Encyclopedia of Global Resources part 54 potx

entry and therefore is not covered here. Radon gas, which is ubiquitous, is an end product of uranium decay that is radioactive and emanates from soil, rocks, and hot springs in areas where uranium and thorium are found. Primary Uses The primary uses of these gases are in arc welding, neon lights, fluorescent lights, and lasers. They are also used as Geiger counters and inert atmospheres. Technical Definition The inert or noble gases are Group VIIIA of the peri- odic table of the elements. They are colorless, taste- less, and odorless monoatomic gases. Description, Distribution, and Forms Neon (abbreviated Ne), atomic number 10, has three naturally occurring stable isotopes: neon 20 (90.51 percent), neon 21 (0.27 percent), and neon 22 (9.22 percent). The atomic weight is 20.183, with a boiling point of −246° Celsius and a melting point of −249° Celsius. Argon (Ar),atomic number18, has three naturally occurring stableisotopes: argon 40(99.600 percent), argon 38 (0.0632 percent), and argon 36 (0.3364 percent). The atomic weight is 39.944, with a boilingpoint of −186°Celsius and amelting point of −189° Celsius. Krypton (Kr), atomic number 36, has six naturally occurring stable isotopes (78, 80, 82, 83, 84, and 86), of which 84 is the most abundant (57.0 percent). The atomic weight is 83.80, with a boiling point of −157° Celsius and a melting point of −153° Celsius. One isotope that has been studied, krypton 85, is mainly gen- erated in uranium reprocessing plants but also in nu- clear reactors andas aproduct ofspontaneous fission. The study concluded that the concentration could grow to the point that krypton 85 could produce as much radiation exposure for humans as is the natural background radiation. The outcome of this could be an increase in skin cancer. Xenon (Xe),atomic number54, has ninenaturally occurring stable isotopes. The atomic weight is 131.30, with a boiling point at −112° Celsius and a melting point of −107° Celsius. Although argon has been found in certain igneous rocks with helium, and all the gases have been found in water from hot springs, the atmosphere is still the major source of the noble gases. Dry air contains 0.937 percent (9,370 parts per million) argon, 18 parts per million neon, 1.1 part per million krypton, and 0.086part permillionxenon. Thehigher concentration of argon is thought to be because radioactive potassium 40 decays to argon. The group has been called rare gases or inert gases, but since the atmosphere is almost 1 percent argon, and because krypton and xenon are not totally inert, the name “noble gases” has gained favor. The noble gases are always found as inert, monoatomic gases. Although compounds of xenon and krypton have been formed, they can be formed only under extreme conditions; no compounds occur naturally. Radon (Rn), atomic number 86, is a decay product of radium and occurs in nature as a very dense, odorless, colorless, and highly radioactive gas. Its radioac- tivity, ubiquity, and tendency to accumulate in homes makes it a health hazard and a major contributor to lung cancer. History In 1785, Henry Cavendish found that a very small portion (less than 1/120) of the air could not be reacted in the experiments that reacted oxygen and nitrogen. This clue was not followed, however, and it was 1882 before a noble gas was discovered by Lord Rayleigh and Sir William Ramsay. In experiments to measure the density of gases, Rayleigh found that the density of nitrogen from ammonia and that from air with the oxygen removed were not the same. Ramsay then studied atmospheric nitrogen. By reacting the nitrogen with red-hot magnesium, he isolated a small amount of much denser gas. When its spectrum was examined there were lines that did not match any known element. This new element was named argon, from the Greek word for idle or lazy, because of its inert nature. Ramsay suspected that another element might exist between argon and helium (which had been discovered inthe Sun in1868), astheir atomic weightsof 40 and 4 were so different. In May of 1898, Ramsay and Morris William Travers allowed liquid air to boil away gradually until only a small amount was left. They removed the nitrogen and oxygen with red-hot copper and magnesium. When they examined the spectrum,there were newlines. Thisnew element was named krypton, from the Greek word for hidden. Krypton was a new element of the group, but it was not the one for which they had searched. In June, 1898, Ramsay and Travers liquefied and solidified an argon sample. Instead of keeping the last gas to boil 478 • Gases, inert or noble Global Resources away (which had led them to krypton), they kept the first fraction. When they examined the spectrum it produced, they found a blaze of crimson light unlike that of any other element. The new element was named neon, from the Greek word for new. Ramsay and Travers continued their search for elements using a new liquid-air machine supplied by Ludwig Mond. By repeated fractionation of krypton, a still heavier gas was extracted in July, 1898.The spectrum identified it as a new element, which they called xenon, from the Greek for stranger. It has been