Classifications of Igneous Rocks Classifications of Igneous Rocks
Chapter 2
Chapter 2
Classification of Igneous Rocks Classification of Igneous Rocks
Method #1 for plotting a point with the components: 70% X, 20% Y, and 10% Z on triangular diagrams.
Figure 2-1a. from your text: An Introduction to Igneous and Metamorphic Petrology, John Winter.
X = 100%
X = 0%
Discussion:
Normalization
If total does not add to 100%, normalize by multiplying each term by
100/(X + Y + Z)
Normalization Example Normalization Example
Normalization
If total does not add to 100%, normalize by multiplying each term by
100/(X + Y + Z)
Ex 1. A sample contains X = 9 g. Qtz, Y = 2.6 g Plag, Z= 1.3 g Microcline What are the percentages by weight?
Sol’n: Multiply each by
100/ (9 + 2.6 + 1.3) = 7.752
9 x 7.752 = 69.76, 2.6 x 7.752 = 20.15 1.3 x 7.752 = 10.1 percents
total 99.99% close enough to 70, 20,
10 percents respectively
IUGS IUGS
Classification of Classification of
Phaneritic Phaneritic
Igneous Rocks Igneous Rocks
Figure 2-2. A classification of the phaneritic igneous rocks. a. Phaneritic rocks with more than 10% (quartz + feldspar + feldspathoids). After IUGS. From your text: An Introduction to Igneous and Metamorphic Petrology, John Winter, Prentice Hall.
The rock must contain a total of at least 10% of the minerals below.
Renormalize to 100%
(a)
Quartz-rich Granitoid
90 90
60 60
20 20
Alkali Fs.
Quartz Syenite Quartz Syenite
Quartz
Monzonite Quartz Monzodiorite Syenite Monzonite Monzodiorite (Foid)-bearing
Syenite
5
10 35 65
(Foid)-bearing
Monzonite (Foid)-bearing Monzodiorite
90
Alkali Fs.
Syenite
(Foid)-bearing Alkali Fs. Syenite
10
(Foid) Monzosyenite (Foid
) Sye nite
(Foid) Monzodiorite
(Foid) Gabbro
Qtz. Diorite/
Qtz. Gabbro
5
10
Diorite/Gabbro/
Anorthosite (Foid)-bearing Diorite/Gabbro
60
(Foid)olites
Quartzolite
Granite Grano- diorite
Tona lite
Alkali Feldspar Granite
Q
A P
F
60
Define Tonalite, Monzonite,
Syenite based on this.
International Union of Geological Sciences Don’t use “foid”
in a rock name.
Use the actual
Feldspathoid
mineral name
Classification of Classification of Aphanitic Igneous Aphanitic Igneous
Rocks Rocks
Figure 2-3. A classification and nomenclature of volcanic rocks. After IUGS. From your text:
An Introduction to Igneous and Metamorphic Petrology, John Winter, Prentice Hall.
(foid)-bearing
Trachyte (foid)-bearing
Latite (foid)-bearing Andesite/Basalt
(Foid)ites
10
60 60
35 65
10
20 20
60 60
F
A P
Q
Rhyolite Dacite
Trachyte Latite Andesite/Basalt
Phonolite Tephrite
Define Dacite, Trachyte, Latite and Phonolite and Tephrite
based on this
Classification of Aphanitic Igneous Rocks Classification of Aphanitic Igneous Rocks
Figure 2-4. A chemical classification of volcanics based on total alkalis vs. silica. After Le Bas et al.
(1986) J. Petrol., 27, 745-750. Oxford University Press.
Classification of Pyroclastic Igneous Rocks Classification of Pyroclastic Igneous Rocks
Figure 2-5. Classification of the pyroclastic rocks. a. Based on type of material. After Pettijohn (1975) Sedimentary Rocks, Harper & Row, and Schmid (1981) Geology, 9, 40-43. b. Based on the size of the material. After Fisher (1966) Earth Sci. Rev., 1, 287-298. From your text: An
Introduction to Igneous and Metamorphic Petrology, John Winter, Prentice Hall.
