Economic geology is the study of fuels, metals, and other materials from the earth that are of interest to industry or the economy in general. It is concerned with the distribution of resources, the costs and benefits of their recovery, and the value and availability of existing materials. These materials include ore (rocks or minerals possessing economic value) as well as fossil fuels, which embrace a range of products from petroleum to coal. Rooted in several subdisciplines of the geologic sciences—particularly geophysics, structural geology, and stratigraphy—economic geology affects daily life in myriad ways. Masonry stones and gasoline, gypsum wallboard (sometimes known by the brand name Sheetrock) and jewelry, natural gas, and table salt—these and many more products are the result of efforts in the broad field known as economic geology.
Some sources of information in the geologic sciences use a definition of "economic geology" narrower than the one applied here. Rather than including nonmineral resources that develop in and are recovered from a geologic environment—a category that consists primarily of fossil fuels—this more limited definition restricts the scope of economic geology to minerals and ores. Given the obvious economic importance of fossil fuels such as petroleum and its many byproducts as well as coal and peat, however, it seems only appropriate to discuss these valuable organic resources alongside valuable inorganic ones.
The concept of economic geology as such is a relatively new one, even though humans have been extracting metals and minerals of value from the ground since prehistoric times. For all their ability to appreciate the worth of such resources, however, premodern peoples possessed little in the way of scientific theories regarding either their formation or the means of extracting them.
The Greeks, for instance, believed that veins of metallic materials in the earth indicated that those materials were living things putting down roots after the manner of trees. Astrologers of medieval times maintained that each of the "seven planets" (Sun, Moon, and the five planets, besides Earth, known at the time) ruled one of the seven known metals—gold, copper, silver, lead, tin, iron, and mercury—which supposedly had been created under the influence of their respective "planets."
The first thinker who attempted to go beyond such unscientific (if imaginative) ideas was a German physician writing under the Latinized name Georgius Agricola (1494-1555). As a result of treating miners for various conditions, Agricola, whose real name was Georg Bauer, became fascinated with minerals. The result was a series of written works, culminating with De re metallica (On the nature of minerals, 1556, released postthumusly), that collectively initiated the modern subdiscipline of physical geology. (It is worth noting that the first translators of Metallica into English were Lou [d. 1944] and Herbert Clark Hoover [1874-1964]. The couple published their translation in 1912 in London's Mining Magazine, and the husband went on to become the thirty-first president of the United States in 1929.)
Rejecting the works of the ancients and all manner of fanciful explanations for geologic phenomena, Agricola instead favored careful observation, on the basis of which he formed verifiable hypotheses. Regarded as the father of both mineralogy and economic geology, Agricola introduced several ideas that provided a scientific foundation for the study of Earth and its products. In De ortu et causis subterraneorum (1546), he critiqued all preceding ideas regarding the formation of ores, including the Greek and astrological notions mentioned earlier as well as the alchemical belief that all metals are composed of mercury and sulfur. Instead, he maintained that subterranean fluids carry dissolved minerals, which, when cooled, leave deposits in the cracks of rocks and thus give rise to mineral veins. Agricola's ideas later helped form the basis for modern theories regarding the formation of ore deposits.
In De natura fossilium (On the nature of fossils, 1546), Agricola also introduced a method for the classification of "fossils," as minerals were then known. Agricola's system, which categorizes minerals according to such properties as color, texture, weight, and transparency, is the basis for the system of mineral classification in use today. Of all his works, however, the most important was De re metallica, which would remain the leading textbook for miners and mineralogists during the two centuries that followed. In this monumental work, he introduced many new ideas, including the concept that rocks contain ores that are older than the rocks themselves. He also explored in detail the mining practices in use during his time, itself an extraordinary feat in that miners of the sixteenth century tended to guard their trade secrets closely.
Of all known chemical elements, 87, or about 80%, are metals. The latter group is identified as being lustrous or shiny in appearance and malleable or ductile, meaning that they can be molded into different shapes without breaking. Despite their ductility, metals are extremely durable, have high melting and boiling points, and are excellent conductors of heat and electricity. Some, though far from all, register high on the Mohs hardness scale, discussed later in the context of minerals.
