Classifications of Igneous Rocks
Chapter 2
Classification of Igneous Rocks
X = 100%
Discussion:
Normalization
If total does not add
to 100%, normalize
by multiplying each
term by
100/(X + Y + Z)
X = 0%
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.
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
(a)
IUGS
Classification of
Phaneritic
Igneous Rocks
Define Tonalite,
Monzonite,
Syenite based on this.
The rock must contain a total of
at least 10% of the minerals below.
Renormalize to 100%
Q
Quartzolite
90
90
Quartz-rich
Granitoid
60
60
Granodiorite
Granite
Alkali Fs.
Quartz Syenite
Alkali Fs.
Syenite
20
20
Quartz
Monzonite
Quartz
Syenite
5
10
A
Syenite
(Foid)-bearing
Syenite
35
Monzonite
(Foid)-bearing
Monzonite
Quartz
Monzodiorite
65
Monzodiorite
(Foid)-bearing
Monzodiorite
10
(Foid)-bearing
Alkali Fs. Syenite
(Foid)
Monzosyenite
5 Diorite/Gabbro/
90
Anorthosite
P
10 (Foid)-bearing
Diorite/Gabbro
(Foid)
Monzodiorite
International Union of Geological Sciences
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.
Qtz. Diorite/
Qtz. Gabbro
60
60
(Foid)olites
F
Don’t use “foid”
in a rock name.
Use the actual
Feldspathoid
mineral name
Q
Classification of
Aphanitic Igneous
Rocks
60
60
Rhyolite
Define Dacite, Trachyte, Latite
and Phonolite and Tephrite
based on this
Dacite
20
20
Trachyte
Latite
35
A
10
(foid)-bearing
Trachyte
65
(foid)-bearing
Latite
Phonolite
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.
Andesite/Basalt
(foid)-bearing
Andesite/Basalt
10
Tephrite
60
60
(Foid)ites
F
P
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
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

Common Silicate minerals

Nesosilicates – Independent Tetrahedra

Olivine
(Mg,Fe)2SiO4
• High temperature Fe-Mg silicate (typical
mantle mineral - formed 100’s km in Earth
• Individual tetrahedra linked together by iron
and magnesium ions
• Forms small, rounded crystals with no
cleavage
High interference
colors
No consistent
cleavages
Classification of Minerals

Common Silicate minerals
Pyroxene Group Single Chain Inosilicates
 for example (Mg,Fe)SiO3

• Single chain structures involving iron and
magnesium, chains weakly paired
• Two distinctive cleavages at nearly 90 degrees
• Augite is the most common mineral in the
pyroxene group
Classification of Minerals

Common Silicate minerals


Looks stringy
Amphibole Group Double Chain Inosilicates
Ca2(Fe,Mg)5Si8O22(OH)2
• Double chain structures involving a variety of
ions
• Two perfect cleavages exhibiting angles of , e.g.
124 and 56 degrees in Hornblende.
• Hornblende is the most common mineral in the
amphibole group
Pleochroic in Plane Polarized Light
Distinguish Hornblende from
Pyroxene Group by cleavage
Pyroxene Crystal
Two Cleavage Faces
at about 90 degrees
Hornblende Crystal
56 and 124 degree
Cleavages
90o
Cleavage in Pyroxenes
It isn’t perfect in all slices
Cleavage in
Amphiboles
Looking down the c-axis
Looking down the c-axis
Amphiboles

Amphiboles such as Hornblende are pleochroic in
Plane Polarized Light. Hornblende is monoclinic.
With crossed polars, they have inclined extinction,
i.e. they go dark at an angle to ONE of their
cleavage planes
“Clinopyroxenes” (monoclinic pyroxenes)
also have inclined extinction,
but are not pleochroic in PPL

Any monoclinic mineral has one inclined
extinction when rotating with crossed polars
http://www.youtube.com/watch?v=1DSqh5oEYOE
Classification of Minerals

Common Silicate minerals

Mica Group Phyllosilicates
• Sheet structures that result in one direction of perfect
cleavage
• Biotite is the common dark colored mica mineral. Has
wavy “bird’s eye extinction”
• Muscovite is the common light colored mica mineral .
Can have undulose extinction.
• https://www.youtube.com/watch?v=dvDankgGBIs
KAl3Si3O10(OH)2
Muscovite
In 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_
Orders of Interference colors
3-D (Framework) Tectosilicates
Quartz SiO2
Quartz




Undulose (aka “undulatory”) extinction
1o gray in standard thin section 30mm
http://www.youtube.com/watch?v=O1I-_YdgaHg
Forms late in igneous, fills in gaps between earlier xtals
Identifying minerals with a
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


Common Silicate minerals
Tectosilicates

Feldspar Group
• Most common mineral group
• 3-dimensional framework of tetrahedra exhibit two
directions of perfect cleavage at 90 degrees
• K-spars (potassium feldspar) and Plagioclases (sodium
to calcium feldspar solutions) are the two most
common groups
• Pearly to vitreous Luster
Potassium feldspar
KAlSi3O8
Note Pearly Luster
http://www.youtube.com/watch?v=7-KZREqrh44
Tartan twins in Microcline.
Microcline is the low TP version of K-spars KAlSi3O8
Microcline is Triclinic, Orthoclase is Monoclinic
Perthitic Texture, Microcline plus exsolved Alb
Plagioclase feldspar
(Ca,Na)AlSi3O8
Note the Twinning, seems to have ‘stripes’
http://www.youtube.com/watch?v=gLcVT_6y-M
Labradorite
Albite NaAlSi3O8
Glass (magma cooled to fast for crystals to form)
Plagioclase (Anorthite) xtals in basaltic glass. Crossed Polars
The glass is isotropic and so stays extinct under crossed polars,
i.e. it is black in all orientations.
Garnet
Garnet is also Isotropic, and has a very high refractive index, so
cracks stand out strongly. Under crossed polars it stays dark.
Looks like a squashed
pink tomato
Pink garnet (PPL)
Indistinct cleavage
Garnet inclusion (crossed polars)
Next week: Chapter 3 Textures of Igneous Rocks