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Chapter 4
Rocks and Minerals:
Documents that Record Earth's History
What can Minerals Tell Us?
1. Minerals may contain radioactive elements that can be used for
radiometric age dating.
2. Minerals that crystallize from magmas and lavas can provide
information about temperatures, as well as viscosity of the
magma, type of volcano, and tectonic setting.
3. Minerals that form under metamorphic conditions can provide
information about temperatures and pressures, from which we
can determine the depth at which metamorphism occurred, and
information about the history of the formation of mountain ranges.
4.
5.
Minerals that form by evaporation in arid climates can tell us
about paleoclimatic conditions. Since some climates are
controlled by latitude, we can make general inferences about
latitude.
Minerals that form in sea water tell us about the nature of ancient
seas.
What can Minerals Tell Us?
6. Minerals which contain iron can record the orientation of the
Earth's magnetic field, which yields information on latitude,
and provides evidence for drifting continents, sea floor
spreading, and movement and reversal of the Earth's
magnetic poles.
7. Minerals in sedimentary rocks can provide information on the
tectonic setting, amount of relief, paleoclimate, and types of
rocks that are eroding in the source area.
8. Minerals can also tell us about the changing chemistry of the
atmosphere, for example, the presence or absence of
oxygen.
Minerals
By definition, minerals are:
1. Naturally occurring
2. Inorganic
3. Solid
4. Definite chemical composition
5. Orderly internal crystal structure
Minerals
Each mineral has different physical and
chemical properties, which make it possible for
us to identify the different species of minerals.
Some Physical Properties of Minerals
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•
•
•
•
•
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color
streak
luster
hardness
density
crystal form
cleavage
•
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fracture
magnetism
reaction to acid
taste
flexibility
feel
Physical Properties of Minerals
• Color - the color or range of colors of a mineral as it appears to
the eye in reflected light.
• Examples:
– Quartz may be colorless, white, pink, purple, dark brown, green or blue.
– Pyrite is always gold colored.
Physical Properties of Minerals
• Streak - the color of a mineral when it is ground to a powder.
Streak color may be quite different from the whole mineral
color.
• Examples:
– Hematite may be silver or gray, but it has a reddish brown streak.
– Pyrite is gold, but is has a black streak.
Physical Properties of Minerals
• Luster - the character of the light reflected from the mineral. A
mineral may have a metallic luster or a non-metallic luster.
Physical Properties of Minerals
• Hardness - the resistance of a mineral to scratching.
• Hardness is measured on a scale of 1 - 10 called Mohs
Hardness Scale.
• Hardness of minerals can also be compared to common objects
(fingernail, copper penny, nail, glass).
Mohs Hardness Scale
Talc (softest)
Gypsum← fingernail
Calcite← penny (copper)
Fluorite← nail
Apatite← glass
Orthoclase feldspar (potassium feldspar)
Quartz
Topaz
Corundum
Diamond (hardest)
Physical Properties of Minerals
• Density - how heavy a mineral is for its size.
• The mass of a mineral divided by its volume is a measure of its
density. Usually given as specific gravity (mass compared to
water).
• Examples:
– Quartz has a density of 2.65 g/cm3.
– Gold has a density of 19.3 g/cm3.
Physical Properties of Minerals
• Crystal form - some minerals are in the form of
crystals. Crystal shape is related to the structural
arrangement of atoms within the mineral.
• Crystals enlarge through addition of ions to their
surfaces as they crystallize.
• Perfect crystals are rare because minerals typically
grow close together in confined spaces, producing a
mass of interlocking crystals.
• A crystal which form in a large space may develop
perfect crystal faces.
Physical Properties of Minerals
• Cleavage - the tendency of a mineral to break along
flat surfaces related to planes of weakness in its
crystal structure. Minerals can be identified by the
number of cleavage planes they exhibit, and the
angles between them.
• Examples:
– Some minerals tend to cleave or break into flat
sheets (the micas: muscovite and biotite).
– Others break into cubes (halite), or into rhombs
(calcite and dolomite).
Physical Properties of Minerals
• Fracture - irregular breakage, not related to planes of
weakness in the mineral.