The bonds that metals form with each other, or with nonmetals, are known as ionic bonds, the strongest type of chemical bond. Even within a metal, however, there are extremely strong, nondirectional bonds. Therefore, though it is easy to shape metals, it is very difficult to separate metal atoms. Obviously, most metal are solids at room temperature, though this is not true of all: mercury is liquid at ordinary temperatures, and gallium melts at just 85.6°F (29.76°C). Generally, however, metals would be described as crystalline solids, meaning that their constituent parts have a simple and definite geometric arrangement that is repeated in all directions. Crystalline structure is important also within the context of minerals as well as the rocks that contain them.
Whereas there are only 87 varieties of metal, there are some 3,700 types of mineral. There is considerable overlap between metals and minerals, but that overlap is far from complete: many minerals include nonmetallic elements, such as oxygen and silicon. A mineral is a substance that appears in nature and therefore cannot be created artificially, is inorganic in origin, has a definite chemical composition, and possesses a crystalline internal structure.
The term organic does not refer simply to substances with a biological origin; rather, it describes any compound that contains carbon, with the exception of carbonates (which are a type of mineral) and oxides, such as carbon dioxide or carbon monoxide. The fact that a mineral must be of nonvarying composition limits minerals almost exclusively to elements and compounds—that is, either to substances that cannot be broken down chemically to yield simpler substances or to substances formed by the chemical bonding of elements. Only in a few highly specific circumstances are naturally occurring alloys, or mixtures of metals, considered minerals.
Minerals are classified into eight basic groups:
The first group, native elements, includes metallic elements that appear in pure form somewhere on Earth; certain metallic alloys, alluded to earlier; as well as native nonmetals, semimetals, and minerals with metallic and nonmetallic elements. The native elements, along with the six classes that follow them in this list, are collectively known as nonsilicates, a term that emphasizes the importance of the eighth group. (For more about the nonsilicates, as well as other subjects covered in the present context, see Minerals.)
The vast majority of minerals, including the most abundant ones, belong to the silicates class, which is built around the element silicon. Just as carbon can form long strings of atoms, particularly in combination with hydrogen (as we discuss in the context of fossil fuels later in this essay), silicon also forms long strings, though its "partner of choice" is typically oxygen rather than hydrogen. Together with oxygen, silicon—known as a metalloid because it exhibits characteristics of both metals and nonmetals—forms the basis for an astonishing array of products, both natural and man-made, which we examine in brief later.
From the list of parameters first developed by Agricola has grown a whole array of characteristics by which minerals are classified. These characteristics also can be used to evaluate an unknown mineral and thus to determine the mineral class within which it fits. One such parameter is the type of crystal of which a mineral is composed. Though there are thousands of minerals, there are just six crystal systems, or basic geometric shapes formed by crystals. Crystallographers, mineralogists concerned with the study of crystal structures, are able to identify the crystal system (the simplest being isometric, or cubic) by studying a good specimen of a mineral and observing the faces of the crystal and the angles at which they meet.
Minerals also can be identified by their hardness, defined as the ability of one mineral to scratch another. Hardness can be measured by the Mohs scale, introduced in 1812 by the German mineralogist Friedrich Mohs (1773-1839), which rates minerals from 1 (talc) to 10 (diamond.) Though it is useful for geologists attempting to identify a mineral in the field, the Mohs scale is not considered helpful for the industrial testing of fine-grained materials, such as steel or ceramics. For such purposes, the Vickers or Knoop scales are applied. These scales (named, respectively, after a British company and an American official) also have an advantage over Mohs in that they offer a precise, proportional scale in which each increase of number indicates the same increase in hardness. By contrast, on the Mohs scale, an increase from 3 to 4 (calcite to fluorite) indicates an additional 25% in hardness, whereas a shift from 9 to 10 (corundum to diamond) marks an increase of 300%.
Other properties significant in identifying minerals are color; streak, or the appearance of the powder produced when one mineral is scratched by a harder one; luster, the appearance of a mineral when light reflects off its surface; cleavage, the planes across which a mineral breaks; fracture, the tendency to break along something other than a flat surface; density, or ratio of mass to volume; and specific gravity, or the ratio between the mineral's density and that of water. Sometimes minerals can be identified in terms of qualities unique to a specific mineral group or groups: magnetism, radioactivity, fluorescence, phosphorescence, and so on. (For more about mineral characteristics, see Minerals.)
A rock is an aggregate of minerals or organic material, which can appear in consolidated or unconsolidated form. Rocks are of three different types: igneous, formed by crystallization of molten minerals, as in a volcano; sedimentary, usually formed by deposition, compaction, or cementation of weathered rock; and metamorphic, formed by alteration of preexisting rock. Rocks made from organic material are typically sedimentary, an example being coal.
