Ex 10 Mineral Resources

Transcription

1 103 Exercise 10 Mineral Resources James S. Reichard Georgia Southern University Student Name Section In this lab you will: explore the connection between society and its use of mineral and rock resources. Of special interest will be how our use of these resources is dependent upon their physical and chemical properties. Background Reading and Needed Supplies Prior to doing this exercise you should read Chapter 3 (Earth Materials) and Chapter 12 (Mineral and Rock Resources) in the textbook. You will need a calculator, ruler, and colored markers to complete this exercise. Part I Mineral Properties and Applications Recall that a mineral is defined as a naturally occurring, inorganic solid composed of one or more elements in which the individual atoms are arranged in an orderly manner called a crystalline structure. As illustrated in Figure 10.1, atoms are assembled in a three-dimensional pattern that repeats itself throughout the structure. Figure 10.1 All minerals have a unique combination of crystalline structure and chemical composition. Shown below is the mineral halite (NaCl), or common table salt. In addition to being crystalline, all minerals have a definite chemical composition. This means is that only certain elements are allowed into a crystalline structure. For example, the chemical formula for the mineral pyrite (fool's gold) is FeS 2, where only iron (Fe) and sulfur (S) atoms are allowed into the crystalline structure. The formula also tells us that there are exactly two sulfur atoms for every iron atom in the mineral pyrite. Interestingly, geologists have identified over 4,000 minerals on planet Earth. What makes each mineral unique is the fact that no two minerals have the same combination of structure and chemical composition. This is important because a mineral's composition and internal structure is what determines its physical properties, such as melting point, hardness, and density. Next we will look at some key mineral properties and how humans have used these properties for practical applications.

2 104 Hardness The physical property known as hardness is defined as the ability to resist scratching. When one substance is harder than another it means it can scratch or cut the softer substance. This process of material being removed by scratching is referred to as abrasion. A common example is when you rub up against something hard, such as concrete or carpeting, and some of your skin is removed. Clearly, our skin easily undergoes abrasion because it is quite soft compared to most substances. 1) The mineral we call diamond is the hardest known substance on Earth. Explain why attaching small pieces of diamond to the tips of saw blades and drill bits makes the best type of tools for cutting through hard objects. 2) The graph in Figure 10.2 shows the absolute hardness of several minerals (y-axis) plotted against their relative hardness (x-axis) on a 1 to 10 scale called the Mohs scale. Notice the considerable difference in the absolute hardness of diamond and the next hardest mineral, corundum. Using the fact that diamond's absolute hardness is about 8,000 kg/mm 2, calculate how many times harder diamond is than each of the following minerals: corundum (Al 2 O 3 ) 1,800 kg/mm 2 quartz (SiO 2 ) 750 kg/mm 2 calcite (CaCO 3 ) 135 kg/mm 2 gypsum (CaSO 4 2H 2 O) 32 kg/mm 2 Figure 10.2 The absolute hardness of select minerals plotted against their relative hardness. *Based on data from Winchell, H., 1945, The Knoop Microhardness Tester as a Mineralogical Tool, American Mineralogist, v. 30, pp

3 105 3) Explain why small bits of corundum (Al 2 O 3 ) would be used instead of diamond for making sandpaper to polish hardened steel, whose relative hardness is around 7. 4) Finely-ground calcite is often used to make household cleaners advertized as being mildly abrasive. Explain why calcite-based cleaners would be best for cleaning plastic materials, whose relative hardness is usually in the range of 3 to 4. 5) Gypsum is a very soft mineral (Figure 10.2) and the main component in drywall (also called sheetrock and wallboard). Drywall is used extensively in the interior of modern homes and businesses. During construction, sheets of drywall are normally cut into smaller pieces and then nailed to a building's wood or metal framing to form walls and ceilings. Explain how gypsum's softness makes it ideal for this type of application. Cleavage and Fracture Another important physical property of minerals is how they break into smaller pieces when subjected to a sufficiently strong force, such as a blow by a falling hammer or rock. Some minerals will break along weakness planes in their crystalline structure, producing flat surfaces known as cleavage planes. Others have no weakness planes within their structure, causing them to break along irregular surfaces called fractures. The photos in Figure 10.3 illustrate the difference between cleavage and fracture surfaces. Humans have learned to take advantage of the way certain minerals cleave and others fracture. Figure 10.3 A broken piece of the mineral galena (A) shows several sets of cleavage planes, whereas chert (B) produces only irregular fracture surfaces. (A) (B)