known for some time that clathrates, organic hydroxy compounds with large cavities, would contain (but not bond to) the larger noble gases (argon, krypton, and xenon), but it was not until 1962 that compounds of the noble gases were first made by Neil Bartlett. Most of the compounds are xenon, but a few are krypton with fluorine or oxygen. No compounds of neon or argon have been prepared. Obtaining the Noble Gases The noble gases are obtained as a by-product of the liquefaction and separation of air. Dry carbon- dioxide-free air is liquefied and distilled. The volatile fraction contains nitrogen, neon, and helium. The remaining liquid of oxygen, argon, krypton, and xenon is fractionated to yield argon contaminated with oxygen. The oxygen is removed by reaction with hot copper-copper oxide. Further separation of the gases is achieved by selective adsorption and desorption with charcoal. Some argon is obtained as a by-product in the production of ammonia (NH 3 ). The argon is an impurity in the nitrogen and hydrogen gases. About 635,000 metric tons of argon are obtained an- nually. Smaller amounts of the other gases are collected. Uses of Noble Gases The main use of argon is as an inert atmosphere for high-temperature metallurgicalwork.It isalso usedto fill incandescent lamps. The inert atmosphere allows the filamentto burnfor along periodof timebefore it burns out. Argon is also used in lasers and Geiger counters (radiation counters). The naturally occurring presence ofargon isotopesis used todate geological formations. There are two methods that use the amount of argon isotopes to date materials in the mil- lions of years range.One method uses the argon 40 to argon 39 ratio; the other uses the argon 40 to potas - sium 40 ratio. All the noble gases are used in discharge tubes (neon lights). Each gas produces a particular color— for example, red by neon and blue by xenon. Other colors can be produced by a combination of gases. The neon-light industry was started by Georges Claude in theearly 1900’s andgrew into alargeindus- try. Fluorescent tubes are also filled with the noble gases, but the color of the tube depends on the phos- phor coat on the inside of the tube. The denser noble gases, especially argon, have been used to fill the space between layers of glass in thermal insulating windows. Neon is also used in fog lights, television tubes, lasers, andvoltage detectors. Kryptonis used in flashbulbs and ultraviolet lasers. The wavelength of one isotope of krypton is the standard for the metric system. Xenon is also used in ultraviolet lamps, sun- lamps, paint testers, projection lamps, and electronic flashes. Radon has been used in radiation therapy to treat cancers but for the most part has been super- seded by radionuclides. It also has some uses in re- search. C. Alton Hassell Further Reading Greenwood, N. N., and A. Earnshaw. “The Noble Gases: Helium, Neon, Argon, Krypton, Xenon, and Radon.” In Chemistry of the Elements. 2d ed. Bos- ton: Butterworth-Heinemann, 1997. Henderson, William. “The Group18 (Noble Gas) Ele- ments: Helium, Neon, Argon, Krypton, Xenon, and Radon.” In Main Group Chemistry. Cambridge, England: Royal Society of Chemistry, 2000. Israël, H., and G. W. Israël. Trace Elements in the Atmo- sphere. AnnArbor,Mich.: AnnArbor Science,1974. Krebs, Robert E. The History and Use of Our Earth’s Chemical Elements: A Reference Guide. Illustrations by Rae Déjur. 2d ed. Westport, Conn.: Greenwood Press, 2006. Ojima, Minoru, and Frank A. Podosek. Noble Gas Geo- chemistry. 2d ed. New York: Cambridge University Press, 2002. Porcelli, Donald, Chris J. Ballentine, and Rainer Wieler, eds. Noble Gases in Geochemistry and Cosmo- chemistry. Columbus, Ohio: Geochemical Society, 2002. Stern, Rudi. The New Let There Be Neon. Enlarged and updated ed. Cincinnati, Ohio: ST, 1996. Weeks, Mary Elvira. Discovery of the Elements. 7th ed. New materialaddedby HenryM. Leicester.Easton, Pa.: Journal of Chemical Education, 1968. Global Resources Gases, inert or noble • 479 Web Sites Universal Industrial Gases, Inc. Argon (Ar) Properties, Uses, Applications: Argon Gas and Liquid Argon http://www.uigi.com/argon.html Universal Industrial Gases, Inc. Properties, Applications and Uses of the “Rare Gases”: Neon, Krypton, and Xenon http://www.uigi.com/rare_gases.html See also: American Gas Association; Atmosphere; Haber-Bosch process; Helium; Hydrogen; Nitrogen and ammonia; Oxygen. Gasoline and other petroleum fuels Categories: Energy resources; products from resources Gasoline isthe mostimportantproduct from petroleum and is the dominant transportation fuel in the world. Other petroleum products with important fuel uses include kerosene (usually refined to jet fuel), diesel oil for railway locomotives and trucks, and heating oils. Background Petroleum is the source of nearly all the world’s transportation fuels:gasoline forautomobiles, lighttrucks, and light aircraft; jetfuel for airplanes; anddiesel fuel for locomotives, heavy trucks, and agricultural vehicles. Heating oils (also called fuel oils or furnace oils) are used for domestic heating and industrial process heat; they are also usedin oil-fired electric generating plants. Petroleum