Rocks have possessed economic importance from a time long before "economics" as we know it existed—a time when there was nothing to buy and nothing to sell. That time, of course, would be the Stone Age, which dates back practically to the beginnings of the human species and overlapped with the beginnings of civilization some 5,500 years ago. In the hundreds of thousands of years when stone constituted the most advanced toolmaking material, humans developed an array of stone devices for making fire, sharpening knives, killing animals (and other humans), cutting food or animal skins, and so on.
The Stone Age, both in the popular imagination and (with some qualifications) in actual archaeological fact, was a time when people lived in caves. Since that time, of course, humans have generally departed from the caves, though exceptions exist, as the United States military found in 2001 when attempting to hunt for terrorists in the caves of Afghanistan. In any case, the human attachment to stone dwellings has taken other forms, beginning with the pyramids and continuing through today's masonry homes. Nor is rock simply a structural material for building, as the use of gypsum wallboard, slate countertops, marble finishes, and graveled walkways attests. And, of course, construction is only one of many applications to which rocks and minerals are directed, as we shall see.
As noted earlier, the focus of economic geology is on both rocks and minerals, on the one hand, and fossil fuels, on the other. The latter may be defined as fuel (specifically, coal, oil, and gas) derived from deposits of organic material that have experienced decomposition and chemical alteration under conditions of high pressure. Given this derivation from organic material, by definition all fossil fuels are carbon-based, and, specifically, they are built around hydrocarbons—chemical compounds whose molecules are made up of nothing but carbon and hydrogen atoms.
Theoretically, there is no limit to the number of possible hydrocarbons. Carbon forms itself into apparently limitless molecular shapes, and hydrogen is a particularly versatile chemical partner. Hydrocarbons may form straight chains, branched chains, or rings, and the result is a variety of compounds distinguished not by the elements in their makeup or even (in some cases) by the numbers of different atoms in each molecule, but rather by the structure of a given molecule.
Among the various groups of hydrocarbons are alkanes or saturated hydrocarbons, so designated because all the chemical bonds are filled to their capacity (that is, "saturated") with hydrogen atoms. Included among them are such familiar names as methane (CH4), ethane (C2H6), propane (C3H8), and butane (C4H10). The first four, being the lowest in molecular mass, are gases at room temperature, while the heavier ones—including octane (C8H18)—are oily liquids. Alkanes even heavier than octane tend to be waxy solids, an example being paraffin wax, for making candles.
With regard to octane, incidentally, there is a reason why its name is so familiar, while that of heptane (C7H16) is not. Heptane does not fire smoothly in an internal-combustion engine and therefore disrupts the engine's rhythm. For this reason, it has a rating of zero on a scale of desirability, while octane has a rating of 100. This is why gas stations list octane ratings at the pump: the higher the content of octane, the better the gas is for one's automobile.
In a hydrocarbon chain, if one or more hydrogen atoms is removed, a new bond may be formed. The hydrocarbon chain is then named by adding the suffix yl—hence such names as methyl, ethyl, and so on. This indicates that the substance is an alkane, and that something other than hydrogen can be attached to the chain; for example, the attachment of a chlorine atom could yield methyl chloride. Two other large structural groups of hydrocarbons are alkenes and alkynes, which contain double or triple bonds between carbon atoms. Such hydrocarbons are unsaturated—in other words, if the double or triple bond is broken, some of the carbon atoms are then free to form other bonds. Among the products of these groups is the alkene known as acetylene, or C2H2, used for welding steel. In addition to alkanes, alkenes, and alkynes, all of which tend to form carbon chains, there are the aromatic hydrocarbons, a traditional name that actually has nothing to do with smell.
All aromatic hydrocarbons contain what is known as a benzene ring, which has the chemical formula C6H6 and appears in characteristic ring shapes. In this group are such products as naphthalene, toluene, and dimethyl benzene. These last two are used as solvents as well as in the synthesis of drugs, dyes, and plastics. One of the more famous (or infamous) products in this part of the vast hydrocarbon network is trinitrotoluene, or TNT. Naphthalene is derived from coal tar and used in the synthesis of other compounds. A crystalline solid with a powerful odor, it is found in mothballs and various deodorant disinfectants.