4 106 6) Figure 10.4 illustrates how the crystalline structure of graphite contains sheets of carbon atoms held together by strong chemical bonds. Between the individual sheets, however, are very weak bonds. This allows graphite to break quite easily, which is why it is one of the softest minerals. Humans have learned how to utilize graphite's softness by grinding the mineral into a powder and mixing it with a little clay to make pencils erroneously called "lead" pencils as they do not contain lead. Describe what you think happens on an atomic scale when you push a pencil across a piece of paper such that it leaves a thin trace of graphite. Figure 10.4 The crystalline structure of graphite consists of strong sheets of carbon atoms held together by much weaker chemical bonds. 7) Unlike graphite, the mineral known as chert or flint (SiO 2 ) is quite hard and will produce irregular fracture surfaces (Figure 10.3b). In freshly broken chert, the edge along where two fracture surfaces meet can be extremely sharp. Describe how ancient humans made use of chert's hardness and ability to form fracture surfaces.

5 107 Density The density of a substance is defined as its mass divided by its volume. A baseball for example is denser than a Nerf ball because it has more mass packed into roughly the same volume. As with the properties of hardness and cleavage, humans have developed practical applications for certain minerals based on their density. One application where mineral density is important is the familiar hand tool we call a hammer. Here a heavy object is attached to a wooden shaft and used to transfer kinetic energy to another object. Hammers are excellent tools for crushing, splitting, or shaping objects and for driving nails. For thousands of years people used rounded stones to make hammers because they were readily available in river beds. Once humans learned how to extract metallic iron by heating iron-oxide minerals (e.g., Fe 2 O 3 ) in a hot fire, iron hammers eventually replaced those made of stone. In this section we will use following equations to examine, in a quantitative manner, why iron makes for a more effective hammer than does stone: mass density = volume ( mass ) ( velocity ) kinetic energy = 8a) Suppose you made a hammer from a rounded stone whose volume was 500 cm 3 (size of a half-liter bottle). Assuming crustal rocks have an average density of 2.8 g/cm 3, calculate the mass of this stone in kilograms (kg). Be sure to show your math, including the units. 2 2 b) Calculate the kinetic energy in joules (kg-m 2 /s 2 ) that you could transfer to another object if you were to swing the stone hammer with a velocity of 5 m/s. 9a) This time you make 500 cm 3 hammer out of iron, whose density is 7.9 g/cm 3. Determine the mass of iron in kilograms (kg). Be sure to show your math, including the units. b) If you were to swing your iron hammer with the same velocity of 5 m/s, calculate the kinetic energy in joules (kg-m 2 /s 2 ) you could transfer to another object. 10) Suppose that a prehistoric person was able to switch from a stone to an iron hammer. Describe what the energy difference between the hammers would mean, in practical terms, to the user.

6 108 Part II Consumption of Mineral Resources From Table 10.1 you can see that each American is responsible for using over 20,000 lbs (10 tons) of stone, sand, and gravel on a per-capita basis each year. These basic rock materials account for 87% of all U.S. mineral consumption. Coming in a distant second is cement, where per-capita consumption is 841 lbs, or 3.5% of all U.S. mineral consumption. The other major non-metallic resources are salt, phosphate rock, and clays. With respect to metals, iron is clearly the most widely used. Although the use of aluminum, copper, lead, zinc, and gold is relatively small in terms of weight, these metals nonetheless have very important applications in society. In this section we will examine some of the ways in which society uses both nonmetallic and metallic mineral resources. Table 10.1 U.S. per capita consumption rates of various mineral resources. Basic Rock Resources Source: Mineral Information Institute, ) At 20,000 lbs (10 tons) per year, stone, sand, and gravel resources represent the vast majority of U.S. mineral consumption. However, no one personally uses 10 tons of these basic rock materials around their home each year. Explain then where most of this rock material is likely being used.