• Some minerals, such as quartz and olivine, do not have
cleavage. They have a type of fracture called conchoidal
fracture. Conchoidal fracture produces curved breakage
surfaces, as seen on arrowheads or chipped glass.
Physical Properties of Minerals
• Magnetism - A few minerals are magnetic. They are attracted
to a magnet, or they act as a natural magnet, attracting small
steel objects such as paperclips.
• Example:
– Magnetite.
Physical Properties of Minerals
• Reaction to acid - The carbonate minerals react with
diluted hydrochloric acid (HCl) by effervescing or
fizzing, producing bubbles of carbon dioxide gas.
• Examples:
– Calcite fizzes readily in hydrochloric acid.
– Dolomite will fizz if it is first scratched and
powdered.
Physical Properties of Minerals
• Taste - Some minerals have a distinctive taste.
• Example:
– Halite has a salty taste. It is used as table salt.
Physical Properties of Minerals
• Flexibility - Some minerals can be bent.
• Examples:
– Muscovite and biotite mica are elastic. When bent they return to their original
shape.
– Gypsum is flexible. It bends and stays bent.
Physical Properties of Minerals
• Feel - Some minerals have a distinctive feel to the
fingers.
• Example:
– Talc has a soapy feel.
Rock-Forming Minerals
• There are more than 3000 minerals on the Earth, but only a few
are common and make up most of the rocks.
• The common rock-forming minerals can be divided into two
groups:
– Silicates
– Non-silicates.
Silicate Minerals
• Earth's crust is dominated by 2 chemical elements:
– Oxygen (46.6% by weight)
– Silicon (27.7% by weight)
– These elements help make up the dominant group
of rock-forming minerals, the silicate minerals.
• Examples: quartz, feldspar, mica
Silicate Minerals - Structure
• The silicate minerals are based on a crystal structure that
involves four oxygen atoms arranged in pyramid-like shape,
surrounding a smaller silicon atom.
• This structure is called the silicate tetrahedron.
Silicate Minerals - Feldspar
• Dominant mineral in Earth's crust.
• Two directions of cleavage at 90o
• Flat, glassy rectangular surfaces.
• Color may be white, pink, gray, green.
• Common in igneous rocks such as granite and basalt.
Silicate Minerals - Feldspar
Two major types:
• Orthoclase (potassium feldspar) - KAlSi3O8
• Plagioclase - A range of compositions with sodium and
calcium.
– Calcium-rich = anorthite (CaAl2Si2O8)
– Sodium-rich = albite (NaAlSi3O8)
Silicate Minerals - Quartz
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Second-most abundant mineral in Earth's crust.
Color varies - colorless, white (milky quartz), gray to brown (smoky quartz), pink
(rose quartz), purple (amethyst), blue, or green.
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Hard (scratches glass)
Glassy luster
Conchoidal fracture.
Six-sided, elongated
crystals.
Silicate Minerals - Quartz
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Common in granite
Resists weathering; common in some sands in humid areas
Major constituent of quartz sandstone and quartzite.
Chert is composed of microcrystalline quartz.
Silicate Minerals - Mica
• Perfect cleavage in one direction causing it
to split into thin sheets.
• Two types:
– Muscovite - Colorless or silver-colored
mica.
– Biotite - Black or dark brown mica
(contains Mg and Fe).
Silicate Minerals - Olivine
• Olive green color
• Glassy texture.
• No cleavage.
• Conchoidal fracture.
• Contains Mg and Fe.
• Main constituent of the ultramafic rock,
peridotite (birthstone = peridot).
Non-silicate Minerals
Non-silicate minerals comprise about 8% of the Earth's crust.
Carbonate minerals are the most widespread.
Types:
– native elements
– oxides
– sulfides
– sulfates
– carbonates
– halides
– phosphates, etc.
Carbonate minerals
• Calcium carbonate.
– Calcite (CaCO3)
– Aragonite (CaCO3) different crystal form
• Calcium magnesium carbonate.
– Dolomite (CaMg(CO3)2)
Calcite
• Main constituent of limestone and marble.