The organic material that has decomposed to create the hydrocarbons in fossil fuels comes primarily from dinosaurs and prehistoric plants, though it just as easily could have come from any other organisms that died in large numbers a long, long time ago. To form petroleum, there must be very large quantities of organic material deposited along with sediments and buried under more sediment. The accumulated sediments and organic material are called source rock.
What happens after accumulation of this material is critical and depends a great deal on the nature of the source rock. It is important that the organic material—for example, the vast numbers of dinosaurs that died in a mass extinction about 65 million years ago (see Paleontology)—not be allowed simply to rot, as would happen in an aerobic, or oxygen-containing, environment. Instead, the organic material undergoes transformation into hydrocarbons as a result of anaerobic chemical activity, or activity that takes place in the absence of oxygen.
Good source rocks for this transformation are shale or limestone, provided the particular rocks are composed of between 1% and 5% organic carbon. The source rocks should be deep enough that the pressure heats the organic material, yet not so deep that the pressure and temperature cause the rocks to undergo metamorphism or transform them into graphite or other non-hydrocarbon versions of carbon. Temperatures of up to 302°F (150°C) are considered optimal for petroleum generation.
Once generated, petroleum gradually moves from the source rock to a reservoir rock, or a rock that stores petroleum in its pores. A good reservoir rock is one in which the pore space constitutes more than 30% of the rock volume. Yet the rock must be sealed by another rock that is much less porous; indeed, for a seal or cap rock, as it is called, a virtually impermeable rock is preferred. Thus, the best kind of seal-forming rock is one made of very small, closely fitting pieces of sediment, for instance, shale. Such a rock is capable of holding petroleum in place for millions of years until it is ready to be discovered and used.
People have known about petroleum from pre-history, simply because there were places on Earth where it literally seeped from the ground. The modern era of petroleum drilling, however, began in 1853, when an American lawyer named George Bissell (1821-1884) recognized its potential for use as a lamp fuel. He hired "Colonel" Edwin Drake (1819-1880) to oversee the drilling of an oil well at Titusville, Pennsylvania, and in 1859 Drake struck oil. The legend of "black gold," of fortunes to be made by drilling holes in the ground, was born.
In the wake of the development and widespread application of the internal-combustion engine during the latter part of the nineteenth and the early part of the twentieth centuries, interest in oil became much more intense, and wells sprouted up around the world. Sumatra, Indonesia, yielded oil from its first wells in 1885, and in 1901, successful drilling began in Texas—the source of many a Texas-sized fortune. An early form of the company known today as British Petroleum (BP) discovered the first Middle Eastern oil in Persia (now Iran) in 1908. Over the next 50 years, the economic importance and prospects of that region changed considerably.
With the vast expansion in automobile ownership that began following World War I (1914-1918) and reached even greater heights after World War II (1939-1945), the value and importance of petroleum soared. The oil industry boomed, and, as a result, many geologists found employment in a sector that offered far more in the way of financial benefits than university or government positions ever could. Today geologists assist their employers in locating oil reserves, not an easy task because so many variables must line up to produce a viable oil source. Given the cost of drilling a new oil well, which may run to $30 million or more, it is clearly important to exercise good judgment in assessing the possibilities of finding oil.
The oil industry has been fraught with environmental concerns over the impact of drilling (much of which takes place offshore, on rigs placed in the ocean); possible biohazards associated with spills, such as the one involving the Exxon Valdez in 1989; and the effect on the
At the present rate of consumption, oil reserves will be exhausted by about the year 2040, but this takes into account only reserves that are considered viable today. As exploration continues, the tapping of United States reserves, such as those in Alaska, will become more and more profitable, leading to increased exploitation of U.S. resources and decreased dependence on oil produced by Middle Eastern states, many of which openly or covertly support terrorist attacks against the United States. In the long run, however, it will be necessary to develop new means of fueling the industrialized world, because petroleum is a nonrenewable resource: there is only so much of it underground, and when it is gone, it will not be replaced for millions of years (if at all).
In the meantime, however, petroleum—a mixture of alkanes, alkenes, and aromatic hydrocarbons—makes the world (or at least the industrialized world) go 'round. Petroleum itself is a raw material from which numerous products, collectively known as petrochemicals or petroleum derivatives, are obtained. Through a process termed fractional distillation, the petrochemicals of the lowest molecular mass boil off first, and those having higher mass separate at higher temperatures.
