Chapter 3 - Minerals and Rocks

Learning Objectives
After reading and studying this chapter, students should

* Understand minerals in terms of their chemistry and internal structure
* Know the major groups of important rock-forming minerals and their environmental significance
* Understand the rock cycle and how it interacts with plate tectonics
* Know the three rock laws
* Know the basic rock types and their environmental significance
* Know the basic rock structures

Chapter 3 Summary
This chapter focuses on minerals, rocks, rock structures, and the environmental importance thereof. The chapter opens with discussions of mineral characteristics, bonding, and mineral groups, then focuses on the rock cycle and definitions of the three main rock groups. Following a summary of three main rock laws, the chapter describes in detail the three main rock groups, their main subgroups, their properties, and their environmental significance. The chapter closes with a discussion of rock strength and deformation, as well as the main types of rock structures.

Chapter 3 Outline
I. Minerals
A. Definition
1. a naturally occurring, solid Earth material formed by geologic processes
B. Atomic chemistry
1. Atoms and elements
a. all matter is composed of atoms
b. an element is a chemical substance composed of identical atoms
2. Conceptual model of an atom
a. atoms are composed of three subatomic particles, protons, neutrons and electrons
b. protons (positive charge) and neutrons (neutral charge) in nucleus
c. electrons (negative charge) found outside nucleus in orbitals or shells
d. number of protons is the atomic number
e. almost entire mass is in nucleus
f. electrons can be lost or gained, forming charged ions
3. isotopes
a. two atoms of same element with different numbers of neutrons
b. isotopes distinguished by atomic mass
C. Minerals
1. formal definition: element or chemical compound that is naturally formed, is normally a solid, has a characteristic chemical formula, and normally has crystalline structure
a. minerals can be composed of single elements or as compounds of multiple elements
2. minerals and chemical bonding
a. four types of chemical bonds
1. covalent: atoms share electrons
2. ionic: atoms attracted by opposite charges imparted by gain or loss of electrons
3. Van der Waals: weak attraction between chains or sheets of atoms
4. metallic: electrons shared by all atoms of the solid mass rather than by specific atoms
b. bonding determines important mineral properties
3. crystalline structure of minerals
a. crystalline structure is the orderly, regularly repeating geometric patterns of atoms
b. unit cell is smallest unit of the geometric pattern
c. crystal lattice is the framework that defines the regular geometric pattern of atoms in a crystal
II. Important rock forming minerals
A. Common minerals
1. more than 4000 known minerals
a. only a few dozen are common constituents of rocks at or near Earth’s surface
2. hand specimen identification involves appearance and physical properties
a. mineral properties summarized in Appendix A
3. weathering
a. physical and chemical breakdown of rocks at or near Earth’s surface
b. important in forming sediments and soils
B. Silicates
1. make up 98% of mineral mass of Earth’s crust
a. 45% of crust is oxygen, 27% is silicon
b. silicon and oxygen combine with several other elements to make up most silicate minerals
c. silicon-oxygen tetrahedron is the building block of silicates
1. tetrahedra combine in a variety of patterns (e.g., sheets, chains) to form various types of silicates
2. Quartz
a. SiO2 with network structure of silicon and oxygen atoms
b. most abundant mineral in crust
c. recognized by hardness and conchoidal fracture
d. variety of colors determined by impurities
3. Feldspars
a. aluminosilicates containing silicon, oxygen and aluminum, combined with potassium, sodium, and calcium
b. network crystal structure
c. most abundant group of rock-forming minerals
d. two main types are alkali feldspars and plagioclase feldspars
4. Mica
a. sheets of Si-O tetrahedra
b. includes muscovite (colorless, potassium and aluminum rich) and biotite (ferromagnesian)
5. other important ferromagnesian minerals
a. olivine
b. pyroxene
c. amphibole
d. all tend to be weathered easily
C. Other important rock-forming minerals
1. oxides
a. metal atoms combined with oxygen
b. includes many ores, especially of iron and aluminum
2. carbonates
a. metal ions combined with carbonate ion
b. calcite is most important environmentally
c. chemical weathering of carbonate-bearing rocks produces caverns and sinkholes
3. sulfide minerals
a. metal ions combined with sulfur
b. includes iron pyrite and important ore minerals
c. associated with environmental degradation when exposed to oxygen and water to produce sulfuric acid
4. native elements
a. minerals composed of single elements
b. includes gold, silver, copper, and diamonds
III. Rock cycle
A. Rock defined
1. an aggregate of one or more minerals
B. The rock cycle
1. the rock cycle is a worldwide rock-recycling system linking subsurface and surface processes
a. produces three main groups of rocks
2. three main groups of rocks
a. igneous: crystallization of molten rock