fuels are a vital component of the energy economies of industrialized nations. The first stepin making all petroleum fuels is distillation of the petroleum or crude oil. Kerosene, diesel oil, and heating oils require comparatively little refining thereafter to be ready for marketing. Consider- able effort is put into gasoline production both to en- sure adequate engine performance and to guarantee that sufficient quantities will be available to meet market requirements. Gasoline The most important characteristic of gasoline is its combustion performance. When gasoline is ignited in the cylinder, the pressure rises as combustion pro - ceeds. The pressure can, potentially, get so high that the remaining unburned gasoline-air mixture deto- nates rather than continuing to burn smoothly. The explosion, which can readily be heard, is usually called “engine knock.” Engine knock puts undue me- chanical stresses on the engine components, is waste- ful of fuel (which the driver will experience as reduced mileage), and reduces engine performance, such as acceleration. Several factors contribute to engine knock. One is the compression ratio of the engine—the ratio of volumes of the cylinder when the piston is at the upward and downward limits of its stroke. Generally, the higher the compression ratio, the more powerful the engine and the greater the acceleration andtop speed ofthe car.A highercompres- sion ratio results in higher pressures inside the cylinder at thestartof combustion. If the cylinder pressure is higher to begin with, the engine is more likely to knock. A second characteristic affecting knocking tendency is the nature of the fuel. The dominant family of chemical components of most gasolines is the paraffins. These compounds contain carbon atoms arranged in chains, either straight (the normal paraffins) or with branches (isoparaffins). Normal paraffins have a great tendency to knock, whereas branched paraffins do not. An octane rating scale was established by assigning the normal paraffin heptane the value 0 and the isoparaffin “iso-octane” (2,2,4-trimethylpentane) the value 100. The octane rating of a gasoline is found by comparing its knocking characteristics (in a carefully calibrated and stan- dardized test engine) to the behavior of a heptane/ iso-octane blend. The percentage of iso-octane in a blend having the same knocking behavior of the gasoline being tested is the octane number of the gasoline. Gasoline is sold in three grades, a regular gasoline with octane number 87, a premium gasoline of about 93 octane, and a medium grade of about 89 octane. Another important propertyofgasoline isits ability to vaporizein the engine,measured bythe vapor pressure of the gasoline. Gasoline with high vapor pressure contains a large number of components that va- porize easily. This is desirable for wintertime driving in cold climates,since easy vaporization helps starting when the engine is cold. It is not desirable for driving in hotweather, because thegasoline could vaporizein the fuel system before it gets to the engine, leading to 480 • Gasoline and other petroleum fuels Global Resources the problem of vapor lock, which temporarily shuts down the engine. Oil companies adjust the vapor pressure of their gasolines depending on the region of thecountry,the local climate,and the seasonof the year. Many process streams within a refinery are blended to produce the gasolines that actually appear on the market. Gaseousmolecules thatwould be by-products of refining can be recombined to produce gasoline in processes called alkylation or polymerization. Some gasoline, called straight-run gasoline, comes directly from distillation of the petroleum. Refinery streams of little value can be converted into high-octane gasoline by catalytic cracking. The octane numbers of straight-run gasoline, or a related product called straight-runnaphtha, can be enhanced bycatalytic re- forming. Other refinery operations can also yield small amounts of material boiling in the gasoline range. Various of these streams are blended to make products of desired octane, vaporpressure, and other characteristics. Environmental concerns about gasoline have cen- tered on theemission of unburned hydrocarbons (including evaporation from fuel tanks), carbonmonox- ide and nitrogen oxide emissions from combustion, and the presence of aromatic compounds, some of which are suspected carcinogens and contribute to smoke or soot formation. These concerns have led to the development of reformulated gasolines. One aspect of production of reformulated gasoline is in- creased vapor pressure, which retards evaporation. A second is removal of aromatic compounds; removal actually complicates formulation because aromatics have desirably high octane numbers. A third step istheaddition ofoxygen-containing compounds, oxygenates, which serve several purposes: They reduce the flame temperature, for example, and change the combustion chemistry to reduce formation of carbon monoxide and nitrogen oxides. Oxygenates also have high octane numbers, so they can make up forthe loss ofaromatics. An exampleof an oxygen- ate useful in reformulated gasoline is methyl tertiary- butyl