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Chapter 2
Atoms, Elements, and Minerals
Index

Rocks & Minerals
Rock – Naturally formed, consolidated material
composed of grains of one or more
minerals.
(There are a few
exceptions to this definition.)
Fig. 2.1
Mineral – A naturally occurring, inorganic,
crystalline
solid that has a definite
chemical composition.
Fig. 2.2


Atoms
Element – A substance that cannot be broken down to
other
substances by ordinary chemical
methods. Each atom of
an element possesses the
same number of protons.
Atom – Smallest possible particle of an element that retains
the
properties of that element.
Proton – A subatomic particle that contributes mass and a
single
positive electrical charge to an atom.
Neutron – A subatomic particle that contributes mass to an
atom
and is electrically neutral.
Electron – A single, negative electric charge that
contributes
virtually no mass to an atom.
Fig. 2.3


Inner Atom
Nucleus – Protons and neutrons form the nucleus
of an
atom. Although the nucleus occupies
an
extremely tiny fraction of the volume
of the
entire atom, practically all the
mass of the
atom is concentrated in
the nucleus.
Isotopes – Atoms (of the same element) that have
different numbers of neutrons but
the same
number of protons. (in
the nucleus)


Atomic . . . .
Atomic mass number – The total number of
neutrons
and
protons in an atom.
Atomic number – The total number of protons in
an
atom.
Atomic weight – The sum of the weight of the
subatomic
particles in an average
atom of an
element, given in
atomic mass units.
Chemical Activity
– Many geological processes can be explained as chemical
reactions. Some rocks form as a result of chemical
reactions between substances. Understanding a few basic
concepts of chemistry will clarify why and under what
conditions chemical reactions occur.
Atoms that are not electrically neutral tend to react (or
combine) with other atoms to neutralize the electrical
imbalance. Each atom not only seeks electrical neutrality
but wants each of its shells to be full of electrons. The
innermost shell is full when it possesses 2 electrons. Outer
shells each require 8 electrons for an atom to be nonreactive (this is true for the second and third shells;
elements having additional shells are more complicated.)
• Fig. 2.4


Ions
Ions – An electrically charged atom or group
of atoms.
Fig. 2.5
Bonding


Chemical Composition of the
Earth’s Crust
Pg. 33, Table 2.1


The Important Minerals
It is useful to be able to associate the names of important
minerals with the physical properties that identify them of course,
what constitutes an “important” mineral depends on your
perspective. To a miner or prospector, an important mineral is one
that is commercially valuable (and is, by implication, relatively
uncommon). A “rock hound” is interested in collecting any mineral
that is pretty or unusual. A gemologist specializes in those
varieties of minerals that are of gem quality (diamonds, emeralds,
etc.). A mineralogist is a scientist who studies the chemistry and
crystallographic structure of minerals. In this book, the minerals
we regard as important are those that help us understand the
nature of the earth. We are particularly interested in rock-forming
minerals because they make up most of the rocks of the earth’s
crust.


– Continued
Of the several thousand identifiable minerals on earth, most
are rare and not important to geology (many occur at only a
single site on the globe). Only a few hundred are classified
as rock-forming minerals. Even most of these are relatively
uncommon in comparison with the few minerals that make
up the vast bulk of the earth’s crust. The five minerals
groups listed in the upper third of table 2.2 account for well
over 90% of the earth’s crust. These are the minerals whose
names recur most often in this book.


The Physical Properties of
Minerals
Color
External Crystal Form
Streak
Cleavage
Luster
Fracture
Hardness
Specific Gravity
Simple Chemical Tests