7 109 12) There are limited reserves that can currently be mined economically for many of the mineral resources listed in Table Do you think that the reserves of stone, sand, and gravel are also limited? Explain why or why not. Limestone and Cement Concrete and mortar are cement-based products that are of great importance in modern societies. The basic raw material for making cement is the sedimentary mineral called calcite (CaCO 3 ), which is the main component of limestone rock and shells of marine organisms. Humans long ago discovered that placing crushed limestone or sea shells (CaCO 3 ) in a hot fire produces a powdery residue known as lime (CaO), or cement. When lime cement is mixed with water and allowed to dry, it results in a hard rock-like material. People learned that the strength of this rocky material could greatly be enhanced by adding solid particles (stones, seashells, sand, etc.) to the cement while it is still wet. The term concrete refers to the strong rock-like substance produced by adding coarse particles to wet cement, whereas mortar is a mixture of sand and wet cement that forms a granular paste for holding individual bricks together in a wall. Note that modern cement products use what is known as Portland cement, which contains additional ingredients that allow it to harden more quickly and without necessarily being exposed to the atmosphere. The use of cement-based concrete ultimately replaced cut blocks of stone as a major building material. Concrete structures not only had the strength of those made of cut rock, but were also easier to build. No longer would stone have to be quarried, cut, and hauled to the construction site. The ingredients for concrete (cement, water, and small stones) could be transported separately in small loads and then mixed onsite. This gave humans the ability to erect large structures by pouring sections of concrete into forms of about any shape or size. 13a) Based on the chemical formulas of calcite (CaCO 3 ) and lime (CaO), what common compound do you suspect is released into the atmosphere when crushed limestone (calcite) is heated and converted into lime? b) Explain how the production of cement products is contributing to the problem of global warming. 14) Because natural rainwater is slightly acidic, it slowly dissolves concrete as well as monuments composed of calcite (limestone and marble). In many areas, the acidity of rainfall has greatly increased due to the release of sulfur dioxide (SO 2 ) gas that forms when sulfurrich minerals in coal undergo combustion. What do you suspect this so-called "acid rain" is doing to our concrete highways and bridges?

8 110 15a) It is often stated that when the environmental costs associated with the mining and burning of coal are considered, coal is no longer the cheapest means of producing electricity. Explain who pays for the coal-related deterioration of society's highways and bridges. b) What do you think would happen if the costs associated with replacing our concrete infrastructure were included in the price of coal, making coal-generated electricity more expensive? Metallic Minerals Humans have discovered many important applications for the metals listed in Table For example, because iron (Fe) is strong and quite abundant, it is used to make large quantities of structural steel. Although aluminum (Al) is not as strong as iron, its lower density makes it ideal in applications where weight is a critical factor, such as airplanes and fuel-efficient vehicles. Another extremely useful property of metals is their ability to conduct electricity. For example, copper (Cu) is used in the wiring that carries electricity throughout our homes and cars. It is also used in the circuits of countless electronic devices, including cell phones, computers, and televisions. The modern society we have come to know would simply not exist were it not for the unique properties of metals. Table 10.2 World mineral production and projected lifetime of reserves. Source: Data from U.S. Geological Survey Mineral Commodity Summaries, 2008.

9 111 16) Since silver (Ag) is a better electrical conductor than copper, provide an explanation as to why copper is used rather than silver in most applications. Hint, see Table ) The photo in Figure 10.5a shows a standard copper wire for carrying electricity to outlets and switches in our homes and businesses. In addition to being a good electrical conductor, copper is also highly ductile, which means it tends not to break when drawn out or stretched. Explain why being ductile is important when it comes to electrical wiring. Hint: examine photo (A) below. Figure 10.5 Photo (A) illustrates how a copper wire can be bent and twisted due to the metal being ductile; (B) shows a different type of wire with fine, insulated strands of copper. (A) (B) 18) Notice that the wire in Figure 10.5b actually consists of several fine strands of insulated copper wire. If the function of the wire in photo (A) is to supply electricity throughout a building, what do you suppose is the purpose of the bundled set of insulated wires in (B)?

10 112 19) From Table 10.3 you can see that the overall U.S. recycling rate of major metals is about 48%. Compare this to 28% U.S. recycling rate for glass and plastics. Provide one or more reasons why metals are far more likely to be recycled in the U.S. than glass and plastics. Table 10.3 Recycling rates of various metals in the U.S. Source: Data from U.S. Geological Survey Minerals Yearbook, 2006.