Reference:
Chapters 2, 3, 6, 7; Appendix A & B
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Unit 3:
Topic 3.2
 identifying the different compositions of Igneous rock.
 relating Bowen's reaction series to mineral composition.
 identifying and explaining how different Igneous rocks can have the same composition.
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Classification of Igneous Rocks:
 Igneous rocks are further classified according to:
1. Texture
 Describes the appearance of an igneous rock, based on the size, shape and arrangement of interlocking crystals.
2. Mineral Composition
 Is the mineral makeup of an igneous rock based on the chemical composition of the magma.
Text
Reference: pages 61-75
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Igneous Magma Compositions:
 Mainly silicate minerals.
 Determined by composition of magma from which it crystallized.
 Magma has mainly 8 elements:
 Si, O , Al, Ca, Na, K, Mg, Fe
*Most abundant constituents
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Igneous Magma Compositions:

.
The minerals that are present in any
.
igneous rock depends on the chemical composition and the environment
(temperature of the magma) from which the molten rock crystallizes.
 N. L. Bowen discovered that different minerals form at different temp..
 Bowen also noted that minerals will react with the magma to produce the next mineral in the reaction series, this is known as Bowen’s Reaction Series .
Ca-Rich
Plagioclase Mafic
Na-Rich
Plagioclase
Intermediate
Felsic
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Igneous Magma Compositions:
 Olivine and Ca-rich plagioclase feldspar are the first to crystallize at very high temperatures and these minerals are often found in mafic igneous rocks.
 Orthoclase feldspar and quartz crystallize at lower temperatures and are found in felsic igneous rocks
 Therefore, igneous rocks are classified according to their mineral makeup as
Mafic, Intermediate, or Felsic .
Ca-Rich
Plagioclase Mafic
Na-Rich
Plagioclase
Intermediate
Felsic
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Igneous Magma Compositions:
 Magma
 cools
 solidifies
 forms silicate minerals
 Two major silicate mineral groups:
 DARK Silicates
 LIGHT Silicates
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Igneous Magma Compositions:
Silicate Group Rich In
DARK
LIGHT
Fe and/or Mg
Si, K, Na, Ca
Poor In
Si
Examples olivine pyroxene amphibole biotite
Fe and/or Mg quartz muscovite feldspars
• Orthoclase
• plagioclase
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Igneous Magma Compositions:
Mafic magnesium + iron • High in Mg and Fe.
• Dark and Dense
• Found in Ocean Crust
Felsic feldspar + silica
(quartz)
• High in Si.
• Lighter and Less Dense
• Found in Continental Crust
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Igneous Magma Compositions:
 3 main compositions:
 “Mafic” : ma gnesium + fe rrum
:high in Mg, Fe. Dark, dense
 Basaltic
 “Intermediate”  Andesitic
 Granitic
 “Felsic” : fel dspar + si licate
:high in Si, lighter, less dense
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Mafic Magma Composition:
 contain magnesium and iron rich minerals
 form at high temperatures 1200 E C
 found mainly in oceanic crust
 form most volcanic rocks (Basalt)
 low percentage of silica, very fluid magmas
 forms dark colored minerals, thus rocks are dark color
 minerals include; olivine, pyroxene, plagioclase feldspar
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Mafic Magma Composition: zooming in basalt gabbro
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Intermediate Magma Composition:
 contains 25% dark silicate minerals (Amphibole,
Pyroxene,and Biotite Mica)
 forms at moderate temperatures, approximately
800 E C
 found associated with volcanic activity confined to the margins of the continents
 form mainly volcanic intermediate rocks (Andesite)
 high percentage of silica, very viscous (thick) magmas
 contains both light and dark colored minerals
 minerals include; Amphibole, Pyroxene, Biotite Mica
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Intermediate Magma Composition: zooming in andesite diorite
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Felsic Magma Composition:
 contains orthoclase feldspar and quartz (silica)
 forms at lower temperatures, approximately 600 E C
 found mainly in continental crust
 form mainly plutonic rocks (Granite)
 high percentage of silica, very viscous (thick) magmas
 forms light colored minerals, thus rocks are light colored
 minerals include; orthoclase and plagioclase feldspars, .
quartz, and muscovite mica
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Felsic Magma Composition: zooming in rhyolite granite
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Felsic Magma Composition: obsidian pumice
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Mineral Composition:
Classification Chart
Composition Felsic
(Granitic)
Major
Minerals
Quartz
Orthoclase
Feldspar
Color Light
Plutonic Granite
Volcanic Rhyolite
Intermediate
(Andesitic)
Amphibole
Feldspars
Medium
Diorite
Andesite
Mafic
(Basaltic)
Pyroxene
Plagioclase
Feldspar
Dark
Gabbro
Basalt
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Mineral Composition:
What are the similarities and differences between the following rock pairs?
Gabbro and
Basalt
Similarity
 composition
 both Mafic
Difference
 texture
 form in diff. environments
Diorite and
Andesite
Granite and
Rhyolite
 composition
 both Intermediate
 composition
 both Felsic
 texture
 form in diff. environments
 texture
 form in diff. environments
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What is the correct match for magma composition and environment?
Magma
Composition
(A) Felsic
(B) Felsic
(C) Mafic
(D) Mafic
Environment continental crust ocean crust mountain interior transform fault
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Which pair of rocks have the same mineral composition?
(A) andesite and granite
(B) andesite and rhyolite
(C) basalt and diorite
(D) basalt and gabbro
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Take the time and complete the following questions . . .
(Solutions to follow)
Question:
Explain how mineral composition could be used to classify igneous rocks.
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Question:
Explain how mineral composition could be used to classify igneous rocks.
Mineral Composition:
If the igneous rock is light color and consist of the minerals quartz and orthoclase feldspar, it is classified as felsic.
If the igneous rock is dark color and consist of the minerals pyroxene and plagioclase feldspar, it is classified as mafic.
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Overview of Points covered:
 Bowen’s reaction series.
Composition of Igneous Rocks:
3 Types …..
Mafic  Dark color, high density, ocean crust.
 Gabbro and Basalt
Intermediate  medium color and density, cont. margins.
 Diorite and Andesite
Felsic  Light color, low density, continental crust.
 Granite and Rhyolite
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