Learning Objectives

After reading this module, students should be able to

• know the importance of minerals to society
• know factors that control availability of mineral resources
• know why future world mineral supply and demand is an important issue
• understand the environmental impact of mining and processing of minerals
• understand how we can work toward solving the crisis involving mineral supply

Importance of Minerals

The earth’s crust is composed of many kinds of rocks, each of which is an aggregate of one or more minerals. In geology, the term mineral describes any naturally-occurring solid substance with a specific composition and crystal structure. A mineral’s composition refers to the kinds and proportions of elements making up the mineral. The way these elements are packed together determines the structure of the mineral. More than 3,500 different minerals have been identified. There are only 12 common elements (oxygen, silicon, aluminum, iron, calcium, magnesium, sodium, potassium, titanium, hydrogen, manganese, phosphorus) that occur in the earth’s crust. They have abundances of 0.1 percent or more. All other naturally occurring elements are found in very minor or trace amounts.

Silicon and oxygen are the most abundant crustal elements, together comprising more than 70 percent by weight. It is therefore not surprising that the most abundant crustal minerals are the silicates (e.g. olivine, Mg2SiO4), followed by the oxides (e.g. hematite, Fe2O3).

Other important types of minerals include: the carbonates (e.g. calcite, CaCO3) the sulfides (e.g. galena, PbS) and the sulfates (e.g. anhydrite, CaSO4). Most of the abundant minerals in the earth’s crust are not of commercial value. Economically valuable minerals (metallic and nonmetallic) that provide the raw materials for industry tend to be rare and hard to find. Therefore, considerable effort and skill is necessary for finding where they occur and extracting them in sufficient quantities.

Mineral resources are essential to our modern industrial society and they are used everywhere. For example, at breakfast you drink some juice in a glass (made from melted quartz sand), eat from a ceramic plate (created from clay minerals heated at high temperatures), sprinkle salt (halite) on your eggs, use steel utensils (from iron ore and other minerals), read a magazine (coated with up to 50% kaolinite clay to give the glossy look), and answer your cellphone (containing over 40 different minerals including copper, silver, gold, and platinum). We need minerals to make cars, computers, appliances, concrete roads, houses, tractors, fertilizer, electrical transmission lines, and jewelry. Without mineral resources, industry would collapse and living standards would plummet. In 2010, the average person in the U.S. consumed more than16,000 pounds of mineral resources¹ (see Table Per Capita Consumption of Minerals). With an average life expectancy of 78 years, that translates to about1.3 million pounds of mineral resources over such a person’s lifetime. Here are a few statistics that help to explain these large values of mineral use: an average American house contains about 250,000 pounds of minerals (see Figure Mineral Use in the Kitchen for examples of mineral use in the kitchen), one mile of Interstate highway uses 170 million pounds of earth materials, and the U.S. has nearly 4 million miles of roads. All of these mineral resources are nonrenewable, because nature usually takes hundreds of thousands to millions of years to produce mineral deposits. Early hominids used rocks as simple tools as early as 2.6 million years ago. At least 500,000 years ago prehistoric people used flint (fine-grained quartz) for knives and arrowheads. Other important early uses of minerals include mineral pigments such as manganese oxides and iron oxides for art, salt for food preservation, stone for pyramids, and metals such as bronze (typically tin and copper), which is stronger than pure copper and iron for steel, which is stronger than bronze.

ECONOMIC VALUE OF MINERALS

Minerals that are of economic value can be classified as metallic or nonmetallic. Metallic minerals are those from which valuable metals (e.g. iron, copper) can be extracted for commercial use. Metals that are considered geochemically abundant occur at crustal abundances of 0.1 percent or more (e.g. iron, aluminum, manganese, magnesium, titanium). Metals that are considered geochemically scarce occur at crustal abundances of less than 0.1 percent (e.g. nickel, copper, zinc, platinum metals). Some important metallic minerals are: hematite (a source of iron), bauxite (a source of aluminum), sphalerite (a source of zinc) and galena (a source of lead). Metallic minerals occasionally but rarely occur as a single element (e.g. native gold or copper).