ether. Jet Fuel Jet fuel is produced by refining and purifying kerosene. Kerosene is a useful fuel, particularly for some agricultural vehicles, but the most important fuel use of kerosene today is for jet aircraft engines. Because many jetplanes flyat high altitudes,where theoutside air temperature is well below zero, the flow character - istics of the fuel at very low temperature are critical. When the fuel is cooled, large molecules of paraffins settle out from the fuelas a waxy deposit.The temperature at which the formation of this wax first begins, noticeable as a cloudy appearance, is called the cloud point. Eventually a fuel can be cooled to an extent where it cannot even flow, not even to pour from an open container.This characteristictemperature isthe pour point. Smoke emissions from jet engines are an environmental concern. The “smoke point” measures an important property of jet fuel combustion. Aromatics are the most likely compounds to produce smoke, while paraffins have the least tendency. A jet fuel with a low smoke point will have a high proportion of paraffins relative to aromatics. The sulfur content of jet fuel can be important, both to limit emissions of sulfur oxide to the atmosphere and because some sulfur compounds are corrosive. Both sulfur and aromatics contents ofjet fuel canbe reduced bytreating withhy- drogen in the presence of catalysts containing cobalt, or nickel, and molybdenum. Diesel Fuel A familiar automobileengine operates by ignitingthe gasoline-air mixture with a spark plug. Diesel engines operate differently: They have nospark plugs,but rely on compression heating of the air in the cylinder to ignite the fuel. A diesel engine has a much higher compression ratio than a comparable spark-ignition engine. Inacrudesense, adieselengine actuallyoper- ates by knocking. The desirable composition for diesel fuel is essentially the inverse of that for gasoline: Normal paraffins are ideal components, while isoparaffins and aromatics are not. The combustion behavior of a diesel fuel is measured by the cetane number, basedon ablend ofcetane (hexadecane),assigned a value of 100, and alpha-methylnaphthalene, assigned 0, as the test components. A typical diesel fuel for automobile and light truck engines would have a cetane rating of about 50. Many of the physical property characteristics of jet fuel are also important for diesel fuel, including the cloud and pour points and the flow characteristics (viscosity) at low temperature. Sulfur and aromatic compounds are a concern. Aromatics are particularly undesirable because they are the precursors to the formation of soot. As environmental regulations con - tinue to become more stringent, refiners will face ad - Global Resources Gasoline and other petroleum fuels • 481 ditional challenges to reduce the levels of these com - ponents in diesel fuels. Heating Oils Heating oils,alsocalled furnaceoils orfuel oils, are often graded and sold on the basis of viscosity. The grades are based on a numerical classification from number 1 to number 6 (though there is no number 3 oil). As the number increases, so do the pour point, the sulfur content, and the viscosity. Number 1 oil is comparable to kerosene. Number 2 is an oil commonly used for domestic and industrial heating. Both have low pour points and sulfur contents and are produced from the distillation of petroleum. The other oils (numbers 4-6) are obtained by treating the resid- uum from the distillation process. They are some- times called bunker oils because they have such high viscosities that they may have to be heated to have them flow up, from the storage tank, or bunker, and into the burners in the combustion equipment. Harold H. Schobert Further Reading Berger, Bill D., and Kenneth E. Anderson. Modern Pe- troleum: A Basic Primer of the Industry. 3d ed. Tulsa, Okla.: PennWell Books, 1992. Black, Edwin. Internal Combustion: How Corporations and Governments Addictedthe World to Oil andDerailed the Alternatives. New York: St. Martin’s Press, 2006. Conaway, Charles F. The Petroleum Industry: A Nontech- nical Guide. Tulsa, Okla.: PennWell Books, 1999. Kunstler, James Howard. The Long Emergency: Sur- viving the Converging Catastrophes of the Twenty-first Century. New York: Atlantic Monthly Press, 2005. Middleton, Paul. A Brief Guide to the End of Oil. Lon- don: Constable and Robinson, 2007. Mushrush, George W., and James G. Speight. Petro- leum Products: Instability and Incompatibility. Wash- ington, D.C.: Taylor & Francis, 1995. Royal Dutch Shell, comp. The Petroleum Handbook. 