Geology 200

Study Guide

Unit 2:
Minerals and Rock Types

Overview

This unit begins a study of minerals and the different types of rock found on the Earth’s surface. Section 1 introduces the basic chemical concepts you will need to know to understand the structure of minerals. Section 2 discusses the chemical composition of the Earth’s crust, and introduces the concept of crystallinity, using the silicate minerals—the most abundant on Earth—as an example. The physical and chemical characteristics of minerals form the subject of Section 3, and Section 4 is devoted to a brief discussion of the major classes of rocks: igneous, sedimentary, and metamorphic.

Objectives

After completing this unit, you should be able to

1. define each of the terms listed below:
   • element
   • atom
   • ion
   • proton
   • neutron
   • electron
   • atomic mass number
   • atomic number
   • atomic weight
   • isotope
2. describe the structure of an atom, and explain how ions are formed.
3. describe four mechanisms of bonding.
4. explain the principal difference between solids, liquids, and gases.
5. describe the major building blocks of silicate minerals and the main arrangement of these building blocks.
6. list five defining characteristics of a mineral.
7. explain the law of constancy of interfacial angles.
8. define polymorphism.
9. explain ionic substitution, and discuss its effects.
10. list and explain the major physical properties of minerals, and describe a mineral using these properties.
11. identify the relative hardness of the minerals that make up the Mohs’ hardness scale.
12. list the three major groups of rocks, and identify the most important minerals in each rock group.
13. list the diagnostic properties of the major rock-forming minerals, and identify these minerals in hand specimens.

Section 1: Atoms, Elements, and the States of Matter

In this section you will learn that rocks are aggregates of minerals; and minerals, in turn, are composed of atoms. We can predict many of the properties of minerals if we understand the properties of their atoms. For example, atoms that contain many protons and neutrons are heavy, so minerals composed of these atoms will also be relatively heavy. For example, barium (Ba, atomic weight 137) in the mineral barite, imparts a high specific gravity to this mineral (i.e., it’s heavy). Similarly, some atoms combine to form very strong bonds (another property). For example, carbon-carbon bonds are very strong and impart their strength to diamond.

Examine the periodic table of elements given in Appendix D of the textbook. Elements that fall within the same vertical column of the table require the same number of electrons to complete their outer shell, and thus have similar chemical properties. Group VIIIA comprises the inert elements, those whose outer shells are completely full of electrons; these elements are very stable and rarely combine with others. Each element in Group Ia has a single electron in its outer shell: to reach its most stable form, it must lose that electron. Therefore, the most stable form for an element in Group Ia exists as an ion with a single positive charge; sodium Na⁺ is a member of Group Ia. Conversely, each member of Group VIIa needs one additional electron to fill its outer shell; it is most stable as an ion with a single negative charge (e.g., chlorine, Cl–). Group Ia elements and Group VIIa elements have a strong tendency to combine by forming ionic bonds, as in sodium chloride, NaCl. (Note: positively charged ions are known as cations (pronounced “cat-ions”); negatively charged ions are anions (pronounced “ann-ions”).

Matter can exist in one of three states: solid, liquid, or gas. Atoms that form a solid are arranged in a rigid framework. In a liquid, the basic particles are in random motion, but they are packed closely together. In a gas, the particles are in random motion, but individual atoms or molecules are separated by empty spaces and are far apart (comparatively). The melting of a solid or the evaporation of a liquid can occur as a result of an increase in temperature or a decrease in pressure. As temperature increases, the atoms or molecules in a solid move faster and can break free of the bonds holding them. As bonds are broken, the rigid framework of a solid is disrupted, causing melting; in a liquid, the particles move farther apart, causing vaporization. Pressure affects the space in which particles can move. Higher pressures confine the particles and cause an increase in structuring. As pressure is released, the particles have more space to move, and melting or vaporization can occur.

Reading Assignment

Plummer, C. C., Carlson, D. H., & Hammersley, L. Physical Geology (15th ed.).

• “Atoms and Elements” (pp. 28-32, up to “Crystalline Structures”).

Study Questions

1. Define each of the terms listed below.
   1. element
   2. atom
   3. ion
   4. isotope
2. Distinguish between atomic mass number, atomic number, and atomic weight. Few elements have atomic weights that are integers (i.e., single whole numbers). Why?
3. Define each of the parts of the atom listed below.
   1. proton
   2. neutron
   3. electron
4. How does an ion form?
5. Distinguish between cations and anions, and describe how each is formed.
6. Were the outer shell of each of the following atoms “full,” it would contain eight electrons. Referring to the periodic table in Appendix D, give the charge of each element in its most stable ionic form.