Nonmetallic minerals are valuable, not for the metals they contain, but for their properties as chemical compounds. Because they are commonly used in industry, they are also often referred to as industrial minerals. They are classified according to their use. Some industrial minerals are used as sources of important chemicals (e.g. halite for sodium chloride and borax for borates). Some are used for building materials (e.g. gypsum for plaster and kaolin for bricks). Others are used for making fertilizers (e.g. apatite for phosphate and sylvite for potassium). Still others are used as abrasives (e.g. diamond and corrundum).

Mineral Resource Principles

A geologist defines a mineral as a naturally occurring inorganic solid with a defined chemical composition and crystal structure (regular arrangement of atoms). Minerals are the ingredients of rock, which is a solid coherent (i.e., will not fall apart) piece of planet Earth. There are three classes of rock, igneous, sedimentary, and metamorphic. Igneous rocks form by cooling and solidification of hot molten rock called lava or magma. Lava solidifies at the surface after it is ejected by a volcano, and magma cools underground. Sedimentary rocks form by hardening of layers of sediment (loose grains such as sand or mud) deposited at Earth’s surface or by mineral precipitation, i.e., formation of minerals in water from dissolved mineral matter. Metamorphic rocks form when the shape or type of minerals in a preexisting rock changes due to intense heat and pressure deep within the Earth. Oreis rock with an enrichment of minerals that can be mined for profit. Sometimes ore deposits (locations with abundant ore) can be beautiful, such as the giant gypsum crystals at the amazing Cave of the Crystals in Mexico (see Figure Giant Gypsum Crystals). The enrichment factor, which is the ratio of the metal concentration needed for an economic ore deposit over the average abundance of that metal in Earth’s crust, is listed for several important metals in the Table Enrichment Factor. Mining of some metals, such as aluminum and iron, is profitable at relatively small concentration factors, whereas for others, such as lead and mercury, it is profitable only at very large concentration factors. The metal concentration in ore (column 3 in Table Enrichment Factor) can also be expressed in terms of the proportion of metal and waste rock produced after processing one metric ton (1,000 kg) of ore. Iron is at one extreme, with up to 690 kg of Fe metal and only 310 kg of waste rock produced from pure iron ore, and gold is at the other extreme with only one gram (.03 troy oz) of Au metal and 999.999 kg of waste rock produced from gold ore.

Formation of Ore Deposits

MINERAL DEPOSITS

Minerals are everywhere around us. For example, the ocean is estimated to contain more than 70 million tons of gold. Yet, it would be much too expensive to recover that gold because of its very low concentration in the water. Minerals must be concentrated into deposits to make their collection economically feasible. A mineral deposit containing one or more minerals that can be extracted profitably is called an ore. Many minerals are commonly found together (e.g. quartz and gold; molybdenum, tin and tungsten; copper, lead and zinc; platinum and palladium). Because various geologic processes can create local enrichments of minerals, mineral deposits can be classified according to the concentration process that formed them. The five basic types of mineral deposits are: hydrothermal, magmatic, sedimentary, placer and residual.

Hydrothermal mineral deposits are formed when minerals are deposited by hot, aqueous solutions flowing through fractures and pore spaces of crustal rock. Many famous ore bodies have resulted from hydrothermal deposition, including the tin mines in Cornwall, England and the copper mines in Arizona and Utah. Magmatic mineral deposits are formed when processes such as partial melting and fractional crystallization occur during the melting and cooling of rocks. Pegmatite rocks formed by fractional crystallization can contain high concentrations of lithium, beryllium and cesium. Layers of chromite (chrome ore) were also formed by igneous processes in the famous Bushveld Igneous Complex in South Africa.

Several mineral concentration processes involve sedimentation or weathering. Water soluble salts can form sedimentary mineral deposits when they precipitate during evaporation of lake or seawater (evaporate deposits). Important deposits of industrial minerals were formed in this manner, including the borax deposits at Death Valley and Searles Lake, and the marine deposits of gypsum found in many states.

