CHAPTER 5 - ATOMS, ELEMENTS, AND MINERALS
Overview
A rudimentary knowledge of chemistry is required to fully understand much of
physical geology, and rocks and minerals in particular. The relation of rocks
and minerals introduces this chapter, and mineral is defined as "a naturally
occurring solid that is crystalline (which is to say that it has a periodically
repeating arrangement of atoms) and has a specific chemical composition."
Atoms, atomic structure, and their relationship to elements are discussed,
followed by a brief examination of chemical activity and ions. Bonding is
treated in Box 5.2 (see below).
The distribution of the eight most common elements in the crust is
summarized in Table 5.1. The importance of silicate minerals, abundance of
aluminum, and rarity of other economic minerals is related to elemental
abundance in the crust. Crystallinity is discussed in some detail with
supporting figures, and introduces the silicon-oxygen tetrahedron. Silicate
structure is described fully with Figures 5.7-5.10 illustrating isolated silicates,
chain (single and double) silicates, sheet silicates, and framework silicates.
The section on minerals repeats their definition, requiring that they be
crystalline solids, occur naturally, and have a specific chemical composition,
followed by a more detailed discussion of each characteristic of that definition.
Important, rock-forming minerals are noted, and summarized in Table 5.2.
Silicates, including quartz, the feldspars, pyroxenes, amphiboles, micas
comprise 90% of the crust. Clays, nonsilicates, Calcite, and ore minerals are
noted as well.
Physical Properties are discussed, in detail. These include color, streak, luster,
hardness, external crystal form, cleavage, fracture, specific gravity, special
properties (smell, taste, striations, magnetism), other properties (double
refraction, effects of polarized light, x-ray defraction), and chemical tests.
Learning Objectives
1. Rocks are naturally formed, consolidated material composed of grains or
one or more minerals. Minerals are crystalline (orderly three-dimensionalarrange of atoms).
2. Elements are substances that cannot be broken down by ordinary chemical
methods. Atoms are the smallest particles of elements. They are constructed
of protons, neutrons (forming the nucleus) and electrons. Atomic mass
number, atomic number and atomic weight control the "character" of an
element, particularly its isotopes.
3. Chemical activity is related to ions and their bonding.
4. Eight elements comprise 98% of the weight of the crust. Oxygen accounts
for half the weight of the crust. Silicon is the second most abundant element in
the crust and silicate minerals, combinations of oxygen and silicon, are the
most common in the crust.
5. Crystalline substances have a three-dimensional, regularly repeating,
orderly pattern of their anions. The silica tetrahedron is the basic "building
block" of most common (silicate) minerals. Silicate structure reflects the
arrangement of silica tetrahedra and the numbers of shared oxygens. These
structures include: isolated silicate structure (no shared oxygens), chainsilicates (two shared oxygens), sheet silicates (three shared oxygens), and
framework silicates (four shared oxygens).
6. Minerals are naturally occurring solids that are crystalline (which is to say
that it has a periodically repeating arrangement of atoms) and have a specific
chemical composition. Specific chemical composition reflects the orderly
internal arrangement of atoms. Zoning further reflects the orderly
arrangement.
7. A small number of rock-forming minerals comprise most of the crust. Five
mineral groups (feldspar, quartz, pyroxene, amphibole, and mica) account for
greater than 90% of the earth's crust. Feldspars are the most common crustal
mineral, while olivine is the most abundant mineral in the earth as a whole.
Nonsilicates are either native elements or are classified by their negative ion.
These include ore minerals or commercial value.
8. Physical properties are used to identify minerals. These include color,
streak, luster, hardness, external crystal form, cleavage, fracture, specific
gravity, special properties (smell, taste, striations, magnetism), and other
properties (double refraction, effects of polarized light, x-ray defraction).
Chemical tests can be used to identify minerals.
Boxes
5.1 IN GREATER DEPTH -WATER AND ICE - MOLECULES AND CRYSTALS:
Water is abundant on earth and life would be impossible without it. Water is an
unusual substance. The molecule is asymmetrical and polarized. Thus, water is
the universal solvent because other substances are attracted to water.