6th ed. New York: Elsevier,1983. Speight, James G. The Chemistry and Technology of Petro- leum. 4th ed. Boca Raton, Fla.: CRC Press/Taylor & Francis, 2007. Yergin, Daniel. The Prize: The Epic Quest for Oil, Money, and Power. Newed.New York: TheFree Press,2008. See also: Oil and natural gas chemistry; Oil industry; Petroleum refining and processing; Propane; Trans - portation, energy use in. Gems Category: Mineral and other nonliving resources Where Found Mineral gems occur within the Earth’s crust and are widely distributed on the planet. The most important source of the world’s diamonds is the African continent. Emerald hasbeen found primarilyon the South American continent, particularly near Bogotá, Co- lombia. Because sapphire and ruby are color varieties of the same mineral, corundum, they frequently occur in thesame regions. Historically,rubiesand sapphires have been found in Sri Lanka, Burma (Myanmar), Thailand, and Cambodia. Primary Uses All naturally occurring gems are primarily used for jewelry, ornamentation, or decorative purposes. The most beautiful, durable, and uncommon gems are frequently embedded in or dangle from works of gold, silver, or platinum. Historically, kings and queens, aristocrats, popes, and other important soci- etal figures wore these ornaments. In modern society anyone who can afford to purchase the jewels may ac- quire them. Synthetic gems are used in electronics, drills, and cutting tools. Technical Definition A gemstoneis agem that hasbeen cut,ground, orpol- ished from a large rock. Attributes that impart mag- nificent beauty to a gem include flawless crystalline structure, uniformity and intensity of color (or uncommon color), hardness, durability, and extent of transparency and refractivity. All minerals of the Earth, including gems, come from rock. The geological events that yield igneous rock produceall theprecious gemsand mostsemipre- cious gems. Igneous rock is formed upon the cooling of hot molten lava (magma). During this cooling process, the liquid rock solidifies. Description, Distribution, and Forms Gems are minerals of beauty, rarity, and durability. Like all minerals, gems have a definite chemical composition in which the atoms are arranged in a specific pattern. Repetition of this pattern generates a crystal - line shape that imparts a characteristic color, luster, hardness, and transparency to each gem. The tradi - 482 • Gems Global Resources tional precious gems are diamond, emerald, ruby, and sapphire, but any gem may be considered precious if it is uncommonly beautiful. Gems are subdivided into two categories: organic gems (such as pearls, amber, and coral) and mineral gems (from rock or of other geological origin). Or- ganic gems are derived from living or once living or- ganisms. Aside from the traditionalprecious stones— diamond, emerald, ruby, and sapphire—familiar mineral gems include aquamarine, garnet, jade, olivine, topaz, turquoise, and manyforms ofquartz.These “semiprecious” gems have a combination of beauty and affordability that makes them both desirable and marketable. Diamond is found in ultrabasic rock and alluvial deposits. (Alluvial deposits are the deposits that re- main after the physical wearing of rock.) Ultrabasic rock isigneous rock (volcanic)that isessentially made of silicate minerals or ferromagnesian minerals such as olivine, hornblende, augitite, and biotite (mica). Also found in igneous rock are corundum, the mineral of emerald, commonly in hexagonal, elongated, and broadcrystals, aswellas beryl,the mineralof ruby and sapphire, often shaped as bipyramids or barrel- shaped hexagons. Diamonds are valued based upon the overall quality of the diamond, which isassessed by the “four C’s”: color, cut, carat, and clarity. Most gemologists agree that a colored diamond is the rarest gem of all. Transi- tion metal ions,when present in traceamounts within the crystalline structure, impart a light color to an oth- erwise colorless diamond. Colored diamonds range from blue, blue-white, and blue-green to red and yellow. Because the cut of a gem is critical to maximizing both beauty and value, cutting must be done by ex- perts who know exactly where and how to cut a stone to reveal the optimal brilliance (or “fire”) of the crystal’s refractive planes. The size of adiamond isexpressed incarats, where one carat weighs 200 milligrams. In addi- tion tocolor, “fire,”and size,the degreeof flawlessness and the hardness of a stone are importantin determining its finalmar- ket value. The other precious gems are as- sessed for market value in a manner simi- lar to that of diamond. Two of the semiprecious gems, garnet and olivine, are silicate minerals. Garnet stones are often rhombo-dodecahedral (twelve-sided) or hexaoctahedral shapes in nature; these intriguing crystals occur in colors of red, green, and black. Olivine crystals are perfect small cubes with colors of green to brown-green. Garnet occurs commonly in the Earth’s crust in metamorphic rock (formed by the action of heat and pressure) rather than in igneous (volcanic) rock, the origin of most gems. Colorless diamond is made exclusively of carbon atoms arranged in rigid tetra- hedrons. Yet blue, yellow, and other colors of diamond also occurin nature. Chemists discovered that the trapping of certain transition metal ions in the crystal as it forms can result in coloration of the stone. The same fact holds for other precious gems. For example, the mineral corun - Global Resources Gems • 483 Properties of Gem Minerals Gem Material Hardness Specific Gravity Refractive Index Amber 2-2½ 1.05 1.54 Beryl 7½-8 2.67-2.85 1.57-1.58 Chrysoberyl 8½ 3.73 1.746-1.755 Corundum 9 4.00 1.76-1.77 Diamond 10 3.52 2.42 Feldspar 6-6½ 2.55-2.75 1.5-1.57 Garnet* 7 3.88 1.784 Hematite 5½-6½ 5.20 Jade* 6½ 3.15 41.645 Lapis lazuli 5-6 2.4-3.05 1.50 Malachite 