This document provides an overview of the classification of igneous rocks. It discusses several key criteria used for classification, including texture, mode of occurrence (intrusive vs extrusive), and chemical composition based on silica and alumina content. Texture types include phaneritic, aphanitic, porphyritic, glassy, and pyroclastic. Mode of occurrence divides rocks into plutonic (intrusive) and volcanic (extrusive) types. Chemical classification schemes analyze silica content to categorize rocks as felsic, intermediate, or mafic, and also consider silica and alumina saturation states. Diagrams are provided illustrating these classification approaches. Examples of different rock types are also briefly described,

1. Classification Of Igneous Rocks
Saif Elden Abo Khashaba
Prepared by
Supervised by
Prof.Dr. Elmetwally Mohammed Lebda
Kafrelsheikh University
Faculty of science
Geology Department
2018

2. Classification of igneous rocks
 Classification of igneous rocks is one of the most confusing
aspects of geology.
 This is partly due to historical reasons, partly due to the nature of
magmas, and partly due to the various criteria that could
potentially be used to classify rocks.
 Igneous Rocks Are Most Classified Into Groups On The Basis Of :
Textures
Mode of occurrence
Chemical composition

3. Classification according to textures
 Phaneritic: rocks with mineral grains that are large enough to be identified by eye.
Slowly cooled intrusive rocks. Ex: gabbro and granite
 Aphanitic: rocks with grain too small to be identified by eye. Rapidly solidified extruded
magma and marginal facies of shallow intrusions. Ex: basalt and rhyolite .
 Porphyritic: bimodal grain size distribution. The large grains (phenocrysts) imbedded in
fine grains (ground mass)
 Glassy: no crystals formed due to very rapid cooling
 Pyroclastic: including the rocks come from explosive eruptions of lava into the air
resulting in fragmental, typically glassy material which fall as volcanic
ash, lapilli and volcanic bombs.
 Pegmatitic: very large xtals (cm to 10s of cm); i.e., slowly cooled, Forms veins or layers
within plutonic body

4. Classification according to mode of occurrence
A. Plutonic rocks (Intrusive):
• Igneous rocks of deep-seated origin
• They have phaneritic texture
• Generally medium to coarse grained in size (individual
crystal can be distinguished with the naked eyes).
• Examples: gabbro, diorite and granite

5. Classification according to mode of occurrence
B. Volcanic rocks (extrusive):
• Igneous rocks formed at or very near to the surface of the earth.
• They have aphanitic texture.
• very fine grained or glassy rocks (Most of the individual
crystals cannot be distinguished with naked eyes).
• Examples are basalt, andesite .and rhyolite

6. Classification according to chemical composition
1-Silica percentage (% SiO2)
 Felsic: feldspar + silica
~55-70% silica, K-feldspar > 1/3 of feldspars present
light-colored silicate minerals — Continental crust
 Intermediate: between felsic and mafic
~55-65% silica, plag > 2/3 of feldspars present
Na-rich plag predominates over Ca-rich plag
 Mafic: magnesium + ferric iron
~45-50% silica; Ca-rich plag dominant feldspar
dark silicate minerals — Oceanic crust
 Ultramafic: >90% mafic minerals, silica < 45%, few or no feldspars Mantle-derived

7. 2- Silica saturation
The silica saturation concept can thus be used to divide rocks in silica
undersaturated, silica saturated, and silica oversaturated rocks. The first and last
of these terms are most easily seen.
This is an easy concept which leads to a simple classification that actually utilizes the
mineralogy of the rock.
 Rocks are classified into 3 groups:
 Saturated: Are rocks which do not contain either quartz or feldspathoids.
 Oversaturated: Are rocks containing quartz.
 Undersaturated: Are rocks which contain feldspathoids (no quartz).
In addition to Qz and feldspathoids, other minerals can be used to indicate the level of
silica saturation in a rock.

8. 3-Alumina (Al2O3) Saturation
The concept of alumina saturation is based on whether or not there is an
excess or lack of Al to make up the feldspars.
 Note that Al2O3 occurs in feldspars in a ratio of 1 Al to 1 Na, 1K, or 1 Ca:
 KAlSi3O8 -- 1/2K2O : 1/2Al2O3
 NaAlSi3O8 -- 1/2Na2O : 1/2Al2O
 CaAl2Si2O8 -- 1CaO : 1Al2O3
 Three possible conditions exist:
1. If there is an excess of Alumina over that required to form feldspars, we say
that the rock is peraluminous.This condition is expressed chemically on a
molecular basis as:
Al2O3 > (CaO + Na2O + K2O).