Molecules are more tightly packed in liquid water than in ice, which forms
hexagonal crystals by sharing hydrogen atoms and floats even though it is the
solid phase. The fact that ice floats makes the climate of the earth habitable. If
ice sank, bodies of water would eventually freeze solid. Water also expands
when it freezes, which promotes mechanical weathering.
5.2 - IN GREATER DEPTH -BONDING: The attraction between ions involves a
balancing of electromagnetic charges, and orderly internal-arrangement of
atoms characterizes all crystalline substances. Four types of bonding are
recognized: 1) ionic, with fixed atom positions and transferred electrons, is
typical of halite and is the most common in minerals; 2) covalent, with
adjacent atoms sharing electrons, is typical of diamond; 3) metallic,
characterized by free movement of electrons between atoms, is not important
in geology; and 4) very weak attractive forces, such as those that keep
adjacent sheets of micas and graphite together. Graphite and diamond are one
of the softest and the hardest minerals, yet they have the same composition
(pure carbon). The difference between those two minerals reflects bonding.
This box concludes with reference to a website called Mineral Web that
provides illustrations of crystal structures in 3-D.
5.3 - ENVIRONMENTAL GEOLOGY – IS ASBESTOS REALLY HARMFUL TO
HUMANS? Studies suggest that the threat from exposure to chrysotile or
white asbestos, as opposed to blue and brown amphibolite asbestos, is
overstated. The greatest threat of asbestosis and lung cancer seems to have
been to asbestos workers who didn't wear protective clothing or those who
smoked, while non-occupational exposure risk is less than being struck by
lightning or killed in an automobile accident. Residents living near asbestos
mines in Canada and the United States did not have a higher incidence of
cancer than the overall population. White asbestos fibers dissolve in lung
tissue over a year, while fibers of blue and brown asbestos and fiberglass, now
replacing asbestos, would not dissolve in a person's lifetime and represent a
health threat. Removal of white asbestos (about 95% of domestic use) from
public buildings is probably unnecessary.
5.4 - ENVIRONMENTAL GEOLOGY -CLAY MINERALS THAT SWELL: Clay
minerals are sheet silicates with a variety of uses. Kaolinite is used as a food,
additive, particularly to thicken fast-food "milk" shakes. Montmorillonite
adsorbs water between its silicate layers. This characteristic causes expansion
and can cause problems for building-foundations, although it may be useful in
sealing cracks in bedrock or concrete.
5.5 – IN GREATER DEPTH – BANCROFT: MINERAL CAPITAL OF CANADA:
The Bancroft area of southern Ontario is host to many valuable mineral
resources including sodalite, talc, gold, mica, fluorite, beryl, uranium, graphite
and iron. These minerals have been mined in the area since the mid-1800’s.
Bancroft’s mineral wealth is due to deformation of volcanic rocks during the
Grenvillian orogeny some 1.1 billion years ago. Erosion of the Grenville
Mountains over time has exposed the mineral rich rocks at or close to the
ground surface where they can be easily mined.
Short Discussion/Essay
1. Explain the general public's confusion with the term mineral.
2. How might it be argued that water is actually a mineral?
3. How does silicate structure explain why mafic rocks are silica-poor and felsic
rocks are silica-rich?
4. Why are gold and diamonds valuable?
5. How can graphite and carbon have the same composition?
Longer Discussion/Essay
1. Why are oxygen and silicon the most common elements in the earth's
crust?
2. Why are silicates the most common minerals in the earth's crust?
3. If all minerals are crystalline, why don't they always form crystals?
4. How does silicate structure control the physical properties of silicate
minerals, particularly cleavage?
5. How do minerals actually form the three major rock types?
Selected Readings
Mineralogy is a basic subdiscipline of geology and there are many textbooks treating this area.
Some articles of interest:
Kirshner, R. P. 1994."The earth's elements," Scientific American 271:58-65.
Mueche, G.K. and Moller, P. 1988."The not-so-rare earths," Scientific American
258:72-77