Minerals with a high specific gravity (e.g. gold, platinum, diamonds) can be concentrated by flowing water in placer deposits found in stream beds and along shorelines. The most famous gold placer deposits occur in the Witwatersrand basin of South Africa. Residual mineral deposits can form when weathering processes remove water soluble minerals from an area, leaving a concentration of less soluble minerals. The aluminum ore, bauxite, was originally formed in this manner under tropical weathering conditions. The best known bauxite deposit in the United States occurs in Arkansas.

Ore deposits form when minerals are concentrated—sometimes by a factor of many thousands—in rock, usually by one of six major processes. These include the following: (a) igneous crystallization, where molten rock cools to form igneous rock. This process forms building stone such as granite, a variety of gemstones, sulfur ore, and metallic ores, which involve dense chromium or platinum minerals that sink to the bottom of liquid magma. Diamonds form in rare Mg-rich igneous rock called kimberlite that originates as molten rock at 150–200 km depth (where the diamonds form) and later moves very quickly to the surface, where it erupts explosively. The cooled magma forms a narrow, carrot-shaped feature called a pipe. Diamond mines in kimberlite pipes can be relatively narrow but deep (see Figure A Diamond Mine). (b) Hydrothermal is the most common ore-forming process. It involves hot, salty water that dissolves metallic elements from a large area and then precipitates ore minerals in a smaller area, commonly along rock fractures and faults. Molten rock commonly provides the heat and the water is from groundwater, the ocean, or the magma itself. The ore minerals usually contain sulfide (S^2-) bonded to metals such as copper, lead, zinc, mercury, and silver. Actively forming hydrothermal ore deposits occur at undersea mountain ranges, called oceanic ridges, where new ocean crust is produced. Here, mineral-rich waters up to 350°C sometimes discharge from cracks in the crust and precipitate a variety of metallic sulfide minerals that make the water appear black; they are called black smokers (see Figure Black Smokers). (c) Metamorphism occurs deep in the earth under very high temperature and pressure and produces several building stones, including marble and slate, as well as some nonmetallic ore, including asbestos, talc, and graphite. (d) Sedimentary processes occur in rivers that concentrate sand and gravel (used in construction), as well as dense gold particles and diamonds that weathered away from bedrock. These gold and diamond ore bodies are called placer deposits. Other sedimentary ore deposits include the deep ocean floor, which contains manganese and cobalt ore deposits and evaporated lakes or seawater, which produce halite and a variety of other salts. (e) Biological processes involve the action of living organisms and are responsible for the formation of pearls in oysters, as well as phosphorous ore in the feces of birds and the bones and teeth of fish. (f) Weathering in tropical rain forest environments involves soil water that concentrates insoluble elements such as aluminum (bauxite) by dissolving away the soluble elements.

Mining and Processing Ore

There are two kinds of mineral mines, surface mines and underground mines. The kind of mine used depends on the quality of the ore, i.e., concentration of mineral and its distance from the surface. Surface mines include open-pit mines, which commonly involve large holes that extract relatively low-grade metallic ore (see Figure Open Pit Mine), strip mines, which extract horizontal layers of ore or rock, and placer mines, where gold or diamonds are extracted from river and beach sediment by scooping up (dredging) the sediment and then separating the ore by density. Large, open-pit mines can create huge piles of rock (called overburden) that was removed to expose the ore as well as huge piles of ore for processing. Underground mines, which are used when relatively high-grade ore is too deep for surface mining, involve a network of tunnels to access and extract the ore. Processing metallic ore (e.g., gold, silver, iron, copper, zinc, nickel, and lead) can involve numerous steps including crushing, grinding with water, physically separating the ore minerals from non-ore minerals often by density, and chemically separating the metal from the ore minerals using methods such as smelting (heating the ore minerals with different chemicals to extract the metal) and leaching(using chemicals to dissolve the metal from a large volume of crushed rock). The fine-grained waste produced from processing ore is called tailings. Slag is the glassy unwanted by-product of smelting ore. Many of the nonmetallic minerals and rocks do not require chemical separation techniques.