3½-4 3.34-3.95 1.66-1.91 Opal 5-6½ 2.15 1.45 Peridot 6½-7 3.34 1.654-1.690 Quartz* 7 2.63 1.541 Spinel 8 3.60 1.72 Spodumene 6-7 3.18 1.66-1.676 Topaz 8 3.53 1.61-1.62 Tourmaline 7-7½ 3.06 1.624-1.644 Turquoise 5-6 2.76 1.61-1.65 Zircon* 6½ 4.35 1.88 *Average for a variety of types. Source: Data are derived from Sybil P. Parker, ed., McGraw-Hill Concise Encyclopedia of Science and Technology, 2d ed., 1989. dum (Al 2 O 3 ) is an oxide of aluminum, a hard, white substance. The presence of transition metal impuri- ties within the corundum crystal results in colorful gems. Specifically, ruby is corundum with chromium cations, which give the crystal a rich red color. Iron and titanium cations cause the brilliant blue of the sapphire, while iron cations give oriental topaz its yellow color. Finally, oriental amethyst acquires its violet color from chromium andtitanium cationswithin the corundum lattice. 484 • Gems Global Resources Data from the U.S. Geological Survey, . U.S. Government Printing Office, 2009.Source: Mineral Commodity Summaries, 2009 720,000 1,100,000 350,000 2,200,000 23,300,000 600,000 6,100,000 230,000 210,000 Carats 30,000,00020 0,000,0015 0,000,00 10,000,000 5,000,000 Tanzania Russia Namibia Guyana Guinea Ghana Sierra Leone South Africa Other countries 10,000,000 230,000 25,000,000 200,000 18,000,000 470,000 100,000 5,400,000 210,000 Congo, Democratic Republic of the Canada Brazil Botswana Australia Angola Central African Republic China Côte d’Ivoire 25 0,000,00 Top Nations Producing Gem Diamonds, 2008 Similarly, silicon dioxide (SiO 2 ), or quartz, is a clear, colorless crystal unless transition metal impuri- ties are present. Then rose, purple, or smoke-gray colors may be produced. Another example is found in the simple arrangement of sulfate (SiO 4 ) tetrahe- drons about a metal cation; when they are around a magnesium cation, olivine (Mg 2 SiO 4 ) results, but when around a zirconium cation, zircon (ZrSiO 4 )is formed. Mineral gems are widely distributed on the Earth. Gems have been found on all continents except Ant- arctica. (Antarctic exploration for this purpose has not yet occurred.) A country that has yielded a great variety and abundance of gems is Sri Lanka. This small island has yielded more than a dozen different types of gemstones. Located just south of the tip of India, which itself is famous for its diamonds and emeralds, Sri Lanka has rich alluvial deposits. The African continenthas been themain source of all known diamonds. Aside from India, other mini- mally productive diamond sources have been Brazil and the Democratic Republic of the Congo. Within the United States, Arkansas has yielded the most diamonds, although the number is very low in compari- son with the other regions mentioned. Even less productive mines have been found on the eastern slopes of the Appalachian Mountains. Rubies are found in Sri Lanka, Burma, Thailand, and Cambodia;sapphiresoccur inthesame regionsas rubies, since both are color varieties of corundum. Both ruby and sapphire stones are found in Russia, China, Germany,India, and Australia as wellas on the African continent. In the United States, a small number of rubies have been found in North Carolina, while Montana has provided a mining site for small but exceptionally brilliant blue sapphires. Emerald has primarily been found on the South American continentnear Bogotá,Colombia. Another important source of emeralds is Siberia; emeralds have also been mined in Brazil,Egypt, Austria, Zimba- bwe, Mozambique, Tanzania, and South Africa. Within the United States pale, muted green emeralds with many flaws have been found in North Carolina. These crystals are poor-quality gems and have little value. Emerald is a color variety of the mineral beryl; another color variety ofberyl is the semiprecious gem aquamarine. Aquamarine is more abundant in the Earth’s crust than emerald and is plentiful in North - ern Ireland, Italy, Russia, Namibia, and Brazil, where the largest (110.5 kilograms) aquamarine stone was found in 1910. Other semiprecious stones are as di - verse in their distribution as the precious gems are, but most of these stones are more common. Synthetic garnets are useful in industry. One synthetic gem, yttrium iron garnet, is used in microwave devices. Another, yttrium aluminum garnet (YAG), is used in lasers as a source of coherent light; it is also used asan artificial gemstone.On themolecular level, the crystalsof synthetic gems are subtlydifferent from those of natural stones. These differences, however, typically evade the untrained eye. For this reason, synthetic gemstones are used in jewelry but cannot be sold as fine jewels. History Since ancient times, gems have been used to adorn the humanbody and createartwork. Althoughno one knows when gems were first discovered, desired, or used, there is archaeological evidence that beads of garnet were worn by people of the Bronze Age five thousand years ago. The Old Testament refers to a variety of gems, including amethyst,diamond, emerald, malachite, and cinnabar. It is known that, in the first century b.c.e., emerald was the preferred stone of Cleopatra VII, the last queen of ancient Egypt. Emeralds