Natural gas separates from petroleum at temperatures below 96.8°F (36°C)—far lower than the boiling point of water. At somewhat higher temperatures, petroleum ether and naphtha, both solvents (naphtha is used in paint thinner), separate; then, in the region between 156.2°F and 165.2°F (69-74°C), gasoline separates. Still higher temperatures yield other substances, each thicker than the one before it: kerosene; fuel for heating and the operation of diesel engines; lubricating oils; petroleum jelly; paraffin wax; and pitch, or tar. A host of other organic chemicals, including various drugs, plastics, paints, adhesives, fibers, detergents, synthetic rubber, and agricultural chemicals, owe their existence to petrochemicals.
It was stated earlier that both carbon and silicon have the tendency to produce long strings of atoms, usually in combination with hydrogen in the first case and oxygen in the second. This is no accident, since silicon lies just below carbon on the periodic table of elements and they share certain chemical features (see Minerals). Just as carbon is at the center of a vast world of hydrocarbons, so silicon is equally important to inorganic substances ranging from sand or silica (SiO2) to silicone (a highly versatile set of silicon-based products), to the rocks known as silicates.
Silicates are the basis for several well-known mineral types, including garnet, topaz, zircon, kaolinite, talc, mica, and the two most abundant minerals on Earth, feldspar and quartz. (Note that most of the terms used here refer to a group of minerals, not to a single mineral.) Made of compounds formed around silicon and oxygen and comprising various metals, such as aluminum, iron, sodium, and potassium, the silicates account for 30% of all minerals. As such, they appear in everything from gemstones to building materials; yet they are far from the only notable products centered around silicon.
Silicone is not a mineral; rather, it is a synthetic product often used as a substitute for organic oils, greases, and rubber. Instead of attaching to oxygen atoms, as in a silicate, silicon atoms in silicone attach to organic groups, that is, molecules containing carbon. Silicone oils frequently are used in place of organic petroleum as a lubricant because they can with stand greater variations in temperature. And because the body tolerates the introduction of silicone implants better than it does organic ones, silicones are used in surgical implants as well. Silicone rubbers appear in everything from bouncing balls to space vehicles, and silicones are also present in electrical insulators, rust preventives, fabric softeners, hair sprays, hand creams, furniture and automobile polishes, paints, adhesives, and even chewing gum.
Even this list does not exhaust the many applications of silicon, which (together with oxygen) accounts for the vast majority of the mass in Earth's crust. Owing to its semimetallic qualities, silicon is used as a semiconductor of electricity. Computer chips are tiny slices of ultrapure silicon, etched with as many as half a million microscopic and intricately connected electronic circuits. These chips manipulate voltages using binary codes, for which 1 means "voltage on" and 0 means "voltage off." By means of these pulses, silicon chips perform multitudes of calculations in seconds—calculations that would take humans hours or months or even years.
A porous form of silica known as silica gel absorbs water vapor from the air and is often packed alongside moisture-sensitive products,
Earlier, it was stated that an ore is a rock or mineral that possesses economic value. This is true, but a more targeted definition would include the adjective metalliferous, since economically valuable minerals that contain no metals usually are treated as a separate category, industrial minerals. Indeed, it can be said that the interests of economic geology are divided into three areas: ores, industrial minerals, and fuels, which we have discussed already.
The very word ore seems to call to mind one of the oldest-known metals in the world and probably the first material worked by prehistoric metallurgists: gold. Even the Spanish word for gold, oro, suggests a connection. When conquistadors from Spain arrived in the New World after about 1500, oro was their obsession, and it was said that the Spanish invaders of Mexico found every bit of gold or silver ore located at the surface of the earth. However, miners of the sixteenth century lacked much of the knowledge that helps geologists today find ore deposits that are not at the surface.
The modern approach uses knowledge gained from experience. As in Agricola's day, much of the wealth possessed by a mining company is in the form of information regarding the means of best seeking out and retrieving materials from the solid earth. Certain surface geochemical and geophysical indicators help direct the steps of geologists and miners searching for ore. Thus, by the time a company in search of ore begins drilling, a great deal of exploratory work has been done. Only at that point is it possible to determine the value of the deposits, which may simply be minerals of little economic interest.