Other Properties

Color
The first thing most people notice about a mineral is its
color. For some minerals, color is a useful property.
Because color is so obvious, beginning students tend to
rely too heavily on it as a key to mineral identification.
Unfortunately, color is also apt to be the most ambiguous
of physical properties. If you look at a number of quartz
crystals, for instance, you may find specimens that are
white, pink, black, yellow, or purple. Color is extremely
variable in quartz and many other minerals because even
minute chemical impurities can strongly influence it.
Obviously, it is poor procedure to attempt to identify
quartz strictly on the basis of color.
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Streak
A pulverized mineral gives a color, called a streak, that
usually is more reliable than the color of the specimen
itself, scraping the edge of a minerals sample across an
unglazed porcelain plate leaves a streak that may be
diagnostic of the mineral. For instance, hematite always
leaves a reddish brown streak though the sample may be
brown or red or silver.
Unfortunately, few of the silicate minerals – the most
common minerals – leave an identifying streak because
most are harder than the porcelain plate.
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Luster
The quality and intensity of light that is reflected from the
surface of a mineral is termed luster. (A photograph cannot
show this quality.)
Luster is either metallic or nonmetallic.
Metallic – Gives a substance the appearance of being made of metal.
NonMetallic
Glassy luster – like glass or porcelain
Earthy luster – like unglazed pottery
of
Silky luster – appearance of
silk
Pearly luster – appearance
pearl
Resinous luster – appearance of resin
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Hardness
The property of “scratchability,” or hardness, can be
tested fairly reliably. For a true test of hardness, the harder
mineral or substance must be able to make a groove or
scratch on a smooth, fresh surface of the softer mineral.
For example, quartz can always scratch calcite or feldspar.
Substances can be compared to Mohs’ hardness scale,
on which ten minerals are designated as standards of
hardness. The softest mineral, talc (used for talcum
powder because of its softness), is designated as 1.
Diamond, the hardest natural substance on earth, is 10 on
the scale.
Fig. 2.13
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Mohs’ Hardness Scale
Table 2.3 on pg. 40
Back
Fig. 2.13
Pg. 41
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External Crystal Form
The crystal form of a mineral is a set of faces that have a
definite geometric relationship to one another. What most
people call a “crystal” is a mineral with well-developed
crystal faces. To geologists a crystal usually means any
homogenous solid that is crystalline (with or without crystal
faces).
Fig. 2.14
Fig. 2.15
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Fig. 2.18
Fig. 2.14
Pg. 41
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Fig. 2.15
Pg. 41
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Fig. 2.18
Pg. 42
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Cleavage
The internal order of a crystal may be expressed externally
by crystal faces, or it may be indicated by the minerals’
tendency to split apart along certain preferred directions.
Cleavage is the ability of a mineral to break, when struck,
along preferred directions.
A mineral tends to break along certain planes because the
bonding between atoms is weaker there.
Cleavage is one of the most useful diagnostic tools because it
is identical for a given mineral from one sample to another.
Cleavage is especially useful for identifying minerals when
they are small grains in rocks.
Fig. 2.19
Fig. 2.20
Back
Fig. 2.19
Pg. 44
Back
Fig. 2.20
Pg. 44
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Fracture
Fracture is the way a substance breaks where not
controlled by cleavage. Minerals that have no cleavage
commonly have an irregular fracture.
Some minerals break along curved fracture surfaces known
as conchoidal fractures (Fig. 2.23). These look like the
inside of a clam or conch shell.
Back
Fig. 2.23
Pg. 45
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Specific Gravity
It is easy to tell that a brick is heavier than a loaf of bread
just by hefting each of them. The brick has a higher
density, weight per given volume, than the bread.
Density commonly expressed as specific gravity, the
ration of a mass of a substance to the mass of an equal
volume of water.
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Other Properties
Properties that are useful in only a few instances include
taste and smell. Halite obviously tastes salty; few other
minerals have any taste at all. An “earthy” smell is
characteristic of some clay minerals when they are
moistened.
Plagioclase feldspar commonly exhibits striations – straight,
parallel lines on the flat surfaces of one of the two cleavage
directions (Fig. 2.24)
The mineral magnetite (an iron oxide) owes its name to its
characteristic physical property of being attracted to a
magnet.
A clear crystal of calcite exhibits an unusual property. If you
place transparent calcite over an image on paper, you will see
two images (Fig. 2.25)
Fig. 2.24
Pg. 46
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Fig. 2.25
Pg. 46
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Simple Chemical Tests
One chemical reaction is routinely used for identifying
minerals. The mineral calcite, as well as some other
carbonate minerals (those containing CO3-2), reacts with a
weak acid to produce carbon dioxide gas. In this test, a
drop of dilute hydrochloric acid applied to the sample of
calcite bubbles vigorously, indicating that CO2 gas is being
formed. Normally this is the only chemical test that
geologists do during field research.
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The Rock Cycle
Fig. 2.27 on pg. 48


Word Example of Rock Cycle
As described in chapter 1, magma is molten rock. Igneous rocks form
when magma solidifies. If the magma is brought to the surface by a
volcanic eruption, it may solidify into an extrusive igneous rock. Magma
may also solidify very slowly beneath the surface. The resulting intrusive
igneous rock may be exposed later after uplift and erosion remove the
overlying rock (as shown in Fig. 1.13) the igneous rock, being out of
equilibrium, may then undergo weathering and erosion, and the debris
produced is transported and ultimately deposited (usually on a sea floor)
as a sediment. If the unconsolidated sediment becomes lithified
(cemented or otherwise consolidated into a rock), it becomes a
sedimentary rock. As the rock is buried by additional layers of sediment
and sedimentary rock, heat and pressure increase. Tectonic forces may
also increase the temperature and pressure. If the temperature and
pressure become high enough, usually at depths greater than several
kilometers below the surface, the original sedimentary rock is no longer in
equilibrium and recrystallizes. The new rock that forms is called a
metamorphic rock. If the temperature gets very high, the rock melts and
becomes magma again, completing the cycle. The cycle can be repeated.
A Plate Tectonic Example
Fig. 2.28 on pg. 49
Beginning

Back to the
Fig. 2.1
Pg. 28
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Fig. 2.2
Pg. 28
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Fig. 2.3
Pg. 30
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Fig. 2.4
Pg. 31
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Fig. 2.5
Pg. 31
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Bonding
Article on pg. 32
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Table 2.2
Pg. 38
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