represented regeneration and spring in some ancient societies. The Incan civilization used the rich green stones to guard sacred temples. Emer- alds and rubies are among the rarest of gems, and carat per carat their monetary value often exceeds that of most colorless diamonds. There is virtually no such thing as a truly flawless emerald, as internal frac- tures mar the interior of the crystals. True ruby (or “oriental ruby”) is the only subclass of corundum to have adistinct categoryof its own.When asterism(the appearance of a six-rayed star) is found in a stone, it is coveted even more; legend has it that asterism con- quers evil forces. Excluding red ruby, all corundum is classified as sapphire, which may range in color from clear yellow, green, and lavender to the traditional cornflower blue. Other corundum gems may also have asterism; the Star of India, a 563-carat, blue-gray stone, is the largest sapphire known. Historically, diamondhas beenthe mostimportant of the precious stones. The word “diamond” is derived from adamas, a Greek term meaning “invinci- ble” or “unconquerable.” The earliest recorded reference to diamond comes from a civilization in India during the fourth century b.c.e. Until the eighteenth century, India was believedto be the only source of di - Global Resources Gems • 485 amonds. Then, in the early 1800’s, small, productive diamond mines were discovered in Brazil. In 1867, the first of the rich South African mines was found. India and Africa have produced the largest and most famous diamonds known, including the Hope, the Victoria-Transvaal, the Cullinan, and the Koh-i-Noor diamonds. In 1902, Auguste Verneuil, a researcher in Paris, was ableto grow red crystalsof beryl in the laboratory. Thus, the first synthetic gem was a ruby. More re- cently, chemists have been able to synthesize diamonds for industrial use. The General Electric Com- pany achieved laboratory synthesis of diamond in 1955. Because events in nature cannot be exactly mimicked in the laboratory, synthetic diamonds lack the aesthetic appeal of naturally occurring stones; therefore they are not used in fine jewelry making. A synthetic substanceused tomimic diamondin jewelry is cubic zirconium (imitation diamond or faux diamond). This material can be synthesized in bulk and at a low cost. Although the durability, refractivity, and transparency of the stone resemble diamond, zirconium lacks its hardness and durability. Obtaining Gems Traditional mining methods have been used to mine gems. Historically, mines were operated through the exploitation of imported slaves or local natives. Typically the laborers were used to dig pits deep into the Earth. The removed earth was pulverized and sifted through, either using water to flush away the gravel or using dry siftingmethods. These techniques were used inthe minesof Africa, Brazil,and Colombia during theeighteenth and nineteenthcenturies. Afri- can mines have become more mechanized, using large drills and other equipment, but mining with manual labor continues in Colombia. As in the past, there are risks of mine collapse and suffocation. Gems can also be collected by sifting through alluvial deposits along the edges of streams and rivers. The gems of India and Sri Lanka have mostly been collected by this method. Miners in Sri Lanka still use the bottoms of their feet to feel for gems within the stones under running river water. In Thailand miners continue to take their boats out in low tide to dredge the mud for gems. Uses of Gems Aside from having aesthetic appeal, some gems are useful in industry and instrumentation. For example, diamond, the hardest substance known, has been used asa cutting tool.In miningand exploratorygeol- ogy, diamonddrills areused tocut through stonesand layers of rock. Additionally, finely powdered diamond is used to grind, shape, and polish large diamond stones as well as other gemstones. Synthetic diamond has replaced the natural gem for industrial tools, while syntheticruby andsapphire are usedto make lasers that emit coherent light and in microwave devices. Ranking just below diamond on the Mohs hardness scale, ruby and sapphire gems are also used in cutting, grinding, and drilling tools. Mary C. Fields Further Reading Bonewitz, Ronald Louis. Rock and Gem. New York: DK, 2005. Chatterjee, Kaulir Kisor. “Gemstones—Miscella- neous.” In Uses of Industrial Minerals, Rocks, and Freshwater. New York: Nova Science, 2009. Hall, Cally. Gemstones. 2dAmerican ed. Photographyby HarryTaylor. NewYork: DorlingKindersley, 2002. Maillard, Robert, Ronne Peltsman, and Neil Grant, eds. Diamonds:Myth, Magic, andReality.New rev. ed. New York: Bonanza Books, 1984. O’Donaghue, Michael. Gems: Their Sources, Descrip- tions, and Identification. 6th ed. Oxford, England: Butterworth-Heinemann, 2006. Read, P. G. Gemmology. 3d ed. Boston: Elsevier/ Butterworth-Heinemann, 2005. Schumann, Walter. Gemstones of the World. 