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Chapter 2
Rocks and Minerals: A First Look
The differences in the
physical properties of
rocks, minerals, and soils
determine their suitability
for different purposes –
extraction of water or of
metals, construction,
manufacturing, waste
disposal, agriculture, and
other uses
Atoms
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Smallest particle into which an element can
be divided while still retaining the chemical
characteristics of that element
Composed of a nucleus surrounded by
electrons
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Nucleus is composed of protons (+) and
neutrons (0)
Number of protons defines the chemical element
and atomic number ( H = 1, He = 2, Li = 3, …)
Number of neutron adds mass to the atom
Number of electrons (-) orbiting nucleus
determined by the number of positively charged
protons;
Negatively charged electrons balance the
positive charges of the protons
Figure 2.1 Schematic drawing of atomic structure
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Element – substance composed of atoms with
the same number of protons
All nuclei, except the simplest hydrogen
atoms, contain neutrons
The number of neutrons is similar to or
somewhat greater than the number of
protons
Isotopes – number of neutrons for an element may
not be the same; variable numbers of neutrons
possible
 Atomic Mass Number is the number of protons and
neutrons in the element’s nucleus
 Some isotopes have more neutrons and are
heavier (carbon-14 has 6 protons and 8 neutrons)
 Some isotopes have fewer neutrons and are lighter
(carbon-12 has 6 protons and 6 neutrons)
Ion
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An atom that is positively charged or
negatively charged
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Anion has gained electrons (-); has more
electrons relative to the number of protons
(+)
Cation has lost electrons (-); has fewer
electrons relative to the number of protons
(+)
The electrical attraction of ions will
cause an ionic bond to form between
oppositely charged ions.
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Na+ + Cl- = NaCl (halite)
Compounds
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Mixing of two or more chemical
elements in particular proportions that
have distinctive physical properties
Elements will bond because of electrical
attraction, forming ionic bonds, or the
atoms may share electrons, forming
covalent bonds
If atoms gain or lose outermost
electrons to form ions – Ionic Bond
Covalent Bond
MINERALS
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By definition a mineral is/has
Naturally occurring
Inorganic
Solid element or compound
Definite chemical composition
Regular, ordered, internal crystal
structure
Figure 2.4B
Figure 2.4C
Identifying Minerals
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The two fundamental characteristics of a
mineral are its chemical composition and
its crystal structure
Analyze the mineral composition
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Measure crystal structure and symmetry
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Technology based
Technology based
Observe and measure physical and special
properties
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Easy for humans to see and recognize
Physical Properties of Minerals
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Crystal form
External expression of a mineral’s
internal structure
 Often interrupted due to
competition for space and rapid
loss of heat
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Quartz crystals
Figure 2.1
A Garnet Crystal
Crystals of Pyrite
Figure 1.11
Physical Properties of Minerals
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Luster
Appearance of a mineral in
reflected light
 Two basic categories
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Metallic
Nonmetallic
Other descriptive terms include
vitreous, silky, or earthy
Galena (PbS) Displays
Metallic Luster
Figure 1.9
Physical Properties of Minerals
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Color
Generally unreliable for mineral
identification
 Often highly variable due to slight
changes in mineral chemistry
 Exotic colorations of certain
minerals produce gemstones
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Quartz (SiO2) Exhibits
a Variety of Colors
Physical Properties of Minerals
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Streak
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Color of a mineral in its powdered
form
•Hardness
Resistance of a mineral
to abrasion or
scratching
All minerals are
compared to a standard
scale called the Mohs
scale of hardness
Physical Properties of Minerals
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Cleavage
Tendency to break along planes of
weak bonding
 Produces flat, shiny surfaces
 Described by resulting geometric
shapes
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Number of planes
Angles between adjacent planes
Muscovite 1 direction
Fluorite, Halite, and Calcite
All Exhibit Perfect Cleavage
Physical Properties of Minerals
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Fracture
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Absence of cleavage when a
mineral is broken
Specific Gravity
Weight of a mineral/weight of an
equal volume of water
 Average value = 2.5–3.0
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Conchoidal Fracture
Figure 1.16
Physical Properties
of Minerals
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Other properties
Magnetism
 Reaction to hydrochloric acid
 Malleability
 Double refraction
 Taste
 Smell
 Elasticity
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Mineral Groups
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Nearly 4000 minerals have been
named
Rock-forming minerals
Common minerals that make up
most of the rocks of Earth’s crust
 Only a few dozen members
 Composed mainly of the eight
elements that make up over 98%
of the continental crust
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Elemental Abundances
in Continental Crust
Figure 1.19
Mineral Groups
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Silicates
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Most important mineral group
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Comprise most rock-forming minerals
Very abundant due to large % of
silicon and oxygen in Earth’s crust
Silicon-oxygen tetrahedron
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Fundamental building block
Four oxygen ions surrounding a much
smaller silicon ion
Figure 2.8
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Silicate group – variety of compounds
based on silicon and oxygen
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Quartz – glass manufacturing
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Feldspar – ceramic manufacturing
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Most common mineral group
Mica
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Composed entirely of O and Si
Muscovite (white mica)
Biotite (black mica)
Clays – used as drilling mud, in building
materials, and as a soil modifier
Ferromagnesian silicates
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Olivine – peridot (semiprecious gem)
Garnet – abrasives; semiprecious gems
Amphibole – industrial products
Important nonsilicate minerals
Comprise only 8% of Earth’s crust
Often occur as constituents in sedimentary rocks
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Carbonates – CO3
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Sulfates – SO4
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Host for many metallic ores (Pb, Cu, Zn, and others)
Oxides – any metal combined with oxygen
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Useful for building materials
Sulfides – S
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Useful for building materials and manufacturing
Iron and aluminum ores
Native elements – minerals composed of single
element
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Carbon as diamond and graphite
Copper, gold, silver, or platinum
Rocks – formed from Minerals
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A solid aggregate of one or more
minerals, or mineral materials
Consists of many mineral grains or
crystals forming a solid mass
Each rock contains a record of its own
history
Three broad categories
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Igneous
Sedimentary
Metamorphic
The Rock Cycle
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Shows the interrelationships among the
three rock types
Rocks of any type can be transformed into
rocks of another type or into another distinct
rock of the same general type through the
geologic processes
Rocks are continually being changed by
geological processes
Full cycle does not always take place due to
"shortcuts" or interruptions
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Through time, geologic processes
acting on older rocks change them into
new and different ones so that, in a
sense, all kinds of rocks are
interrelated
Igneous Rocks
• Magma, at high enough temperatures,
rocks and minerals melt, and the natural
hot, molten rock material is called magma
• Silicates are the most common minerals,
and magmas are thus rich in silica.
