In this lab you will:
examine the physical properties of minerals and how those properties contribute to the usage of mineral and rock resources. You will also explore how we mine for economic deposits of mineral/rock resources.
Background Reading and Needed Supplies
Prior to doing this exercise you should read Chapter 3 (Earth Materials) and review Chapter 12 (Mineral and Rock Resources). You will need a calculator 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 2.1, atoms are assembled in a three-dimensional pattern that repeats itself throughout the structure.
Figure 2.1. All minerals have a unique combination of crystalline structure and chemical composition. Shown above is the mineral halite (NaCl), or common table salt. (rei22967_03_05)
In addition to being crystalline, all minerals have a definite chemical composition. This means 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. What make 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 hardness, strength, and density. Next we will look at some key mineral properties and how humans have used these properties for practical applications. Throughout this lab, you will need to refer to Table 2.1, which lists common mineral resources, their applications, and the properties that make them useful.
Table 2.1. Applications and properties of selected metallic and non-metallic resources. (rei22967_t12_02)
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.
The graph in Figure 2.2 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.
Figure 2.2. 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.) (rei22967_12_02a)
1) 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.
2) Finely-ground calcite is often used to make house-hold 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.
3) Quartz is made up of silicon and oxygen atoms, and because it is a mineral, the atoms are arranged in an orderly fashion. When quartz is heated to a very high temperature, its internal structure is broken down and the mineral melts. If we cool this liquid quickly, the atoms freeze in random arrangements, creating what we refer to as glass. Although glass and quartz are very similar chemically, glass has slightly different properties because it doesn’t have an internal (crystalline) structure. It should be obvious that molten glass can be molded into an almost infinite number of shapes.
a) Give 2 examples of quartz glass that you see around you everyday.
b) What are 2 physical properties of quartz glass that makes it useful for windows?
c) In certain materials, quartz glass is molded into specific shapes. For instance, quartz glass is often formed into a fibrous shape (fiberglass) that is commonly used in roofing shingles and ceiling tiles. What is the purpose of molding quartz into this fibrous shape, particularly for these 2 products? (Hint: imagine what would happen if the glass in roofing shingles were made into rounded spheres instead of long fibers).
4) What key property, or properties, makes titanium an ideal metal for aircraft?
5) What key property of lead makes it ideal for making bullets?
Part II – Consumption of Mineral and Rock Resources
From Table 2.2 you can see that each American is responsible for using nearly 16,000 lbs (8 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 at a distant second is cement, where per-capita consumption is 616 lbs, or 3.4% 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 non-metallic and metallic mineral and rock resources.
Table 2.2. Average yearly U.S. per capita consumption rates of various mineral resources. (rei22967_t12_01)
Basic Rock Resources
6) At 16,000 lbs (8 tons) per year, stone, sand, and gravel resources represent the vast majority of U.S. mineral consumption. However, no one personally uses 8 tons of these basic rock materials around their home each year. Where is most of this rock material likely being used?
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 (it helped build the Roman Empire!). 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.
7a) Based on the chemical formulas of calcite (CaCO3) and lime (CaO), what common gaseous compound do you suspect is released into the atmosphere when crushed limestone (calcite) is heated and converted into lime? Write out the chemical equation to help you find the answer.
b) How is the production of cement products contributing to the problem of global warming? (HINT: It has absolutely nothing to do with the ozone layer)
8) 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 sulfur-rich minerals in coal undergo combustion. What do you suspect this so-called “acid rain” is doing to our concrete highways and bridges?
Metallic Minerals
Humans have discovered many important applications for the metals listed in Table 2.3. 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 2.3. World mineral production and projected lifetime of estimated reserves. (rei22967_t12_04)
9) 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 2.3).
10) Why is copper ideal for electrical wiring?