3d rev. and expanded ed. New York: Sterling, 2007. _______. Handbook of Rocks, Minerals, and Gemstones. Translated by R. Bradshaw and K. A. G. Mills. Bos- ton: Houghton Mifflin, 1993. White, JohnSampson. Mineralsand Gems.Special pho- tography by Chip Clark. Washington, D.C.: Smith- sonian Institution Press, 1991. Zim, Herbert S., and Paul R. Shaffer. Rocks, Gems, and Minerals: A Guide to Familiar Minerals, Gems, Ores, and Rocks. Rev. and updated ed. Revised by Jona- than P. Latimer et al., illustrated by Raymond Perlman. New York: St. Martin’s Press, 2001. Web Sites U.S. Geological Survey Gemstones: Statistics and Information http://minerals.usgs.gov/minerals/pubs/ commodity/gemstones/index.html#mcs 486 • Gems Global Resources U.S. Geological Survey An Overview of Production of Specific U.S. Gemstones http://minerals.usgs.gov/minerals/pubs/ commodity/gemstones/sp14-95/contents.html See also: Abrasives; Aluminum; Beryllium; Crystals; Diamond; Garnet; Geology; Igneous processes, rocks, and mineral deposits; Magma crystallization; Meta- morphic processes, rocks, and mineral deposits; Min- erals, structure and physicalproperties of;Mohs hardness scale; Olivine; Oxides; Pegmatites; Zirconium. General Mining Law Categories: Laws and conventions; government and resources Date: Signed May 10, 1872 The General Mining Law of 1872 was one of several pieces of legislation passed by Congress in the years fol- lowing the Civil War. Its purpose was to combat eco- nomic depression and unemployment by opening up for development the vast federal lands in the West. Amended manytimes overthe years, thislaw continues to govern the exploitation of “hard-rock” minerals in the United States. Background In its original form, the General Mining Law covered all mineral resources on more than 405 million hectares of federal land. Later, it covered only “hard-rock” minerals, thoseassociatedwith igneousand metamorphic rocks. By the Mineral Leasing Act of 1920, the fossil fuels and some minerals were “withdrawn” from coverage under the law. The Common Varieties Min- eral Act of 1955 withdrew sand, gravel, stone, and other commonrocks andminerals. In1976, thelast of the national parks and monuments were withdrawn from coverage, thus protecting them from mining. As a result of these withdrawals, the total land covered under the law was reduced to approximately four hundred million hectares. Provisions The General Mining Law permits U.S. citizens to lay claim to federal land. In exchange, the claimant has only to pay a $100 fee and make minimal annual im - provements (“assessments”) to the land or pay a $100 annual assessment fee. Actual mining need not be done. Claimants possess the right to any mineral deposits below ground; they also possess the right to the exclusive use of theland surface. Claims can be of two types: placer or lode. Placer claims are for 8-hectare sites, whereas lode claims, those designed to exploit localized veins of ore, are for tracts measuring 457 by 183 meters. For a fee of six dollars per hectare (placer claim) or twelve dollars per hectare (lode claim), a claim can be “patented,” or converted to private ownership. Impact on Resource Use Opponents of the law find fault with it in three areas. First, the federal treasury receives no income from minerals taken from lands that belong to the public. Second, the law makes no provision for environmental concerns, which did not exist in 1872. Third, abuses of the law abound, including the resale of claims for thousands of times the original purchase price. Proponents of the law, primarily the major mining companies, argue that while royalties are not paid, mining provides thousands of jobs and significant tax revenue. The mining industry must compete in a global market against companies that exploit cheap labor and are government-subsidized. Whereas the original mining law took no cognizance of environmental concerns, any mining on federal lands is now covered by the same environmental legislation that governs all mining. Proposed modifications to the law revolve around three key issues: royalty payments, patenting, and environmental concerns. Suggestedlevels of royalty pay- ment range from 2 percent on the net value (after taxes and cost) to 8 percent on the gross value of the minerals produced. Either patenting would be elimi- nated or claimants would be allowed to purchase the mining patents for the fair market value of the land surface.Environmental concerns would be addressed by requiring restoration of the land and by using royalty payments to establish a fund for the cleanup of abandoned mine properties. Donald J. Thompson See also: Environmental degradation, resource exploitation and;Mineral Leasing Act;Mineral resource ownership; Mining wastes and mine reclamation; Public lands; United States. Global Resources General Mining Law • 487 . contains 0.937 percent (9,370 parts per million) argon, 18 parts per million neon, 1.1 part per million krypton, and 0.08 6part permillionxenon. Thehigher concentration of argon is thought to be. petroleum fuels Global Resources the problem of vapor lock, which temporarily shuts down the engine. Oil companies adjust the vapor pressure of their gasolines depending on the region of thecountry,the. hydrogen gases. About 635,000 metric tons of argon are obtained an- nually. Smaller amounts of the other gases are collected. Uses of Noble Gases The main use of argon is as an inert atmosphere for high-temperature