CHAPTER 7: ROCKS AND MINERALS
SUMMARY
In this chapter we address issues of scale as we begin our investigation of the basic particles that make up the Earth around us. Combinations of protons, neutrons, and electrons make up atoms that in turn form elements. Atoms of elements are held together by bonds that are difficult to break if the atoms share electrons. The relative strength of these bonds becomes important as we discuss the basic components of rocks known as minerals. The combination of elements forms minerals that are naturally occurring, inorganic compounds with definite chemical composition and uniform atomic structure. Since some minerals are very similar to one another, a variety of properties (e.g., hardness, cleavage, streak) must be used to identify them.
When mixed together in different proportions, these minerals form the rocks around us.
We find that rocks come in three basic varieties: igneous, sedimentary, and metamorphic.
Igneous rocks form in association with volcanic processes where they solidify at or near the surface. The texture (grain size) and composition (color) can be used to tell a story about how and where the igneous rock formed. Sedimentary rocks form from the remains of other rocks that decompose on the surface, from evaporation and precipitation in water, and from the action of organisms. Clastic sedimentary rocks are those that form from the remains of other rocks that break apart and partially dissolve through weathering. Those materials are transported and deposited before being buried and cemented together in a process known as lithification. Particle size is used to classify clastic sedimentary rocks. Chemical sedimentary rocks form when evaporation causes minerals to precipitate out of water and fall to the bottom of the ocean or shallow sea where they also are buried and lithified. Biochemical sedimentary rocks form from the remains of plants or animals or are extracted seawater by the animals to become part of their shells or skeleton. Rocks and the processes that connect them are illustrated by the rock cycle.
Many of these rocks and minerals are used by humans in their everyday lives as they live and build the society in which we live.
7.2: Elements and Atoms: The Basic Building Blocks.
Rocks are made of Minerals. Minerals are made of and categorized by the chemical Elements they contain. An Atom is the smallest particle that retains the characteristic of an Element (so
Elements can be separated into individual Atoms). Atoms can be broken down into Neutrons,
Protons and Electrons.

Figure Above: model of an atom. The dense nucleus consists of protons (positive charge) and neutrons (no charge). Surrounding the nucleus (and occupying most of the volume of an atom) are rapidly moving electrons (negative charge), represented in this model as an electron cloud.
It's important to note that the number of protons in an atom is unique for each element and is called the element's atomic number. If the number of electrons (-) equals the number of protons (+) an atom has no net charge (for example, helium; see Figure 7.6). The outer energy level of many elements are incomplete and may gain or lose electrons to other elements. An ion is an atom that gains or loses electrons and ions have a positive or negative electrical charge.
These atoms or ions of elements can bond together to form minerals (by balancing electrical charges of different ions or by sharing electrons; see page 186 for more).
An ionic bond forms when the positive and negative charges balance creating electrical attraction between the oppositely charged ions. The following example and diagram (below) illustrate this concept. Keep in mind that the first energy level is filled (only has room for) 2 electrons; and the second and third energy levels have space for 8 electrons (each).
Figure Below are atomic models for sodium (Na) and chlorine (Cl). Na has an atomic number of 11 (11 protons and 11 electrons with 2 in the first shell; 8 in the second shell; and 1 in the third shell)...Cl has an atomic number of 17 (17 protons and 17 electrons with 2 in the first shell; 8 in the second shell and 7 in the third shell). The 11th electron of Na is easily lost giving Na a positive charge (11 protons and 10 electrons, so net positive charge) while Cl gains an extra electron to complete its outer shell (which gives Cl a negative charge;
17 protons and 18 electrons with 2 in the inner shell and 8 electrons each in the next two shells). Now the two ions bond together (ionically; opposites attract) to form the mineral
Halite (NaCl). The atomic structure of Halite is cubic (see below).

Also, read about covalent and silicate bonds.
Checkpoint 7.3: The total electrical charges of the ions of the elements in the mineral olivine must balance. From date in Table 7.1, which is the most reasonable formula for the mineral? a) MgSiO2 b) MgSiO4 c) Mg2SiO4 d) Mg4SiO2 when you work on this problem, study Table 7.1 closely first. The ionic charges for each elements is given in this table (for example, Na discussed above shows a + or single positive charge). Also, the number of ions or atoms of each element is given in the chemical formula.
7.3 Minerals.
Read the definition of a mineral closely: Natrually occurring, inorganic solid of one or more elements that have a definite chemical composition and an orderly internal arrangement of atoms
(crystalline structure).