It is estimated that a cubic mile (1.6 km3) of average rock contains about $1 trillion worth of metals, which at first sounds promising—until one does the math. A trillion dollars is a lot of money, but 1 cu. mi. (equal to 5,280 × 5,280 × 5,280 ft., or 1,609 km3) is a lot of space too. The result is that 1 cu. ft. (0.028 m3) is worth only about $6.79. But that is an average cubic foot in an average cubic mile of rock, and no mining company would even consider attempting to extract metals from an average piece of ground. Rather, viable ore appears only in regions that have been subjected to geologic processes that concentrate metals in such a way that their abundance is usually many hundreds of times greater than it would be on Earth as a whole.
Ore contains other minerals, known as gangue, which are of no economic value but which serve as a telltale sign that ore is to be found in that region. The presence of quartz, for example, may suggest deposits of gold. Ore may appear in igneous, metamorphic, or sedimentary deposits as well as in hydrothermal fluids. The latter are emanations from igneous rock, in the form of gas or water, that dissolve metals from rocks through which they pass and later deposit the ore in other locations.
Mining, a means of extracting not only ores but many industrial minerals and fuels, such as coal, is difficult work fraught with numerous hazards. There are short-term dangers to the miners, such as cave-ins, flooding, or the release of gases in the mines, as well as long-term dangers that include such mining-related diseases as black lung (typically a hazard of coal miners). Then there is the sheer mental and emotional stress that comes from spending eight or more hours a day away from the sunlight, in claustrophobic surroundings.
And, of course, there is the environmental stress created by mining—not just by the immediate impact of cutting a gash in Earth's surface, which may disrupt ecosystems on the surface, but myriad additional problems, such as the seepage of pollutants into the water table. Abandoned mines present further dangers, including the threat of subsidence, which make these locations unsafe for the long term.
Higher environmental and occupational safety standards, established in the United States during the last third of the twentieth century, have led to changes in the way mining is performed as well as in the way mines are left when the work is completed. For example, mining companies have experimented with the use of chemicals or even bacteria, which can dissolve a metal underground and allow it to be pumped to the surface without the need to create actual underground shafts and tunnels or to send human miners to work them.
Industrial minerals, as noted earlier, are nonmetal-containing mineral resources of interest to economic geology. Examples include asbestos, a generic term for a large group of minerals that are highly resistant to heat and flame; boron compounds, which are used for making heat-resistant glass, enamels, and ceramics; phosphates and potassium salts, used in making fertilizers; and sulfur, applied in a range of products, from refrigerants to explosives to purifiers used in the production of sugar.
Just one industrial mineral, corundum (from the oxides class of mineral), can have numerous uses. Extremely hard, corundum in the form of an unconsolidated rock commonly called emery has been used as an abrasive since ancient times. Owing to its very high melting point—even higher than that of iron—corundum also is employed in making alumina, a fire-proof product used in furnaces and fireplaces. Though pure corundum is colorless, trace amounts of certain elements can yield brilliant colors: hence, corundum with traces of chromium becomes a red ruby, while traces of iron, titanium, and other elements yield varieties of sapphire in yellow, green, and violet as well as the familiar blue.
We have only begun to scratch the surface, as it were, of the uses to which minerals can be put: after all, everything—literally, every solid object—that people use is either organic in origin or a mineral. The wide array of applications of minerals is clear from the following list of mineral categories, classified by application: abrasives (corundum, diamond), ceramics (feldspar, quartz), chemical minerals (halite, sulfur, borax), and natural pigments (hematite, limonite).
Lime, cement, and plaster comes from calcite and gypsum, while building materials—both structural and ornamental—are products of agate, as well as the two aforementioned minerals. Table salt is a mineral, and so is chalk, as are countless other products. There are rocks, such as granite and marble, used in building, decoration, or artwork, and then there are "rocks"—to use a word that is at once a geologic term and a slang expression—that appear in the form of jewelry.
Out of all minerals, 16 are important for their use as gems: beryl, chrysoberyl, corundum, diamond, feldspar, garnet, jade, lazurite, olivine, opal, quartz, spinel, topaz, tourmaline, turquoise, and zircon. Not all forms of these minerals, of course, are precious. Furthermore, some minerals provide more than one type of gem: corundum, as we have noted, is a source of rubies and sapphires, while beryl produces both emeralds and aquamarines.
Note that many of the precious gems familiar to most of us are not minerals in their own right but versions of minerals. At least one, the pearl, is not on this list because, with its organic origin, it is not a mineral. Certainly not all minerals are created equal: even in the list of 16 just provided, the name diamond stands out, representing a worldwide standard of value. Yet a diamond is nothing but pure carbon, which also appears in the form of graphite and (with a very few impurities) as coke for burning.
