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5 SECTION 2
Silicates
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Tetrahedrons
In 1947, R. Buckminster
Fuller (1895–1983) patented
an architectural structure
called the geodesic dome,
which is based on the
tetrahedron. The dome is
constructed of a network of
interconnected tetrahedra
made of lightweight
material. This design evenly
distributes stress across the
entire structure. The
geodesic dome, free of
interior structural supports,
exhibits a high strength-toweight ratio. Buildings that
employ this architecture
include the Climatron in St.
Louis, Missouri, the
Astrodome in Houston,
Texas, and the United States
Pavilion built for Expo 67 in
Montreal, Canada.
Silicon and oxygen are the two most abundant elements in Earth’s crust, so
it is not surprising that most minerals contain these elements. Minerals
that are compounds including silicon and oxygen are called silicates. A
silicate may also contain one or more metallic elements, such as aluminum
or iron. A few silicates do not contain metal. For example, quartz is
composed only of oxygen and silicon. More than 90 percent of the minerals
in Earth’s crust are silicates.
The basic building block of a silicate is the silica tetrahedron,
consisting of four oxygen atoms packed closely around a silicon atom. This
unit is named for its shape. As shown in the ball-and-stick model below,
imaginary lines connecting the four oxygen atoms form a geometric figure
called a tetrahedron. A silica tetrahedron is held together by chemical
bonds between the silicon atom and the oxygen atoms. All silicates are
composed of these tetrahedra, although the tetrahedra may be arranged in
various ways. The table on the next page shows how silicates are classified
according to the different arrangements of tetrahedral units. Note that the
metals contained in silicate compounds are not considered in the
classification.
Examine 3-D models of common
molecules.
Keycode: ES05031
SILICA TETRAHEDRON
The arrangement of silica tetrahedra
determines many properties of silicate
minerals, including cleavage. Several
arrangements are shown in the table on
page 101. For all but the first
arrangement, oxygen atoms are shared
by adjacent tetrahedra.
Oxygen atom
Silicon atom
BALL-AND-STICK MODEL
SPACE-FILLED MODEL
Crystal Structure and Physical Properties
As a result of their crystalline structures, minerals are solid. The atoms,
ions, and molecules in minerals are closely packed, bound by strong
chemical bonds. An increase in temperature, however, may weaken the
bonds between particles. At high temperatures, minerals melt, becoming
liquids made up of loose groups of particles. At even higher temperatures,
minerals vaporize, becoming gases in which individual particles are far
apart. The temperatures at which a mineral melts and vaporizes are
characteristic of the mineral and can sometimes be used to differentiate
two different minerals of similar appearance.
Crystal structure also determines a mineral’s cleavage, or tendency to
split along definite planes. The planes along which the mineral splits
correspond to planes of weak bonds between the atoms, ions, or molecules
of the mineral. Halite splits into cubes between layers of ions. Quartz, with
its strong network of atoms, does not split along any plane.
V ISUAL T EACHING
Discussion
Have students compare the balland-stick model of the silica
tetrahedron to the space-filled
model. Ask how the models differ.
The ball-and-stick model shows the
bonds and bond angles, as viewed from
the side. The space-filled model shows
how much of the space within the
compound is filled by each atom; it is
viewed from below.
Use Transparency 6.
100
Unit 2 Earth’s Matter
DIFFERENTIATING INSTRUCTION
Hands-On Demonstration
Have students use a microscope or
magnifying lens to observe grains of table salt. Ask: What properties
can you see with and without magnification?
Exercise caution when working with microscope slides. Remind
students that safety is everyone’s responsibility.
100
Unit 2 Earth’s Matter
Support for Physically Impaired Students
If students have
difficulty with manual dexterity, mount magnifying lenses on ring
stands so that students can look at salt and other crystals. Place the
samples in large, flat boxes so that students can more easily move the
boxes.
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CHAPTER
5 SECTION 2
Molecular Structures of Some Common Silicate Minerals
Structure
Cleavage
Mineral
Olivine has no cleavage.
Olivine Group
CLASSZONE.COM
Examine 3-D models of common
molecules.
Keycode: ES0503
Visualizations CD-ROM
Olivine
Beryl has one imperfect
cleavage.
Beryl
V ISUAL T EACHING
Discussion
Make sure students understand
that the silica tetrahedron (shown
in this illustration as a space-filled
model) is the basic building block
of minerals. Point out how in the
more complex structures shown,
the tetrahedra are bound together
by a shared oxygen atom (red). As
a mineral forms, more silica
tetrahedra are layered on. The
weaker the bonds between the
layers of the tetrahedra, the more
likely the mineral is to cleave along
those layers or planes. Cleavage is
classified based on the quality of
the surfaces produced and the ease
of cleaving. A mineral that readily
breaks into large smooth surfaces
shows perfect cleavage. Achieved
with some difficulty, good cleavage
shows smaller steplike surfaces.