b. sedimentary: accumulated layers of sediment from preexisting rocks
c. metamorphic: altered in form and mineral makeup by heat, pressure, and/or fluids
C. Rock cycle and plate tectonics
1. plate tectonics provides several environments for rock formation
a. specific rock forming processes occur at each boundary
2. tectonic processes that drive the rock cycle essential in determining properties of resulting rocks
IV. Three rock laws
A. Fundamental laws are required to understand Earth history
1. cross-cutting relationships
a. a rock is younger than any other rock it cuts
2. original horizontality
a. sedimentary layers are nearly horizontal when deposited
3. superposition
a. oldest layers of sediments are on bottom and youngest layers are on top of a series of layers that have not been overturned
V. Igneous rocks
A. Definition
1. rocks that have crystallized from molten rock (magma)
B. Intrusive igneous rocks
1. cool slowly and crystallizes well below the surface of the Earth
2. individual mineral grains can be seen with naked eye
a. phenocrysts are crystals larger than surrounding crystals
3. inclusions are pieces of surrounding rock incorporated into crystallizing magma
4. batholiths and plutons
a. batholiths are largest masses of igneous rock, often exceeding thousands of cubic kilometers
b. batholiths are composed of smaller intrusions called plutons
5. why magma rises and intrudes other rocks.
a. probable explanation is that once formed, a mass of magma is hotter and less dense than surrounding rocks
b. rise into crust ceases when density differences are equalized
C. Extrusive igneous rocks
1. crystallize at surface of Earth
a. form from lava or pyroclastic debris
2. fine-grained because rapidly cooled
a. porphyritic textures have large crystals surrounded by smaller crystals
3. volcanic breccia
a. lava flow mixed with cemented fragments of broken lava and ash
4. pyroclastic debris forms tuff and agglomerate
D. Igneous rocks and the environment
1. intrusive rocks are generally strong and resistant to weathering
2. lava flows often exhibit columnar jointing and lava tubes, both of which impart weaknesses
3. tuff is generally a soft, weak rock
4. careful field investigation is always necessary before large structures are built on igneous rocks
VI. Sedimentary rocks
A. Definitions and types
1. detrital or clastic
a. form from broken pieces of preexisting rocks
2. chemical
a. deposited when chemical or biochemical processes precipitate dissolved substances
3. diagenesis
a. changes in sediments as a result of burial and fluid passage
B. Sedimentary processes
1. sediment is delivered to sedimentary basin
2. sediment is deposited in strata
3. as basin sinks or sea level rises, several kilometers of sedimentary rocks can be deposited
4. pressure of overlying rocks and precipitation of minerals from pore waters cement the rocks
C. Detrital sedimentary rocks
1. classified according to grain size
a. shale
b. siltstone
c. sandstone
d. conglomerate
D. Chemical sedimentary rocks
1. classified according to chemical composition
a. halite
b. gypsum
c. limestone: most abundant of chemical sedimentary rocks
E. Sedimentary rocks and the environment
1. three primary environmental concerns
a. shale, mudstone, and siltstone are often very weak
b. limestone generally not well suited for human use and activity, because of weathering characteristics
c. cementation may be weak
VII. Metamorphic rocks
A. Definitions
1. rocks changed by heat, pressure and chemically active fluids
2. types of metamorphism
a. high-pressure, low-temperature
b. high-pressure, high-temperature (regional metamorphism)
c. high temperature, low-pressure (contact metamorphism)
B. Foliated metamorphic rocks
1. mineral grains are aligned in parallel layering or banding
2. types
a. slate
b. schist
c. gneiss
C. Nonfoliated metamorphic rocks
1. no alignment of mineral grains
2. types
a. marble
b. quartzite
D. Metamorphic rocks and the environment
1. foundation materials
a. slate is excellent for foundation material and other uses
b. schist is poor because of soft minerals
c. gneiss usually of suitable strength
2. foliation planes are potential planes of weakness
VIII. Rock strength and deformation
A. Definition
1. resistance to failure such as fracturing, sliding, or flowing
a. varies with composition, texture, and location
B. Deformation of Earth materials
1. types of deformation
a. elastic
b. plastic
c. ductile
d. brittle
2. types of strength
a. compressive
b. tensile
c. shear
IX. Rock Structures
A. The term “structural” refers to deformation of rocks or the resulting structures
B. Fractures
1. joints
a. no displacement along fracture
2. faults
a. displacement along fracture
3. problems associated with fractures
a. conduits for fluids
b. zones of weakness
c. fracture is subject to weathering, which widens and weakens it
C. Folds
1. form when layers are shortened by lateral compression
2. types
a. anticline
b. syncline
3. fold belts
D. Unconformities
1. definition and importance
a. significant break or gap in geologic record
b. important in understanding geologic history
c. often form boundary between contrasting rock types
2. types
a. nonconformity
b. angular unconformity
c. disconformity