A diamond is unusual, however, in many respects, including the fact that it is basically ahuge "molecule" composed of carbon atomsstrung together by chemical bonds. The size of this formation corresponds to the size of the diamond, such that a diamond of 1 carat is simply agargantuan "molecule" containing about 1022(10,000,000,000,000,000,000,000, or 10 billiontrillion) carbon atoms. Not only is a diamondrare and (when properly selected, cut, and polished) extremely beautiful, it is also extraordinarily hard. At the top of the Mohs scale, it can cutany other substance, but nothing can cut a diamond except another diamond.
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A mixture of two or more metals.
The smallest particle of an element, consisting of protons, neutrons, and electrons. An atom can exist either alone or in combination with other atoms in a molecule.
The joining through electromagnetic force of atoms that sometimes, but not always, represent more than one chemical element.
A substance made up of atoms of more than one element, chemically bonded to one another.
A process whereby materials become compacted, or experience an increase in density.
A type of solid in which the constituent parts have a simple and definite geometric arrangement that is repeated in all directions.
The process wherebysediment is laid down on the Earth's surface.
Capable of being bent or molded into various shapes without breaking.
The study of fuels, metals, and other materials from the earth that are of interest to industry or the economy in general.
A negatively charged particle in an atom, which spins around the nucleus.
Fuel derived from deposits of organic material that have experienced decomposition and chemical alteration under conditions of high pressure. These nonrenewable forms of bioenergy include petroleum, coal, peat, natural gas, and their derivatives.
Minerals of no economicvalue, which appear in nature with ore. Recognition of certain characteristic combinations can help geologists find ore on the basis of its attendant gangue. (The ue issilent, as in tongue.)
In mineralogy, the ability of one mineral to scratch another. This can be measured by the Mohs scale.
Any organic chemical compound whose molecules are madeup of nothing but carbon and hydrogenatoms.
One of the three principal types of rock, along with sedimentary and metamorphic rock. Igneous rock is formed by the crystallization of molten materials, for instance, in a volcano or other setting where plate tectonicprocesses take place.
Nonmetallic minerals with uses for industry.
The appearance of a mineral when light reflects off its surface. Among the terms used in identifying luster are metallic, vitreous (glassy), and dull.
Substances that are ductile, lustrous or shiny in appearance, extremelydurable, and excellent conductors of heat and electricity. Metals have very high melting and boiling points, and some (thoughfar from all) have a high degree of hardness.
One of the three principal varieties of rock, along with sedimentary and igneous rock. Metamorphic rock is formed through the alteration of preexisting rock as a result of changes intemperature, pressure, or the activity offluids. These changes are known as metamorphism.
A naturally occurring, typically inorganic substance with a specific chemical composition and a crystalline structure. Unknown minerals usually can be identified in terms of specific parameters, such as hardness or luster.
An area of geology devoted to the study of minerals. Mineralogy includes a number of subdisciplines, such as crystallography, or the study of crystal formations within minerals.
A scale introduced in 1812 by the German mineralogist Friedrich Mohs (1773-1839) that rates the hardness of minerals from 1 to 10. Ten is equivalent to the hardness of a diamond and 1 that of talc, an extremely soft mineral.
A group of atoms, usually but not always representing more than one element, joined in a structure. Compounds are typically made up of molecules.
The center of an atom, a region where protons and neutrons are located and around which electrons spin.
A metalliferous rock or mineral possessing economic value.
At one time chemists used the term organic only in reference to living things. Now the word is applied to most compounds containing carbon and hydrogen, thus excluding carbonates (which are minerals), and oxides such as carbon dioxide.
The study of the material components of Earth and of the forces that have shaped the planet. Physical geology is one of two principal branches of geology, the other being historical geology.
A positively charged particle in an atom.
An aggregate of minerals or organic matter, which may be consolidated or unconsolidated.
Material deposited at or near Earth's surface from a number of sources, most notably preexisting rock.
One of the three major types of rock, along with igneous and metamorphic rock. Sedimentary rock usually is formed by the deposition, compaction, and cementation of rock that has experienced weathering. It also may beformed as a result of chemical precipitation.
The color of the powder produced when one mineral is scratched byanother, harder one.
Rock that appears in the form of loose particles, such as sand.