• Shells of some marine organisms.
• Fizzes in hydrochloric acid.
• Has rhombohedral cleavage (three directions not at 90o).
Cleavage fragments
are rhombs.
Dolomite
• Has rhombohedral cleavage like calcite.
• Will fizz in acid only when scratched or powdered.
• Main constituent of sedimentary rock dolostone or dolomite.
• Forms from alteration of limestone through the addition of Mg.
Evaporite minerals
• Halite (NaCl)
• Gypsum (CaSO4 . 2H2O)
• Anhydrite (CaSO4)
Halite
• Major constituent of rock salt
(and table salt).
• Cubic cleavage
• Salty taste.
• Typically colorless to white or
pink.
Gypsum
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Major constituent of rock gypsum.
Used in Plaster of Paris and drywall.
Soft - can be scratched by fingernail.
Typically white or colorless to pink.
Varieties:
– Selenite - clear crystals with rhombohedral cleavage
– Alabaster - fine-grained and massive
– Satin spar - fibrous
Rocks
• A rock is an aggregate of one or more minerals.
• Rocks are the building blocks of the Earth's crust.
Rocks 3 Types
1.
2.
3.
Igneous - Crystallized from hot, molten rock. Examples:
granite, basalt
Sedimentary - Fragments of sediment laid down by water
or wind become compressed or cemented over time
Examples: sandstone, shale, limestone
Metamorphic - Rocks changed by heat and/or pressure or
chemical activity
Examples: gneiss, schist, slate, marble
The Rock Cycle
Through the rock cycle,
one type of rock can be
converted into another.
Igneous Rocks
• The word igneous means "fire-formed."
• Igneous rocks crystallized from hot, molten magma or
lava, as it cooled.
– Magma is hot, molten rock beneath the surface of
the Earth.
– Lava is hot, molten rock which has flowed out on
the surface of the Earth.
• Igneous rocks make up more than 90% of Earth's
crust, by volume.
Extrusive Igneous Rocks
Extrusive or volcanic rocks form from lava, which cooled
on the Earth's surface.
Examples: Basalt, rhyolite, andesite, obsidian
Intrusive Igneous Rocks
Intrusive or plutonic igneous rocks form from magma
which cooled beneath the surface.
Examples: Granite, gabbro, diorite
Cooling History and Grain Size
• The texture of a rock is a description of its grain size.
• Cooling rates influence the texture of the igneous
rock.
• Lava cools much more quickly than magma because
lava is on the surface of the Earth, where temperatures
are much lower than they are at depth.
• Extrusive rocks = quick cooling = fine grained
• Intrusive rocks = slow cooling = coarse grained
Extrusive vs. Intrusive
Rhyolite - fine-grained, extrusive igneous
rock.
Granite - coarse-grained, intrusive igneous
rock.
Igneous Rock Classification
Igneous rocks are classified on the basis of:
1. Texture (or grain size)
2. Composition
Igneous Rock Composition Groups
1.
Silica-rich
2.
Intermediate
Silica-poor
3.
Silica-rich Rocks
1.
2.
3.
High percent silica (quartz).
Light-colored.
Has light-colored minerals such as quartz and potassium
feldspar.
Examples: granite, rhyolite.
Intermediate Rocks
1.
Intermediate in composition between silica-rich and
silica-poor.
Mixture of light and dark minerals.
Examples: diorite, andesite.
2.
Silica-poor Rocks
1.
Iron and magnesium rich.
Dark-colored.
3. Has dark minerals such as olivine, pyroxene, and
amphibole.
Examples: gabbro, basalt.
2.
Very silica-poor Rocks
1. Very iron and magnesium rich.
2. Typically green in color due to abundant olivine.
Example: Peridotite.
Igneous Rock Classification
Silica-rich (silicic)
Intermediate
Silica-poor
(mafic)
Very silica
poor
(ultramafic)
F
i
Rhyolite
Andesite
Basalt
Granite
Diorite
Gabbro
n
e
C
o
a
r
s
e
Peridotite
Basalt
• The most common igneous rock.
