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.
Reference:
Pages 61 - 75
Classification of Igneous Rocks:
Igneous rocks are classified from
 texture
depends on: how fast/slow magma cools
 mineral composition
depends on: chemical makeup of parent magma
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
Igneous 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.
Ca-Rich
Plagioclase
 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.
Mafic
Intermediate
Na-Rich
Plagioclase
Felsic
Igneous 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
Intermediate
Na-Rich
Plagioclase
Felsic
Igneous compositions
Magma
cools
solidifies
forms silicate minerals
Two major silicate mineral groups:
DARK silicates
LIGHT silicates
Igneous compositions
Silicate group
DARK
Rich in
Fe and/or Mg
Poor in
Si
Examples
olivine
pyroxene
amphibole
biotite
LIGHT
Si, K, Na, Ca
Fe and/or Mg
quartz
muscovite
feldspars
orthoclase
 plagioclase
Igneous compositions
Igneous rocks types
 mafic
magnesium + ferrum
 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
Mafic Composition
gabbro
plutonic
basalt
volcanic
zooming in
Mafic Composition
 contain magnesium and iron rich minerals
 form at high temperatures 1200EC
 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
Intermediate Composition
andesite
volcanic
diorite
plutonic
zooming in
Intermediate Composition
 contains 25% dark silicate minerals (Amphibole, Pyroxene,
. and Biotite Mica)
 forms at moderate temperatures, approximately 800EC
 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
Felsic Composition
rhyolite
volcanic
granite
plutonic
zooming in
pumice
volcanic
obsidian
volcanic
Felsic Composition
Felsic Composition
 contains orthoclase feldspar and quartz (silica)
 forms at lower temperatures, approximately 600EC
 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
Igneous Magma Compositions
“mafic”
 basaltic
3 main types
magnesium + ferrum
high in Mg, Fe. Dark, dense
andesitic
 granitic
“felsic”
feldspar + silicate
high in Si, lighter, less dense
Igneous Magma Compositions
magnesium + ferrum
high in Mg, Fe. Dark, dense
“mafic”
 basaltic
andesitic
 granitic
“felsic”
feldspar + silicate
high in Si, lighter, less dense
3 main types
Mineral Composition
Classification Chart
Plutonic
Volcanic
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.