Basis for Classification
• Minerals present in the rock
Rock Classification
Chapter 2, pp. 17-26
– Best for coarse-grained rocks
– Useful for field work
• Chemical Composition
– Works for fine-grained rock
– Expensive and takes time
Mineralogical Classification
Monomineralic Rocks
• Color Index = % of dark minerals
• Plagioclase
Anorthosite
• Felsic < 35% mafic minerals
• Olivine
Dunite
• Mafic = 35% – 90% mafic minerals
• Augite
Clinopyroxenite
• Ultramafic > 90% mafic mineral
• Hypersthene
Orthopyroxenite
QAPF
Diagram
Chemical Classification
• CIPW norm
• Useful for most
Common rocks
• Recalculate the
minerals to
100% QAP or
FAP
– Calculated minerals from
chemical analysis
• Saturation concept
– Si saturation
• Acid to basic
– Al saturation
• Harker-Peacock index
– Alkalies vs calcium
1
Silica Saturation
Acid
SiO2 > 66 %
Intermediate
SiO2 52 to 66 %
Basic
SiO2 45 to 52 %
Ultrabasic
SiO2 < 52 %
Classification of Igneous Rocks
Figure 2-1a. Method #1 for plotting a point with the components: 70% X, 20% Y, and 10% Z on
triangular diagrams. An Introduction to Igneous and Metamorphic Petrology, John Winter, Prentice Hall.
Feldspar Classification
Aluminum Saturation
Based on the feldspar ratio 1:1:3 (NaAlSi3O8)
Peraluminous
Al2O3 > (CaO + Na2O + K2O)
Peralkaline
(Na2O + K2O) > Al2O3
Classification of Igneous Rocks
Figure 2-1b. Method #2 for plotting a point with the components: 70% X, 20% Y, and 10% Z on triangular
diagrams. An Introduction to Igneous and Metamorphic Petrology, John Winter, Prentice Hall.
Pyroxene Classification
2
Classification of Igneous Rocks
Plagioclase
Basalt
Classification
Anorthosite
Figure 2-2. A classification of the phaneritic
igneous rocks. b. Gabbroic rocks. c. Ultramafic
rocks. After IUGS.
90
Ga
lite
cto
Tro
bb
ro
Phases:
Olivine
gabbro
Olivine
Dunite
90
te
bu
rgi
Ha
rz
Pyroxene
Olivine
(b)
e
rlit
eh
Plagioclase-bearing ultramafic rocks
Peridotites
W
Augite (Cpx)
Hypersthene (Hy)
Olivine (Oliv)
Plagioclase (Plag)
Nepheline (Ne)
Lherzolite
40
Pyroxenites
Olivine Websterite
Orthopyroxenite
10
(c)
Websterite
Clinopyroxenite
10
Orthopyroxene
(a)
The rock must contain a total of
at least 10% of the minerals below.
Renormalize to 100%
Q
Q
Quartzolite
90
Classification of
Igneous Rocks
90
Quartz-rich
Granitoid
60
Alkali Fs.
Syenite
Granite
20
20
Quartz
Monzonite
Quartz
Syenite
5
Syenite
10
A
(Foid)-bearing
Syenite
35
Monzonite
(Foid)-bearing
Monzonite
Quartz
Monzodiorite
65 Monzodiorite
Qtz. Diorite/
Qtz. Gabbro
A
P
(Foid)-bearing
Diorite/Gabbro
Latite
35
10
(foid)-bearing
Trachyte
Andesite/Basalt
65
(foid)-bearing
Latite
(foid)-bearing
Andesite/Basalt
P
10
en
Sy
(Foid)
Monzodiorite
(Fo
ite
id)
G
id)
(Foid)
Monzosyenite
abb
ro
(F o
Figure 2-2. A classification of the phaneritic igneous
rocks. a. Phaneritic rocks with more than 10% (quartz +
feldspar + feldspathoids). After IUGS.
60
Phonolite
Tephrite
Figure 2-3. A classification and nomenclature
of volcanic rocks. After IUGS.
60
60
(Foid)olites
60
(Foid)ites
F
Basalt
Discriminator
20
Trachyte
Anorthosite
10
Dacite
20
5 Diorite/Gabbro/
90
(Foid)-bearing
Monzodiorite
10
(Foid)-bearing
Alkali Fs. Syenite
60
Rhyolite
Alk
ali
Alkali Fs.
Quartz Syenite
60
te
ali
F eld
spa
r Gra
nite
60
Granodiorite
n
To
Classification of
Igneous Rocks
Clinopyroxene
F
Basalt Trends
3
Classification of 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.
Si Activity
• αSiO2
• Based on s set of
reactions
• Limits three main
magma series
4