• Ocean crust is dominated by basalt. Covers about 70%
of Earth's surface.
• Islands like Hawaii and Iceland are made of basalt.
• Fine-grained texture
• Dark color because it contains ferromagnesian (Fe and
Mg) minerals, along with feldspar.
Granite
• Earth's continental crust is dominated by granite.
• Coarse-grained texture.
• Light color because it is dominated by light-colored
minerals like quartz and feldspar.
Bowen's Reaction Series
Minerals in igneous rocks
crystallize in a particular
order, at particular
temperatures.
Sedimentary Rocks
• Cover about 75% of the world's land area.
• Form when loose sediment (gravel, sand, silt or clay)
becomes compacted and/or cemented to form rock.
• The process of converting sediment to sedimentary
rock is called lithification.
Sedimentary Rocks
Sediment is deposited in horizontal layers. A major characteristic
of sedimentary rock is layering, also called bedding or strata.
Sedimentary Rocks
Sedimentary rocks contain the fossil record, which
preserves the evolving story of life on Earth.
What can sedimentary rocks tell us?
• Locations of ancient sedimentary environments (seas,
reefs, deltas, beaches, rivers, lakes deserts, glaciers,
and mountains).
• Ancient climates
– humid tropical coal swamps,
– dry windswept deserts,
– glacial ice sheets,
– high temperatures and high sea levels.
Sedimentary rocks contain the fossil record, which
preserves the evolving story of life on Earth.
Sedimentary Rocks
Sedimentary rocks contain the fossil record, which
preserves the evolving story of life on Earth.
Sedimentary rocks also hold the fossil fuels and energy
resources on which our culture depends - coal, oil,
natural gas. Careful reading of the rock record allows
exploration geologists to find these critical resources.
How is sediment formed?
Sediment forms from the weathering and erosion of rocks, as part
of the rock cycle.
Types of Sedimentary Rocks
• Clastic Sedimentary Rocks (also called terrigenous or
detrital)
• Chemical / biochemical Sedimentary Rocks
• Organic Sedimentary Rocks (Coal)
Types of Sedimentary Rocks
1.
Clastic sedimentary rocks (also called terrigenous or detrital)
–
Conglomerate or Breccia
–
Sandstone
–
Siltstone
–
Shale or Claystone
2.
Chemical/biochemical sedimentary rocks
–
–
–
3.
Evaporites
Carbonate sedimentary rocks (limestone and dolostone or dolomite)
Siliceous sedimentary rocks (chert, diatomite)
Organic sedimentary rocks (coal)
–
Peat
–
Lignite
–
Bituminous coal
–
Anthracite coal
Clastic Sedimentary Rocks
Clastic sedimentary rocks are derived from the weathering of
pre-existing rocks, which have been transported to the
depositional basin.
Clastic Texture
• Clasts (larger pieces, such as sand or gravel)
• Matrix (mud or fine-grained sediment surrounding the
clasts)
• Cement (the chemical "glue" that holds it all together)
Types of cement:
• Calcite
• Iron oxide
• Silica
Clastic Sedimentary Rocks Classified by Grain Size
• Gravel - Grain size greater than 2 mm
• Sand - Grain size 1/16 to 2 mm
• Silt - Grain size 1/256 to 1/16 mm
• Clay - Grain size less than 1/256 mm
Clastic Sedimentary Rocks are classified by grain
size
Grain size
Rock name
Gravel
Conglomerate = rounded clasts Breccia = angular clasts
Sand
Sandstone
Silt
Siltstone
Clay
Shale = fissile
Claystone = massive
Chemical/Biochemical Sedimentary Rocks
Form within the depositional basin from chemical
components dissolved in the seawater.
or
Chemicals are removed from seawater and made into
rocks by chemical processes, or biological processes
(such as shell growth).
Chemical/Biochemical Sedimentary Rocks
1.
Evaporites - form from the evaporation of water
2.
Carbonate rocks - form by chemical processes and
biochemical processes
Siliceous rocks - form from chemical processes (silica
replacing limestone) or biochemical processes (silicasecreting organisms)
3.
Evaporites
1.
