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Exercise 10
Mineral and Rock 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 FeS2, 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.
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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/mm2, calculate
how many times harder diamond is than each of the following minerals:
corundum (Al2O3) – 1,800 kg/mm2
quartz (SiO2) – 750 kg/mm2
calcite (CaCO3) – 135 kg/mm2
gypsum (CaSO4 2H2O) – 32 kg/mm2
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. 583-595.
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3) Explain why small bits of corundum (Al2O3) 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)
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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 (SiO2) 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.
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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., Fe2O3) 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:
density =
mass
volume
kinetic energy =
(mass ) × (velocity )2
2
8a) Suppose you made a hammer from a rounded stone whose volume was 500 cm3 (size of a
half-liter bottle). Assuming crustal rocks have an average density of 2.8 g/cm3, calculate the
mass of this stone in kilograms (kg). Be sure to show your math, including the units.
b) Calculate the kinetic energy in joules (kg-m2/s2) 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 cm3 hammer out of iron, whose density is 7.9 g/cm3. 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-m2/s2) 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.
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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.
Source: Mineral Information Institute, 2008.
Basic Rock Resources
11) 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.
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12) There are limited reserves that can currently be mined economically for many of the mineral
resources listed in Table 10.1. 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
(CaCO3), which is the main component of limestone rock and shells of marine organisms.
Humans long ago discovered that placing crushed limestone or sea shells (CaCO3) 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 (CaCO3) 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 (SO2) 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?
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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 10.2.
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.
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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 10.2.
17) 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)?
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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.
Ex 10 – Mineral Resources