MINERAL UTILIZATION

Minerals are not evenly distributed in the earth’s crust. Mineral ores are found in just a relatively few areas, because it takes a special set of circumstances to create them. Therefore, the signs of a mineral deposit are often small and difficult to recognize. Locating deposits requires experience and knowledge. Geologists can search for years before finding an economic mineral deposit. Deposit size, its mineral content, extracting efficiency, processing costs and market value of the processed minerals are all factors that determine if a mineral deposit can be profitably developed. For example, when the market price of copper increased significantly in the 1970s, some marginal or low-grade copper deposits suddenly became profitable ore bodies.

After a potentially profitable mineral deposit is located, it is mined by one of several techniques. Which technique is used depends upon the type of deposit and whether the deposit is shallow and thus suitable for surface mining or deep and thus requiring sub-surface mining.

Surface mining techniques include: open-pit mining, area strip mining, contour strip mining and hydraulic mining. Open-pit mining involves digging a large, terraced hole in the ground in order to remove a near-surface ore body. This technique is used in copper ore mines in Arizona and Utah and iron ore mines in Minnesota.

Area strip mining is used in relatively flat areas. The overburden of soil and rock is removed from a large trench in order to expose the ore body. After the minerals are removed, the old trench is filled and a new trench is dug. This process is repeated until the available ore is exhausted. Contour strip mining is a similar technique except that it is used on hilly or mountainous terrains. A series of terraces are cut into the side of a slope, with the overburden from each new terrace being dumped into the old one below.

Hydraulic mining is used in places such as the Amazon in order to extract gold from hillsides. Powerful, high-pressure streams of water are used to blast away soil and rock containing gold, which is then separated from the runoff. This process is very damaging to the environment, as entire hills are eroded away and streams become clogged with sediment. If land subjected to any of these surface mining techniques is not properly restored after its use, then it leaves an unsightly scar on the land and is highly susceptible to erosion.

Some mineral deposits are too deep to be surface mined and therefore require a sub-surface mining method. In the traditional sub surface method a deep vertical shaft is dug and tunnels are dug horizontally outward from the shaft into the ore body. The ore is removed and transported to the surface. The deepest such subsurface mines (deeper than 3500 m) in the world are located in the Witwatersrand basin of South Africa, where gold is mined. This type of mining is less disturbing to the land surface than surface mining. It also usually produces fewer waste materials. However, it is more expensive and more dangerous than surface mining methods.

A newer form of subsurface mining known as in-situ mining is designed to co-exist with other land uses, such as agriculture. An in-situ mine typically consists of a series of injection wells and recovery wells built with acid-resistant concrete and polyvinyl chloride casing. A weak acid solution is pumped into the ore body in order to dissolve the minerals. Then, the metal-rich solution is drawn up through the recovery wells for processing at a refining facility. This method is used for the in-situ mining of copper ore.

Once an ore has been mined, it must be processed to extract pure metal. Processes for extracting metal include smelting, electrowinning and heap leaching. In preparation for the smelting process, the ore is crushed and concentrated by a flotation method. The concentrated ore is melted in a smelting furnace where impurities are either burned-off as gas or separated as molten slag. This step is usually repeated several times to increase the purity of the metal.

For the electrowinning method ore or mine tailings are first leached with a weak acid solution to remove the desired metal. An electric current is passed through the solution and pure metal is electroplated onto a starter cathode made of the same metal. Copper can be refined from oxide ore by this method. In addition, copper metal initially produced by the smelting method can be purified further by using a similar electrolytic procedure.

Gold is sometimes extracted from ore by the heap leaching process. A large pile of crushed ore is sprayed with a cyanide solution. As the solution percolates through the ore it dissolves the gold. The solution is then collected and the gold extracted from it. All of the refining methods can damage the environment. Smelters produce large amounts of air pollution in the form of sulfur dioxide which leads to acid rain. Leaching methods can pollute streams with toxic chemicals that kill wildlife.

