The document discusses the classification of igneous rocks based on texture, field relations, chemical composition, and mineralogy. It categorizes igneous rocks into extrusive (typically aphanitic or glassy) and intrusive (typically phaneritic) varieties, while also outlining compositional classes such as felsic, intermediate, mafic, and ultramafic. Additionally, it describes chemical classifications based on aluminum saturation and mineralogical composition, referencing the IUGS classification system.

1. Classifications of Igneous Rocks

2. Classification of Igneous Rocks : Bases
texture
field relations
Chemical composition
Mineralogical

3. Classification based on texture
Aphanitic- crystals too small to see by eye
Phaneritic- can see the constituent minerals
Fine grained- < 1 mm diameter
Medium grained- 1-5 mm diameter
Coarse grained- 5-50 mm diameter
Very coarse grained- > 50 mm diameter
Porphyritic- bimodal grain size distribution
Glassy- no crystals formed

5. Field Relations
● Extrusive or volcanic rocks: typically
aphanitic or glassy.
● Many varieties are porphyritic and some have
fragmental (volcaniclastic) fabric.
● Intrusive or plutonic rocks: typically
phaneritic

6. Compositional Classes: Felsic v. Mafic

7. Chemical compositions
● Silica percentage (% SiO2)
● >66%, felsic, granite/rhyolite
● 52-66%, intermediate, diorite/andesite
● 45-52%, mafic, gabboro/basalt
● <45%, ultramafic, peridotite/dunite/komatiite

8. Chemical : Silica vs. alkaline elements
● SiO2 Vs. Na2O+K2O
● Calcic
● Calc-alkalic
● Alkalic-calcic
● Alkalic

9. Classification based on chemical comp
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.

10. Chemical : Aluminum saturation (Al2O3 Vs.
Na2O+K2O+CaO)
● Peraluminous
● Metaaluminous
● Peralkaline

11. If there is an excess of Alumina over that required to form
feldspars, the rock is peraluminous :
This condition is expressed chemically on a molecular
basis as:
Al2O3 > (CaO + Na2O + K2O)
In peraluminous rocks we expect to find an Al2O3-rich
mineral present as a modal mineral such as muscovite
[KAl3Si3O10(OH)2], corundum [Al2O3], topaz
[Al2SiO4(OH,F)2], or an Al2SiO5- mineral like kyanite,
andalusite, or sillimanite.
Peraluminous rocks will have corundum [Al2O3] in the
CIPW norm and no diopside in the norm.
Peraluminous

12. Metaluminous
for which the molecular percentages are as
follows:
Al2O3 < (CaO + Na2O + K2O) and Al2O3 > (Na2O
+ K2O)
These are the more common types of igneous
rocks.
They are characterized by lack of an Al2O3-rich
mineral and lack of sodic pyroxenes and
amphiboles in the mode.

13. Peralkaline
rocks are those that are oversaturated with
alkalies (Na2O + K2O),
and thus undersaturated with respect to
Al2O3.
On a molecular basis, these rocks show:
Al2O3 < (Na2O + K2O)
Peralkaline rocks are distinguished by the
presence of Na-rich minerals

15. classification based on mineralogy
● Determine the modal mineralogy
● IUGS classification

16. Q = % quartz (or other
polymorph)
A = % alkali feldspar (An<5)
P = % plagioclase feldspar
F = % feldspathoids (abbrev.
‘foids’)
classification and nomenclature
of volcanic rocks. After IUGS.

17. IUGS Classification of
Phaneritic/ plutonic
Igneous Rocks
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
Quart
z
Syenite
Quart
z
Monzonite
Quart
z
Monzodiorit
e
Syenite Monzonite Monzodiorite
(Foid)-
bearing
Syenite
5
10 35 65
(Foid)-bearing
Monzonite
(Foid)-
bearing
Monzodiorit
e
90
Alkali Fs.
Syenite
(Foid)-bearing
Alkali Fs. Syenite
10
(Foid)
Monzosyenit
e
(
F
o
i
d
)
S
y
e
n
i
t
e
(Foid)
Monzodiorite
(
F
o
i
d
)
G
a
b
b
r
o
Qtz. Diorite/
Qtz.
Gabbro
5
10
Diorite/Gabbro/
Anorthosit
e
(Foid)-bearing
Diorite/Gabbro
60
(Foid)olites
Quartzolite
Granite Grano
-diorite
T
o
n
a
l
i
t
e
A
l
k
a
l
i
F
e
l
d
s
p
a
r
G
r
a
n
i
t
e
Q
A P
F
60
International Union of Geological Sciences

19. Classification of Mafic Intrusive Rocks

20. A classification of the phaneritic
igneous rocks: Ultramafic rocks. After IUGS.