Classification of Minerals Classification of Minerals
Common Silicate minerals Common Silicate minerals
Nesosilicates – Independent Tetrahedra Nesosilicates – Independent Tetrahedra
Olivine Olivine
• High temperature Fe-Mg silicate (typical High temperature Fe-Mg silicate (typical mantle mineral - formed 100’s km in Earth mantle mineral - formed 100’s km in Earth
• Individual tetrahedra linked together by iron Individual tetrahedra linked together by iron and magnesium ions
and magnesium ions
• Forms small, rounded crystals with no Forms small, rounded crystals with no cleavage
cleavage
(Mg,Fe)
2SiO
4High interference colors
No consistent
cleavages
Classification of Minerals Classification of Minerals
Common Silicate minerals Common Silicate minerals
Pyroxene Group Single Chain Inosilicates Pyroxene Group Single Chain Inosilicates
for example ( for example ( Mg,Fe)SiO Mg,Fe)SiO
33• Single chain structures involving iron and Single chain structures involving iron and magnesium, chains weakly paired
magnesium, chains weakly paired
• Two distinctive cleavages at nearly 90 degrees Two distinctive cleavages at nearly 90 degrees
• Augite Augite is the most common mineral in the is the most common mineral in the pyroxene group
pyroxene group
Classification of Minerals Classification of Minerals
Common Silicate minerals Common Silicate minerals
Amphibole Group Double Chain Inosilicates Amphibole Group Double Chain Inosilicates
Ca Ca
22(Fe,Mg) (Fe,Mg)
55Si Si
88O O
2222(OH) (OH)
22• Double chain structures involving a variety of Double chain structures involving a variety of ions ions
• Two perfect cleavages exhibiting angles of , e.g. Two perfect cleavages exhibiting angles of , e.g.
124 and 56 degrees in Hornblende.
124 and 56 degrees in Hornblende.
• Hornblende Hornblende is the most common mineral in the is the most common mineral in the amphibole group
amphibole group
Pleochroic in Plane Polarized Light
Looks stringy
Hornblende Crystal Hornblende Crystal
56 and 124 degree 56 and 124 degree
Cleavages Cleavages
Distinguish Hornblende from Pyroxene Group by cleavage
Pyroxene Crystal
Two Cleavage Faces
at about 90 degrees
90o
Cleavage in Pyroxenes Cleavage in Pyroxenes
It isn’t perfect in all slices
Cleavage in Amphiboles
Looking down the c-axis
Looking down the c-axis
Looking down the c-axis
Amphiboles Amphiboles
Amphiboles such as Hornblende are pleochroic in Amphiboles such as Hornblende are pleochroic in Plane Polarized Light. Hornblende is monoclinic.
Plane Polarized Light. Hornblende is monoclinic.
With crossed polars, they have inclined extinction, With crossed polars, they have inclined extinction,
i.e. they go dark at an angle to ONE of their i.e. they go dark at an angle to ONE of their
cleavage planes
cleavage planes
“ “ Clinopyroxenes” (monoclinic pyroxenes) Clinopyroxenes” (monoclinic pyroxenes) also have inclined extinction,
also have inclined extinction, but are not pleochroic in PPL but are not pleochroic in PPL
Any monoclinic mineral has one inclined Any monoclinic mineral has one inclined extinction when rotating with crossed polars extinction when rotating with crossed polars
http://www.youtube.com/watch?v=1DSqh5oEYOE
Classification of Minerals Classification of Minerals
Common Silicate minerals Common Silicate minerals
Mica Group Phyllosilicates Mica Group Phyllosilicates
• Sheet structures that result in one direction of perfect Sheet structures that result in one direction of perfect cleavage
cleavage
• Biotite Biotite is the common dark colored mica mineral. Has is the common dark colored mica mineral. Has wavy “bird’s eye extinction”
wavy “bird’s eye extinction”
• Muscovite Muscovite is the common light colored mica mineral . is the common light colored mica mineral . Can have undulose extinction.
Can have undulose extinction.
• https://www.youtube.com/watch?v=dvDankgGBIs https://www.youtube.com/watch?v=dvDankgGBIs
Muscovite
KAl
3Si
3O
10(OH)
2In plane polarized light, Biotite is seen as dark brown to grey against the surrounding mostly colorless minerals. Under crossed polars
"bird's eye " = “mottled” = “wavy”
extinction can easily be seen when the mineral is nearly extinct. Often, the mineral color masks the
interference colors when the mineral is not extinct.
http://www.youtube.com/watch?v=IjUdjGQyWtw http://www.youtube.com/watch?v=Bv3MVkyyxjk
Pleochroic in PPL
http://www.youtube.com/watch?v=-6LEW_H-ccQOrders of Interference colors
Orders of Interference colors
3-D (Framework) Tectosilicates 3-D (Framework) Tectosilicates
Quartz SiO2
Quartz Quartz
Undulose (aka “undulatory”) extinction Undulose (aka “undulatory”) extinction
1 1 o o gray in standard thin section 30 gray in standard thin section 30 m m
http://www.youtube.com/watch?v=O1I-_YdgaHg http://www.youtube.com/watch?v=O1I-_YdgaHg
Forms late in igneous, fills in gaps between earlier xtals Forms late in igneous, fills in gaps between earlier xtals
Identifying minerals with a Identifying minerals with a
Michel-Levy Chart Michel-Levy Chart
If you know the thickness of the thin section, you can narrow
down the possibilities by noting where the interference color of
an unknown crosses the thickness line
Thin section ~30 microns, mineral is dark second order blue, so birefringence about 0.020
Possibilities circled
Feldspars Feldspars
Common Silicate minerals Common Silicate minerals
Tectosilicates Tectosilicates