Mineral Resources and Sustainability Issues

Our heavy dependence on mineral resources presents humanity with some difficult challenges related to sustainability, including how to cope with finite supplies and how to mitigate the enormous environmental impacts of mining and processing ore. As global population growth continues—and perhaps more importantly, as standards of living rise around the world—demand for products made from minerals will increase. In particular, the economies of China, India, Brazil, and a few other countries are growing very quickly, and their demand for critical mineral resources also is accelerating. That means we are depleting our known mineral deposits at an increasing rate, requiring that new deposits be found and put into production. Figure Demand for Nonfuel Minerals Materials shows the large increase in US mineral consumption between 1900 and 2006. Considering that mineral resources are nonrenewable, it is reasonable to ask how long they will last. The Table Strategic Minerals gives a greatly approximated answer to that question for a variety of important and strategic minerals based on the current production and the estimated mineral reserves. Based on this simplified analysis, the estimated life of these important mineral reserves varies from more than 800 to 20 years. It is important to realize that we will not completely run out of any of these minerals but rather the economically viable mineral deposits will be used up. Additional complications arise if only a few countries produce the mineral and they decide not to export it. This situation is looming for rare earth elements, which currently are produced mainly by China, which is threatening to limit exports of these strategic minerals.

A more complex analysis of future depletions of our mineral supplies predicts that 20 out of 23 minerals studied will likely experience a permanent shortfall in global supply by 2030 where global production is less than global demand (Clugston, 2010). Specifically this study concludes the following: for cadmium, gold, mercury, tellurium, and tungsten—they have already passed their global production peak, their future production only will decline, and it is nearly certain that there will be a permanent global supply shortfall by 2030; for cobalt, lead, molybdenum, platinum group metals, phosphate rock, silver, titanium, and zinc—they are likely at or near their global production peak and there is a very high probability that there will be a permanent global supply shortfall by 2030; for chromium, copper, indium, iron ore, lithium, magnesium compounds, nickel, and phosphate rock—they are expected to reach their global production peak between 2010 and 2030 and there is a high probability that there will be a permanent global supply shortfall by 2030; and for bauxite, rare earth minerals, and tin—they are not expected to reach their global production peak before 2030 and there is a low probability that there will be a permanent global supply shortfall by 2030. It is important to note that these kinds of predictions of future mineral shortages are difficult and controversial. Other scientists disagree with Clugston’s predictions of mineral shortages in the near future. Predictions similar to Clugston were made in the 1970s and they were wrong. It is difficult to know exactly the future demand for minerals and the size of future mineral reserves. The remaining life for specific minerals will decrease if future demand increases. On the other hand, mineral reserves can increase if new mineral deposits are found (increasing the known amount of ore) or if currently unprofitable mineral deposits become profitable ones due to either a mineral price increase or technological improvements that make mining or processing cheaper. Mineral resources, a much larger category than mineral reserves, are the total amount of a mineral that is not necessarily profitable to mine today but that has some sort of economic potential.

Mining and processing ore can have considerable impact on the environment. Surface mines can create enormous pits (see Figure Open Pit Mine) in the ground as well as large piles of overburden and tailings that need to be reclaimed, i.e., restored to a useful landscape. Since 1977 surface mines in U.S. are required to be reclaimed, and commonly reclamation is relatively well done in this country. Unfortunately, surface mine reclamation is not done everywhere, especially in underdeveloped countries, due to lack of regulations or lax enforcement of regulations. Unreclaimed surface mines and active surface mines can be major sources of water and sediment pollution. Metallic ore minerals (e.g., copper, lead, zinc, mercury, and silver) commonly include abundant sulfide, and many metallic ore deposits contain abundant pyrite (iron sulfide). The sulfide in these minerals oxidizes quickly when exposed to air at the surface producing sulfuric acid, called acid mine drainage. As a result streams, ponds, and soil water contaminated with this drainage can be highly acidic, reaching pH values of zero or less (see Figure Acid Mine Drainage)! The acidic water can leach heavy metals such as nickel, copper, lead, arsenic, aluminum, and manganese from mine tailings and slag. The acidic contaminated water can be highly toxic to the ecosystem. Plants usually will not regrow in such acidic soil water, and therefore soil erosion rates skyrocket due to the persistence of bare, unvegetated surfaces. With a smaller amount of tailings and no overburden, underground mines usually are much easier to reclaim, and they produce much less acid mine drainage. The major environmental problem with underground mining is the hazardous working environment for miners primarily caused by cave-ins and lung disease due to prolonged inhalation of dust particles. Underground cave-ins also can damage the surface from subsidence. Smelting can be a major source of air pollution, especially SO₂ gas. The case history below examines the environmental impact of mining and processing gold ore.