9. 3. 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 like
aegerine, riebeckite, arfvedsonite , or aenigmatite in the mode.
2. Metaluminous: rocks are those 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 That characterized by lack of an
Al2O3-rich mineral and lack of sodic pyroxenes and amphiboles in the mode

10. 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.

11. Classification according to mineral composition
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.

17. BASALT – With or Without Useful Clues
As with the previous image, the sample on the left exhibits a few slightly larger olivine crystals
[green arrows], and probably has pyroxene as the other mineral. In the right sample, the purple
arrows point to amygdules, which are minerals precipitated in once-empty vesicles. Given that
they may have formed well after the lava cooled, and be unrelated to it, they are no indicator of
the lava’s composition.

18. BASALT – Clues in Vesicular Basalts
The scoria, or scoriaceous basalt, on the left has a few clues to its
chemistry. Many of the vesicles have a lining of pale pea-green
material [green arrows] often derived from the weathering of
olivine, and the iron content is confirmed by the reddish-brown
oxidized (rusted) weathered surface [red arrows]. The right
sample has obvious olivine crystals, perhaps phenocrysts, but no
lining to the vesicles.

19. BASALT – Pillow Basalt from a Mid-Ocean Ridge
Submarine eruption of basalt lavas leads to very rapid cooling of the exterior
surface, to produce a flexible skin that contains the lava. Successive eruptions
look like a stack of pillows, with rounded, convex upper surfaces. On the left
we see the fresh surface (broken and also cut with a rock saw) with the yellow
arrows pointing to the rapidly weathered exposed surface, seen on the right.

20. COLUMNAR BASALT
This outcrop is the Giant’s
Causeway in Ireland; the
Devil’s Post Pile is a
similar exposure in
California.
The polygonal columns result from cooling, contraction, and cracking of thin, widespread
sheets of mafic rock. These occur either by subaerial eruption of fluid basalt at rifts within
continents, or by intrusion of mafic magma as near-surface sills between layers of cooler
sediments.

21. BASALT
This is a porphyritic
basalt, whose
phenocrysts tell us
much about the
composition.
The plagioclase phenocrysts [red arrows] suggest this is mafic, not
ultramafic. As such, the matrix between the phenocrysts is likely a
mixture of pyroxene and plagioclase, without any appreciable olivine.

22. TRACHYTE – an Intermediate Volcanic Rock
The classic intermediate composition volcanic rock is andesite. These samples,
from the Crowsnest Pass, are alkali-rich (K, Na) intermediate rocks, whereas
true andesite would be higher in Ca. The black phenocrysts are garnet [yellow
arrows], with dodecahedral habit (the mafic minerals in intermediate rocks are
more typically amphibole or pyroxene). The pink phenocrysts [blue arrows] are
feldspar.

23. RHYOLITE
The lighter colour
is indicative of the
felsic composition
of this extrusive
rock, though we
cannot distinguish
the minerals.
We expect, based on its felsic composition, that it would have quartz and potassium feldspar,
and perhaps some mafic mineral content. The black spots [red arrows] are in fact not mafic
minerals, but small blobs of black volcanic glass, or obsidian, which itself is felsic.

24. RHYOLITE
This would normally
be called pumice, a
term that describes
the texture rather
than being a proper
rock name used in
classification.
It represents a case in which gas-rich felsic lava has turned into a
sort of sticky froth that has eventually hardened. Flow of this
taffy-like material has produced many glassy strands spanning the
vesicles [blue arrows]. It is otherwise like the previous sample.

25. OBSIDIAN
This rock would have erupted
as a rhyolite lava in the form
of pumice, then collapsed.
Rocks have minerals, which
are crystalline, yet this is
felsic volcanic glass, and is an
exception to the basis for
colour index.
There are no mafic minerals (or, technically, any other for that matter), so this rock has
a Colour Index of zero, even though it is completely black on the fresh surface. The
brown weathered surface is a clue to the presence of iron, dispersed through the glass to
make it black.