   Element	Electrons in the outer shell	Charge in most stable ionic form
   Li	1	______
   O	6	______
   Mg	2	______
   K	1	______
   Al	3	______
   Cl	7	______

7. Describe each of the four types of bonds commonly found in minerals.
8. Define the terms solid, liquid, and gas. What distinguishes a solid from a liquid? —a liquid from a gas?

 Answer Key

Section 2: Silicate Minerals and Crystalline Structures

In this section we examine the structures of minerals. Specifically, though not exclusively, this section looks at the basic components of silicate structures, which are the fundamental building blocks for a majority of the rock-forming minerals. The structures and compositions of minerals are important determinants of their physical and chemical properties.

Reading Assignment

Plummer, C. C., Carlson, D. H., & Hammersley, L. Physical Geology (15th ed.).

• “Crystalline Structures,” “The Silicone-Oxygen Tetrahedron,” and “Nonsilicate Minerals” (pp. 32-37).
• “Variations in Mineral Structures and Compositions” (pp. 37-38).

Study Questions

9. What is a crystalline substance?
10. What is a silicate mineral? What is the chemical formula of quartz?
11. Draw a diagram of the basic building block of silicate minerals. What is the basic building block called, and why?
12. What are the four fundamental configurations of tetrahedral groups in silicate minerals? Give an example of a mineral in each group.

 Answer Key

Section 3: The Nature of Minerals

Different minerals can have different chemical compositions and structures. Interestingly, however, they can also have different chemical compositions but the same structure, or different structures but the same chemical composition.
Minerals with the same structure but different chemical composition are said to be members of a mineral group. Mineral groups tend to form by ionic substitution, in which an element replaces another element of similar size and charge in the crystal structure. An example of a mineral group formed by ionic substitution is the important group of feldspar minerals. Some feldspars, the albites, are characterized by a sodium end member. Feldspars with a calcium end member are orthoclases. (Note: the textbook combines albites and arnothites under the more general name plagioclase feldspar.) All minerals within one mineral group have the same crystal structure, and therefore have many similar physical properties (e.g., cleavage, crystal form, hardness). However, differences in chemical composition do result in some different physical properties (e.g., specific gravity, colour).

Minerals that have the same chemical composition but different crystal structures are called polymorphs. For example, diamond and graphite are both composed of carbon atoms, but since the atoms are bonded in different arrangements, diamond and graphite have very different physical properties.

The laboratory exercise associated with this unit will give you the opportunity to become much more familiar with the physical properties of various minerals.

Reading Assignment

Plummer, C. C., Carlson, D. H., & Hammersley, L. Physical Geology (15th ed.).

• “The Physical Properties of Minerals” (pp. 38-47).
• The Many Conditions of Mineral Formation” (pp. 47-48).

Study Questions

13. What are clay minerals?
14. List the four conditions that a substance must meet to be considered a mineral.
15. Explain the law of constancy of interfacial angles.
16. Explain polymorphism, and give an example.
17. What is ionic substitution?
18. What factors determine whether one ion can substitute for another?
19. What are mineral groups? Identify three different mineral groups.
20. List six of the more significant and observable physical properties of minerals.
21. What is cleavage?

22. List the members of the Mohs’ hardness scale, and describe how the scale is used. Make up your own mnemonic scheme to help you remember the sequence in Mohs’ scale.

    Example: The [talc] Greek [gypsum] cook [calcite] fries [fluorite], apples [apatite], oranges [orthoclase], and carrots [K-feldspar] quarterly [quartz] till [topaz] completely [corundum] done [diamond]. It doesn’t have to make sense, as long as it helps you to remember the names and relative sequence of the minerals!

23. What is specific gravity? On what does it depend?
24. If two samples are different colours, must they necessarily be two different minerals?
25. What is the “streak” of a mineral?

 Answer Key

Section 4: Introduction to Rocks

As you know, rocks are composed of minerals. The most important minerals associated with each of the rock types are identified in the table below. By the end of this unit, you should be able to describe the diagnostic physical properties of each of the minerals listed in the table, and to recognize them in hand specimens. You will find this ability essential when you are asked to examine and identify rocks in the succeeding units.

Table 2.1: Rock types and associated minerals

Reading Assignment

Plummer, C. C., Carlson, D. H., & Hammersley, L. Physical Geology (15th ed.).

• “The Rock Cycle” (pp. 51-53 of Chapter 3).

Study Question

26. Define each of the terms listed below:
    1. magma
    2. igneous rock
    3. sediment
    4. sedimentary rock
    5. metamorphic rock

 Answer Key

Unit 2 Self Test

You have now finished Unit 2, so please complete and submit the associated laboratory exercise and assignment. Instructions can be found in the Assignment Drop Boxes section of the course homepage.