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![Page 1: Igneous rocks are further classified according to; Describes the appearance of an igneous rock, based on the size, shape and arrangement of interlocking.](https://reader030.fdocuments.in/reader030/viewer/2022032805/56649ede5503460f94bee796/html5/thumbnails/1.jpg)
Igneous rocks are further classified according to;
Describes the appearance of an igneous rock, based on the size, shape and arrangement of interlocking crystals.
1. Texture
2. Mineral Composition
Is the mineral makeup of an igneous rock based on the chemical composition of the magma.
Reference:
Pages 61 - 75
Classification of Igneous Rocks:
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Classification of Igneous Rocks:
Igneous rocks are classified from
texture
mineral composition
depends on: chemical makeup of parent magma
depends on: how fast/slow magma cools
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Igneous compositions
•mainly silicate minerals
•determined by composition of magma from
which it crystallized
•magma mainly 8 elements:
Si, O, Al, Ca, Na, K, Mg, Fe
Most abundant constituents
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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.
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.
N. L. Bowen discovered that different minerals form at different temp.
Ca-Rich Plagioclase
Na-Rich Plagioclase
Mafic
Intermediate
Felsic
Igneous compositions
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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.
Therefore, igneous rocks are classified according to their mineral makeup as Mafic, Intermediate, or Felsic.
Orthoclase feldspar and quartz crystallize at lower temperatures and are found in felsic igneous rocks
Ca-Rich Plagioclase
Na-Rich Plagioclase
Mafic
Intermediate
Felsic
Igneous compositions
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Magma
cools
solidifies
forms silicate minerals
Two major silicate mineral groups:
DARK silicates LIGHT silicates
Igneous compositions
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DARK
LIGHTLIGHT
Rich in
Fe and/or Mg
Si, K, Na, CaSi, K, Na, Ca
Poor in
Si
Fe and/or MgFe and/or Mg
Silicate group
olivine
pyroxene
amphibole
biotite
quartzquartz
muscovitemuscovite
feldsparsfeldsparsorthoclaseorthoclase
plagioclaseplagioclase
Examples
Igneous compositions
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Igneous rocks types
mafic
felsic High in Si.
Lighter and Less Dense
Found in Continental Crust
High in Mg and Fe.
Dark and Dense
Found in Ocean Crust
magnesium + ferrum
feldspar + silica (quartz)
Igneous compositions
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Mafic Composition
basaltbasalt
gabbrogabbro
m
a f
i c
m
a f
i c
zooming inzooming in
volc
an
icp
luto
nic
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contain magnesium and iron rich minerals
form at high temperatures 1200C
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
Mafic Composition
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andesiteandesite
dioritediorite
i n
t e
r m
e d
i a
t e
i n
t e
r m
e d
i a
t e
zooming inzooming in
volc
an
icp
luto
nic
Intermediate Composition
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contains 25% dark silicate minerals (Amphibole, Pyroxene, . and Biotite Mica)
forms at moderate temperatures, approximately 800C
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 and . Plagioclase Feldspar
Intermediate Composition
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granitegranite
rhyoliterhyolite
zooming inzooming in
f
e l s
i c
f
e l s
i c
plu
ton
icvolc
an
ic
Felsic Composition
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obsidianobsidian
pumicepumice
f
e l s
i c
f
e l s
i c
volc
an
icvolc
an
ic
Felsic Composition
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contains orthoclase feldspar and quartz (silica)
forms at lower temperatures, approximately 600C
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
Felsic Composition
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3 main types
basaltic
andesitic
granitic
“felsic”“felsic”
“mafic” magnesium + ferrumhigh in Mg, Fe. Dark, dense
feldspar + silicatehigh in Si, lighter, less dense
Igneous Magma Compositions
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basaltic
andesitic
granitic
“felsic”
“mafic”
magnesium + ferrumhigh in Mg, Fe. Dark, dense
feldspar + silicatehigh in Si, lighter, less dense
“felsic”“felsic”
3 main typesIgneous Magma Compositions
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Classification Chart
Mineral Composition
Plutonic
Volcanic
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Sample Problem
Explain how mineral composition could be used to classify igneous rocks.Answer:
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.