Rock salt - composed of halite (NaCl).
2.
Rock gypsum - composed of gypsum (CaSO4 . 2H2O)
3.
Travertine - composed of calcium carbonate (CaCO3)
– a carbonate rock; forms in caves and around hot
springs.
Carbonate Rocks
Limestones
– Micrite (microcrystalline limestone)
– Oolitic limestone
– Fossiliferous limestone
– Coquina
– Chalk
– Crystalline limestone
– Others
2. Dolostones or dolomites
1.
Siliceous rocks
• Diatomite - made of microscopic planktonic organisms
called diatoms. Resembles chalk, but does not fizz in
acid.
• Chert - massive and hard, microcrystalline quartz. May
be dark or light in color. Often replaces limestone.
Does not fizz in acid.
Organic Sedimentary Rocks - Coal
Composed of organic matter (plant fragments). Forms in
swamps.
With increasing depth of burial (temperature and
pressure):
• Peat
• Lignite
• Bituminous coal
• Anthracite coal
Organic Sedimentary Rocks - Coal
• Coal is a fossil fuel. Electric utility companies use
more than 90% of the coal mined in the U.S.
• Chemicals derived from coal are used in making
plastics, tar, synthetic fibers, fertilizers, and
medicines.
• Releases more greenhouse gas than any other fossil
fuel. Contains many other pollutants like uranium and
mercury.
Metamorphic Rocks
•
Metamorphic means "changed form."
•
Metamorphism causes changes in the texture and
mineralogy of rocks.
Rocks are changed or metamorphosed by:
1. High temperatures
2. High pressures
3. Chemical reactions caused by solutions and hot
gases
•
Types of Metamorphism
1.
Contact metamorphism
2.
Alteration of rock by heat adjacent to hot molten lava or
magma.
Regional metamorphism
Alteration of rock over a large area by heat and
pressure due to deep burial or tectonic processes.
Types of Metamorphic Rocks
Metamorphic rocks are separated into two groups on the
basis of texture.
• Foliated
• Non-foliated (or granular)
Foliation = Laminated structure in a metamorphic rock
resulting from the parallel alignment of sheet-like
minerals (usually micas).
Foliated Metamorphic Rocks
In order of increasing grade of metamorphism:
• Slate
• Phyllite
• Schist
• Gneiss
Foliated Metamorphic Rocks
Slate - Mica flakes are microscopic in size. Derived from the
regional metamorphism of shale. Note the relict
sedimentary bedding (vertical).
Phyllite - Mica flakes are very finegrained; other minerals such as
garnet or staurolite may also be
present. Derived from the
regional metamorphism of
shale.
Foliated Metamorphic Rocks
Schist - Mica flakes are visible to the unaided eye.
Derived from the regional metamorphism of shales or
fine-grained volcanic rocks.
Foliated Metamorphic Rocks
Gneiss - Coarse-grained rock with minerals segregated into
light and dark layers or bands. Derived from the regional
metamorphism of high-silica igneous rocks, and muddy
sandstones.
Non-foliated Metamorphic Rocks
Marble - Composed of finely- to coarsely-crystalline
calcite or dolomite. Derived from the metamorphism
of limestone or dolostone. Commonly white or gray.
May be pink.
Non-foliated Metamorphic Rocks
Quartzite - Composed of finely- to coarsely-crystalline
quartz. Derived from the metamorphism of quartz
sandstone.
Non-foliated Metamorphic Rocks
Greenstone - Contains iron and magnesium rich green minerals
such as chlorite and epidote. Fine grained texture. Derived from
the low-grade metamorphism of basalt.
Metamorphic Index Minerals
Certain minerals form during metamorphism, under
specific pressure and temperature conditions. These
minerals can be used as a guide to metamorphic
pressures and temperatures. They are called
metamorphic index minerals.
• Chlorite and muscovite form at relatively low
temperatures.
• Biotite and garnet form at somewhat higher
temperatures and pressures.
• Staurolite and kyanite form at intermediate to high
temperatures and pressures.
• Sillimanite forms at the highest temperatures and
pressures.
Metamorphic Index Minerals