Magmas also contain some dissolved
water and gases, and include some solid
crystals suspended in the melt; iron
content is variable and this variability is the
basis for igneous rock classification
• An igneous rock is a rock formed by the
solidification and crystallization of a
cooling magma
Igneous Rocks
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Usually composed of silicate minerals and
some dissolved gases and water
Molten materials are very hot
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Plutonic rocks form if magma cools inside
earth’s crust (does not flow onto surface);
coarse crystals will grow
Volcanic rocks form if magma flows onto
surface as lava; glass often forms
Felsic;
Intermediate
low iron
light toned
Mafic;
Ultramafic
high iron
dark toned
Volcanic;
extrusive;
aphanitic
Rhyolite
Andesite
Basalt
Eclogite
Plutonic;
intrusive;
phaneritic
Granite
Diorite
Gabbro
Peridotite
Sedimentary Rocks
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Sediments are produced by weathering of preexisting rocks and minerals
Sediments are loose, unconsolidated accumulations
of mineral or rock particles
Sediments are eroded, transported, and deposited
in many sedimentary environments
The sediments will be buried and experience
lithification
Lithification involves compacting the sediments
with burial and cementation of the sediments
forming a sedimentary rock
Sedimentary Rocks
• Gravity plays a role in the formation of all
sedimentary rocks.
• Layering is a very common feature of
sedimentary rocks and is used to identify the
origins of sedimentary rocks.
• Sedimentary rocks can yield information
about the settings in which the sediments
were deposited.
• Sedimentary rocks are formed at or near the
earth’s surface and at temperatures close to
ordinary surface temperatures.
Types of Sedimentary Rocks
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Clastic sedimentary rocks
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Formed by the lithification of mechanically weathered
pieces of rocks and minerals
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Grain sizes range from boulder, gravel, sand, silt, and mud
Grains are continually broken down in size and shape
until deposited
Once deposited these clastic particles a cemented
Chemical sedimentary rocks
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Chemical process occur in water bodies such as lakes,
seas, or oceans
Minerals precipitate from the water and form thick
deposits
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Examples: Halite, Calcite, and Gypsum
Figure 2.12 Sedimentary Rocks
Metamorphic Rocks
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“Changed form” rock
Rock formed from pre-existing rock or minerals
Heat, pressure, and chemically active fluids
cause changes in rock
Heat increases as a rock is buried or is close to a
magma chamber
Pressure increases with burial or collision
between moving continents
Fluids become heated and circulate with burial
or with location near a magma chamber
Metamorphic Rocks
• The temperatures required to form metamorphic
rocks are below magmatic temperatures
• Significant changes can occur in a rock at
temperatures well below melting
• Temperature and pressure can cause the
minerals in the rock to recrystallize
• Pressure may cause the rock to be deformed
• The sources of elevated temperatures of
metamorphism: burial, magma, mountainbuilding, and plate tectonic movement
• The sources of elevated pressures of
metamorphism: burial, mountain-building, and
plate tectonic movement
Types of Metamorphism
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Contact metamorphism – localized
metamorphism of rocks adjacent to a
magma chamber
Regional metamorphism – large
scale stressing and heating of a rock by
deep burial or continental plates moving
and colliding
Common Metamorphic Rocks
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Any kind of preexisting rock (another rock) can
be metamorphosed
Foliation: when a rock is subjected to directed
stress, its minerals form elongated/platy crystals
and line up parallel to each other
Metamorphic rocks without foliation do not
show directed stress
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Marble is metamorphosed limestone
Quartzite is metamorphosed quartz-rich sandstone
Metamorphic rocks with foliation show directed
stress or pressure
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Slate – low grade foliated metamorphic rock
Schist and Gneiss (nice) – high grade metamorphic
rocks
Figures 2.13 Metamorphic rocks have
undergone mineralogical, chemical, and/or
structural change