Cleaving that is difficult and
produces rough surfaces is
imperfect.
Beryl
Diopside has two
perfect cleavages, at
close to 90° angles.
Pyroxene Group
Diopside
Tremolite has one
perfect and one
imperfect cleavage at
close to 60° and 120°
angles.
Amphibole Group
Tremolite
Micas exhibit perfect
cleavage in one
direction.
Mica Group
Muscovite Mica
Feldspar Group
Extend
Make sure students understand
that the crystal faces that result
from mineral growth are distinct
from planes of cleavage. However,
since cleavage does follow the
symmetry of a mineral, planes of
cleavage are often parallel to
crystal faces.
Microcline feldspar has
two good cleavages, at
or near 90° angles.
Too complex to draw
Microcline Feldspar
Chapter 5 Atoms to Minerals
101
Use Transparency 6.
DIFFERENTIATING INSTRUCTION
Reading Support
Make sure students understand the meanings of
the words imperfect and perfect as they relate to cleavage. Then
have them prepare index cards for the structures shown. On one side
of the card, they should list the group name; on the other side they
can draw the molecular structure and give information about cleavage.
Chapter 5 Atoms to Minerals
101
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5 SECTION 2
The hardness of a mineral also depends on the arrangement of its ions,
atoms, or molecules and on the strength of the chemical bonds between
them. A good example of the relationship between hardness and crystalline
structure is the two minerals that consist of the element carbon. When
carbon atoms are arranged in a tetrahedral network, the result is diamond,
the hardest natural mineral. Yet when carbon atoms are arranged in sheets
of hexagonal networks, the result is graphite, a very soft mineral that flakes
easily.
A SSESS
1
naturally occurring crystalline solid
(atoms being arranged in an orderly
pattern), with a definite chemical
composition and inorganic origins
2
Magma process—Magma rises and
begins to cool; atoms, molecules,
and ions move closer together and
bond to form compounds, which
mass together over time; example is
diorite. Evaporation—As water
evaporates, ions crystallize together;
example is halite. Pressure
process—High temperatures and
pressure break down minerals in
rock and they recrystallize in solid
state, rearranging free atoms, ions,
and molecules into new minerals;
example is hornblende schist.
3
Answers should include three of the
following: mineral’s solid form,
cleavage, hardness, and density.
4
The hardness of a mineral depends
on the arrangement of its atoms.
When carbon atoms are arranged in
an interlocking network of silica
tetrahedra, the result is a very hard,
very strong mineral: diamond. When
they are arranged in sheets of
hexagonal networks, the result is a
very soft, slippery mineral: graphite.
5
6
Covalent bond
Diamond
Graphite
CARBON STRUCTURES
Recall that the density of a material is the ratio of its mass to its
volume. Density depends not only on the masses of the atoms in the
mineral but also on how they are arranged. For example, although both
graphite and diamond are made of carbon atoms, the density of graphite is
about 2.2 g/cm3, whereas that of diamond is 3.5 g/cm3.
5.2 Section Review
1
List the five characteristics of a mineral.
2
Describe two ways in which minerals are formed. Include an example
of each process.
3 List three physical characteristics of a mineral that are influenced by
its crystalline structure.
4
metals to which they bond;
temperatures at which they form;
pressures at which they form; speed
at which the formation takes place
Both diamond and graphite are composed purely of carbon atoms.
Explain why diamond is so much harder than graphite.
5 CRITICAL THINKING Silicate minerals have a variety of crystalline
structures, even though they are made from the same building
blocks—silica tetrahedra. What factors do you think affect the
structures of silicate minerals as they are forming?
Answers will vary based on the
region. Remind students that
minerals in their region may have
formed far away and been
transported by water or glaciers.
6
102
GEOGRAPHY What types of minerals are common in the region
where you live? Speculate on the processes by which they might
have formed.
Unit 2 Earth’s Matter
MONITOR AND RETEACH
If students miss . . .
Question 1 Review the characteristics of minerals. (p. 96) Ask students to
explain each characteristic.
Question 2 Have students review mineral formation, including the diagrams.
(p. 97) Have pairs of students retell each process in their own words.
Question 3 Have students reread “Crystal Structure and Physical
Properties.” (pp. 100–102) Have them outline what they read.
102
Unit 2 Earth’s Matter
Question 4 Refer students to the Carbon Structures illustration (p. 102) and
have them compare the crystalline structure of diamond and graphite.
Question 5 Have students reread “How Minerals Form” (p. 97) and
“Silicates.” (p. 100) Tell students to note any information describing factors
that affect the crystalline structure of minerals.
Question 6 Have students research information about regional minerals at
the library or on the Internet.