7.4 Igneous Rocks.
Can be classified as volcanic or plutonic based on whether they form on the Earth's surface
(volcanic) or within the Earth's interior (plutonic).
Figure above is a comparison of volcanic and plutonic rocks. The three rocks types on top
(granite, diorite and gabbro) are plutonic rocks, cool slowly within the earth's interior and have larger grain sizes (crystals of minerals) than volcanic rocks (the three below: rhyolite, andesite and basalt...note the andesite has porphyritic texture: large crystals within a fine-grainded matrix). Rocks high in silica content contain lighter-colored minerals than the low silica rocks
(for example, granite and rhyolite have higher silica content than gabbro or basalt). The chart above shows the minerals and typical percentage of each mineral found in each rock type.
Diorite and andesite (for example) have the same composition but diorite is plutonic and andesite is its volcanic equivalent.
Igneous rocks are classified based on texture (size and arrangement of mineral grains) and color
(indicator of chemical composition). The size of mineral crystals that form depends on how fast the magma cools and solidifies. Volcanic rocks cool rapidly at the surface and grain sizes in volcanic rocks are small. In contrast, plutonic rocks cool slowly beneath the Earth's surface so minerals have plenty of time to grow into large crystals. Occasionally, some volcanic rocks show two cooling histories (producing a porphyritic texture): large crystals grow undeground and

the the magma is extruded to the surface creating a fine-grained matrix surrounding the large crystals (see the Andesite in figure 7.20).
The above information should help you answer Checkpoint 7.10 (page 187) when you get to this question.
7.5: Sedimentary Rocks.
Sedimentary rocks can be divided into three fundamental types based on the materials involved and the process by which each type forms:
*Clastic sedimentary rocks are composed of sediments-rock and mineral fragments that form when rocks break apart at or near the Earth's surface. Clastic sedimentary rocks make up
86 percent of all sedimentary rocks (see clastic sediment sizes and sedimentary rocks in Table
7.5). Sandstone is an example.
*Chemical sedimentary rocks have crystallized from a solution (e.g., seawater) as a result of changing physical conditions (e.g., evaporation). Salt is an example.
*Biochemical sedimentary rocks are formed by the actions of living organisms or are composed of the remains of dead organisms. Chalk and coal are examples.
7.6: Metamorphic Rocks.
Metamorphism means change, and in geology metamorphism relates to changes in mineral composition or texture that occur in solid rocksas a result of increasing pressure or temperature.
For example, limestone becomes marble when heated by igneous intrusions. Two mains types are discussed in your book: Contact metamorphism occurs when rocks come in contact with a heat source (usually a magma body). The rocks are cooked, the composition does not change but the texture does. These rocks do not normally become foliated (see Figure 7.33). Regional metamorphism occurs when rocks undergo increased temperatures and pressures typically associated with the plate tectonic processes that form mountains. Minerals can become aligned parallel to each other and perpendicular to the direction of pressure; this geometric orientation generates an alignment of minerals into sheets termed foliation. The grade of metamorphism increases with higher temperatures and pressures. Table 7.6 shows increasing grade of metamorphic rocks from slate and phyllite to schist and gneiss (increasing metamorphic grade produces lager grain sizes).
7.7: The Rock Cycle and mineral resources.
Concrete is formed by adding cement and water to a mixture of sand and gravel. This could be seen as an analog for the formation of what type of sedimentary rock? a) clastic b) chemical c) biochemical
When answering this question (above) and Checkpoint 7.22 make sure to read on the processes that produce each rock type. For example, how does magma become igneous rock? It must cool and solidify either on the Earth's surface (volcanic) or below the Earth's surface (plutonic).
LEARNING OBJECTIVES
1.
Students will explain concepts related to rocks and minerals.
2.
Students will explain the relationship between atoms, elements, minerals, and rocks.
3.
Students will describe characteristics of minerals.
4.
Students will relate the processes of rock formation to the characteristics of the rock.
5.
Students will correlate rock texture with the processes by which the igneous rock formed.

6.
Students will compare and contrast the features and processes associated with clastic, chemical, and biochemical sedimentary rocks.
7.
Students will compare and contrast contact and regional metamorphism.
8.
Students will explain the links of the rock cycle.