Sustainable Solutions to the Mineral Crisis?

Providing sustainable solutions to the problem of a dwindling supply of a nonrenewable resource such as minerals seems contradictory. Nevertheless, it is extremely important to consider strategies that move towards sustainability even if true sustainability is not possible for most minerals. The general approach towards mineral sustainability should include mineral conservation at the top of the list. We also need to maximize exploration for new mineral resources while at the same time we minimize the environmental impact of mineral mining and processing.

Conservation of mineral resources includes improved efficiency, substitution, and the 3 Rs of sustainability, reduce, reuse, and recycle. Improved efficiency applies to all features of mineral use including mining, processing, and creation of mineral products. Substituting a rare nonrenewable resource with either a more abundant nonrenewable resource or a renewable resource can help. Examples include substituting glass fiber optic cables for copper in telephone wires and wood for aluminum in construction. Reducing global demand for mineral resources will be a challenge, considering projections of continuing population growth and the rapid economic growth of very large countries such as China, India, and Brazil. Historically economic growth is intimately tied to increased mineral consumption, and therefore it will be difficult for those rapidly developing countries to decrease their future demand for minerals. In theory, it should be easier for countries with a high mineral consumption rate such as the U.S. to reduce their demand for minerals but it will take a significant change in mindset to accomplish that. Technology can help some with some avenues to reducing mineral consumption. For example, digital cameras have virtually eliminated the photographic demand for silver, which is used for film development. Using stronger and more durable alloys of steel can translate to fewer construction materials needed. Examples of natural resource reuse include everything at an antique store and yard sale. Recycling can extend the lifetime of mineral reserves, especially metals. Recycling is easiest for pure metals such as copper pipes and aluminum cans, but much harder for alloys (mixtures of metals) and complex manufactured goods, such as computers. Many nonmetals cannot be recycled; examples include road salt and fertilizer. Recycling is easier for a wealthy country because there are more financial resources to use for recycling and more goods to recycle. Additional significant benefits of mineral resource conservation are less pollution and environmental degradation from new mineral mining and processing as well as reductions in energy use and waste production.

Because demand for new minerals will likely increase in the future, we must continue to search for new minerals, even though we probably have already found many of the “easy” targets, i.e., high-grade ore deposits close to the surface and in convenient locations. To find more difficult ore targets, we will need to apply many technologies including geophysical methods (seismic, gravity, magnetic, and electrical measurements, as well as remote sensing, which uses satellite-based measurements of electromagnetic radiation from Earth’s surface), geochemical methods (looking for chemical enrichments in soil, water, air, and plants), and geological information including knowledge of plate tectonics theory. We also may need to consider exploring and mining unconventional areas such as continental margins (submerged edges of continents), the ocean floor (where there are large deposits of manganese ore and other metals in rocks called manganese nodules), and oceanic ridges (undersea mountains that have copper, zinc, and lead ore bodies).

Finally, we need to explore for, mine, and process new minerals while minimizing pollution and other environmental impacts. Regulations and good engineering practices are necessary to ensure adequate mine reclamation and pollution reduction, including acid mine drainage. The emerging field of biotechnology may provide some sustainable solutions to metal extraction. Specific methods include biooxidation (microbial enrichment of metals in a solid phase), bioleaching (microbial dissolution of metals), biosorption (attachment of metals to cells), and genetic engineering of microbes (creating microorganisms specialized in extracting metal from ore).