(A) (B)
Figure 2.3. 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. (Reichard)
Part III – Mining Resources
By now, you should have a good understanding of how useful minerals are to our society. One thing we haven’t addressed is how we obtain these resources. To get an idea of how important earth materials are economically, you’ll need to explore the United States Geological Survey’s Mineral Resource Data System. Use the following steps:
Log on to the USGS Mineral Resource Data System using the link https://mrdata.usgs.gov/mrds/map-us.html
This link should take you to a map of the United States covered in black dots (yes, there really are THAT MANY mine sites in the U.S.). Zoom in on the map and you will notice the black dots change to green and red boxes. Choose a geographic area anywhere in the U.S. and zoom in far enough to click on a red box.
To the left, a pop up window should appear that says “Search results”. This has the mine name and the record ID of the red box you clicked. Click on the blue record ID number. A new window should open with all of the information about the mine.
11) Using the information you just found, fill in the following:
Site Name:
County of site location:
State of site location:
Commodity mined at this site:
Commodity type (metallic or non-metallic):
Operation type (if specified – surface or underground):
If commodity is not an element or mineral, list the rock type:
12) What is this commodity used for?
13) List 2 environmental impacts associated with mining this commodity. You may need to refer to chapter 12 in your textbook.
14) Explain whether or not the economic need for this resource outweighs the environmental impacts.
We mine for ore minerals, or minerals that contain a desirable element or compound that is useful for us (refer to Table 2.1). Ore minerals are rarely found on their own – they are usually part of a rock deposit, mixed in with unusable or undesirable elements/minerals. A deposit will only be mined if there is enough ore to make the deposit economical. In order to determine whether or not the deposit is economical, the amount of the desired element or compound must be determined.
The concentration of a valuable element can be determined by calculating the element’s weight fraction in the mineral. This can be done using the mineral’s chemical formula and the atomic weights of each element within that formula. In pyrite (FeS2), the desirable element is Fe. Below is a step-by-step example of calculating the weight fraction of Fe in pyrite.
Atomic weight of Fe = 55.847 g Fe/mol Fe
Atomic weight of S = 32.06 g S/mol S
Step 1: Count the number of moles, or atoms, of each element in the formula and find the weights of each element using the atomic weights given.
In the chemical formula FeS2, there is 1 mole of Fe and 2 moles of S.
Weight of Fe = 1 mol Fe x 55.847 g/mol Fe = 55.847 g Fe
Weight of S = 2 mol S x 32.06 g/mol S = 64.12 g SStep 2: The total weight is needed, so the weights calculated in step 1 must be added together.
55.847 g Fe
+ 65.12 g S
119.967 g FeS2
Step 3: Fe is the desirable element in this formula, so the weight fraction of Fe is what we are looking for. Divide the weight of Fe calculated in step 1 by the total weight of the mineral, calculated in step 2.
55.847 g Fe = 0.4655 Fe
119.967 g FeS2 1 FeS2This weight fraction shows that for every 1 unit of FeS2, there is 0.4655 units of Fe present. When using the weight fraction of Fe in pyrite in a calculation, you would simply use the number 0.4655. To determine the total number of tons of Fe in a particular deposit of pyrite, you would multiply this weight fraction by the amount of tons of pyrite in the deposit.
For this exercise, you will need the following atomic weights:
Atomic weight of Fe = 55.847 g Fe/mol Fe
Atomic weight of O = 15.9994 g O/mol O15) Calculate the weight fraction of Fe in the mineral magnetite (Fe3O4) using the atomic weights given to you and following the steps from the example above. Show all work and calculate the weight fraction to the 4th decimal place. Keep in mind that you have 3 mol of Fe and 4 mol of O in this formula.
16) As an economic geologist, you have discovered a 700,000 ton deposit that is 35% magnetite. Calculate how many tons of magnetite are in this deposit (you are calculating how much of the mineral is in this deposit, not any specific elements at this point). Show all work.
17) Using the total tons of magnetite you calculated in #16, and the weight fraction you calculated in #15, calculate how many tons of pure Fe are in this deposit. Show all work.
18) Fe is currently ~ $93 per ton. How much is this deposit worth? Show all work.
19) Using the total number of tons in this deposit (700,000) and the number of tons that are pure Fe, and assuming all other material is unusable/not desirable, calculate how many tons of waste material are produced. Show all work.
20) Is this a profitable, economic deposit? Why or why not?