and eventually settle in the liver or bones, where the plutonium might produce cancer tens of years later. The maximum long-term body burden of plutonium 239 believed to be safe is less than one microgram. The most toxic form of plutonium is thought to be fine (10 microns in diameter) airborne particles of plutonium oxide. If inhaled, a significant fraction of such particles could be expected to lodge in the lungs. Estimates based on animal studies suggest that 10 mil- ligrams of plutonium particles lodged in the lungs could cause death in about one month. For compari - son, doses a thousand times smallerof anthrax spores, botulism, or coral snake venom will cause death within 956 • Plutonium Global Resources a few hours or days. With proper precautions, pluto - nium can be handled safely. More than fifty years of monitoring plutonium workers at United States nuclear weapons plants has not found any workers who have suffered serious consequences. Plutonium as a Fuel A common type of nuclear power reactor contains natural uranium that has been enriched in the isotope uranium 235. When struck by aneutron, the rare uranium 235 may fission to produce two daughter nu- clei, a few neutrons, and energy. Plutonium 239 is formed when the more common uranium 238 isotope absorbs a neutron. If left in the reactor, plutonium 239 may also absorba neutron and eitherfission or become plutonium 240. Over half of the plutonium produced in a power reactor does fission, and this fission contributes aboutone-third ofthe totalen- ergy produced in the reactor. It takes nearly 3 million metric tons of coal to produce the same amount of energy as 1 metric ton of plutonium 239. The world stock of civilian plutonium is approximately 1,000 metric tons. Eighty percent of it is tied up in used reactor fuel elements. Plutonium in Nuclear Weapons The bomb dropped on Nagasaki, Japan, by the United States in World War II contained 6.1 kilograms of plutonium and had an explosive yield equal to almost 20,000 metric tons of dynamite. The much greater yield of a hydrogen bomb is triggered by detonating a small plutonium bomb. The military organizations of the world possess about 250 metric tons of plutonium. With the ending of the Cold War, the U.S. Department of Energy and Department of Defense declared that 38 metric tons of weapons-grade plutonium (nearly one-half of its stockpile) was surplus plutonium. The two ways that this plutonium is most likely to be disposed of are to use it as fuel in nuclear reactors or to mixit with radioactive waste andmolten glass in a vitrification process. Other Uses Radioisotope thermoelectric generators (RTGs) use plutonium 238 (with a half-life of 86 years) to heat silicon-germanium junctionswhich thenproduce elec- tricity. Having no moving parts, RTGs can be made to be very sturdy and reliable. RTGs are used to pro- vide electrical power for interplanetary spacecraft such as Galileo and the Voyagers, since solar panels are too inefficient beyond the orbit of Mars. RTGs may also be used to power navigational beacons, remote weather stations, and even cardiac pace- makers. Americium 241 formed by the decay of plutonium 241 is a vital constituent of household smoke detectors. Nuclear Terrorism It seems unlikely that terrorists would use plutonium as a radiological poison, because its toxicity is relatively low. Bernard L. Cohen has calculated that 0.45 kilo- gram of plutonium particles dispersed in a large city in the most effective way might produce twenty-seven fatalities tento forty years later. Chemical or biological weapons are probably easierfor terrorists to obtain. The nerve gas sarin,for example, was used by terrorists in the 1995 Tokyo sub- way attack in which thousands were imme- diately overcome; fifty-five hundred peo- ple were injured, and twelve died. Following the breakup of the Soviet Union, there were several cases of smug - Global Resources Plutonium • 957 A ring of weapons-grade plutonium. (United States Department of Energy/ Los Alamos National Laboratory) . organizations of the world possess about 250 metric tons of plutonium. With the ending of the Cold War, the U.S. Department of Energy and Department of Defense declared that 38 metric tons of weapons-grade. died. Following the breakup of the Soviet Union, there were several cases of smug - Global Resources Plutonium • 957 A ring of weapons-grade plutonium. (United States Department of Energy/ Los Alamos. removal of platinum, and the total world production of this rare metal is only a few metric tons annually. Global Resources Platinum and the platinum group metals • 953 Uses of Platinum Metals Of

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