Transcript Example - HCC Learning Web

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.
What can Minerals Tell Us?
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.
What can Minerals Tell Us?
4. 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.
5. 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.
What can Minerals Tell Us?
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.
2.
3.
4.
5.
Naturally occurring
Inorganic
Solid
Definite chemical composition
Orderly internal crystal structure
Minerals
Each mineral has different physical and
chemical properties, which make it easy for
us to identify the different species of
minerals.
Some Physical Properties
of Minerals
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•
•
color
streak
luster
hardness
density
crystal form
cleavage
•
•
•
•
•
•
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.
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
1.
2.
3.
4.
5.
6.
Talc (softest)
Gypsum← fingernail
Calcite← penny (copper)
Fluorite← nail
Apatite← glass
Orthoclase feldspar (or
potassium feldspar)
7. Quartz
8. Topaz
9. Corundum
10. 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.
• 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
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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
• 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.
• Hard (scratches glass)
• Glassy luster
• Conchoidal fracture.
• Six-sided, elongated
crystals.
Silicate Minerals - Quartz
• 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 - Amphiboles
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Two directions of cleavage, not at 90o
Narrow, elongated crystals
Typically dark in color (black or dark green).
Common in metamorphic rock amphibolite.
Example: Hornblende. Contains Mg and Fe.
Silicate Minerals - Olivine
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Olive green color
Glassy texture.
No cleavage.
Conchoidal fracture.
Contains Mg and Fe.
Main constituent of the ultramafic rock,
peridotite (birthstone = peridot).
Silicate Minerals - Clays
• Group of minerals formed from weathering of
feldspars and some other minerals.
• Very fine-grained
• Dull, earthy luster
• Soft, smooth feel
Example: Kaolinite, a white clay with many economic
uses.
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)
• Calcium magnesium carbonate.
– Dolomite (CaMg(CO3)2)
Calcite
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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.
Aragonite
• Same chemical formula as calcite, but it
has a different crystal structure.
• Fizzes in hydrochloric acid.
• Shells and skeletons of corals and
mollusks (clams and snails).
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
Anhydrite
• Like gypsum, but without the water.
(anhydrous = without water.)
• Forms from the de-watering of gypsum.
• A relatively common sedimentary mineral.
• May be white, gray, colorless or blue.
Rocks
• A rock is an aggregate of one or more
minerals.
• Rocks are the building blocks of the
Earth's crust.
Rocks
1. Igneous - Crystallized from hot, molten
rock. Examples: granite, basalt
2. Sedimentary - Fragments of sediment laid
down by water or wind become
compressed or cemented over time
Examples: sandstone, shale, limestone
3. 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
of the Earth.
• 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.
Cooling History and Grain Size
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
3. Silica-poor
Silica-rich Rocks
1. High percent silica.
2. Light-colored.
3. 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.
2. Mixture of light and dark minerals.
Examples: diorite, andesite.
Silica-poor Rocks
1. Iron and magnesium rich.
2. Dark-colored.
3. Has dark minerals such as olivine,
pyroxene, and amphibole.
Examples: gabbro, basalt.
Very silica-poor Rocks
1. Very iron and magnesium rich.
2. Typically green in color due to abundant
olivine.
Example: Peridotite.
Igneous Rock Classification
F
i
n
e
C
o
a
r
s
e
Silica-rich Intermediate
(silicic)
Silicapoor
(mafic)
Rhyolite
Andesite
Basalt
Granite
Diorite
Gabbro
Very silica
poor
(ultramafic)
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
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.
Weathering of granite
in a humid climate
1. Feldspars undergo hydrolysis to form clay.
2. Biotite and amphibole undergo hydrolysis to
form clay, and oxidation to form iron oxides.
3. Na, Ca, and K ions are lost in solution and
washed away.
4. Small amounts of Si from feldspars, biotite, and
amphibole are lost in solution.
5. Quartz remains as sand grains due to its
resistance to weathering.
Fate of the Weathering Products
• Clay minerals form shale
• Iron oxides form cement, ochre, or iron ore
• Dissolved Na, Ca, and K ions form
limestone, evaporites, or become included
in shale
• Dissolved Si ions form chert, silica
cement, or diatomite
• Unaltered quartz grains form sandstone
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)
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–
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Conglomerate or Breccia
Sandstone
Siltstone
Shale or Claystone
2. Chemical/biochemical sedimentary rocks
3. Organic sedimentary rocks (coal)
Types of Sedimentary Rocks
1. Clastic sedimentary rocks (also called
terrigenous or detrital)
2. Chemical/biochemical sedimentary rocks
– Evaporites
– Carbonate sedimentary rocks (limestone
and dolostone or dolomite)
– Siliceous sedimentary rocks (chert,
diatomite)
3. Organic sedimentary rocks
Types of Sedimentary Rocks
1. Clastic sedimentary rocks (also called
terrigenous or detrital)
2. Chemical/biochemical sedimentary rocks
3. Organic sedimentary rocks
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–
–
–
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 are
Classified by Grain Size
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•
•
•
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.
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
3. Siliceous rocks - form from chemical
processes (silica replacing limestone) or
biochemical processes (silica-secreting
organisms)
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
1. Limestones
–
–
–
–
–
–
–
Micrite (microcrystalline limestone)
Oolitic limestone
Fossiliferous limestone
Coquina
Chalk
Crystalline limestone
Others
2. Dolostones or dolomites
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).
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.
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
Alteration of rock by heat adjacent to hot
molten lava or magma.
2. 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 fine-grained; 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
Non-foliated or granular metamorphic rocks
are composed of equidimensional grains
such as quartz or calcite. There is no
preferred orientation. The grains form a
mosaic.
Non-foliated Metamorphic Rocks
Marble - Composed of finely- to coarselycrystalline 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 coarselycrystalline 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.
Non-foliated Metamorphic Rocks
Hornfels - Very hard, fine-grained rock.
Derived from the contact metamorphism of
shale and other fine-grained rocks.
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.
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
From studies of minerals in metamorphic
rocks it is possible to determine the
conditions under which the rocks formed.
Metamorphic Index Minerals
Recap: The Rock Cycle