The ppt has prepared for undergraduate students of Geology

1. Texture of Igneous Rocks
Dr Rajeeva Guhey,
Prof. Geology
Govt NPG College of Science
Raipur

2. IGNEOUS TEXTURE
• In Igneous rocks the mutual relationship between mineral grains and the
groundmass is known as TEXTURE.
• The fundamental principle behind igneous rock textures is that grain size
is controlled by cooling rate. Thus, rapid cooling at the Earth’s surface of
extrusive molten material, or lava, results in the growth of smaller
crystals, or prevents crystal growth altogether.
• Conversely, slow cooling within the Earth’s crust of intrusive molten
material, called magma, results in the growth of fewer but larger crystals,
because atoms are able to migrate through the liquid to attach
themselves to crystals that have already begun to form. The many
igneous rock textures are simply variations on or modifications of this
principle.
• Textures are useful indicators of cooling and crystallization rates and of phase
relations between minerals and magma at the time of crystallization.

3. 1. CRYSTALLINITY
• This relationship can be studied in
4 ways:
1. Crystallinity:Power of
spontaneous crystallization It is
of three types:
A. Holocrystalline:
Igneous Rock with well
developed crystals.
B. Hemicrystalline: Igneous
Rock with approx. consists of half
crystals (not clear crystals) Ex.
Rhyolite
Rhyolite
C. Holohyaline:
Igneous Rock with full of glassy
matter (fine grained igneous rock).
Ex: Obsidian
Obsidian
Granite

4. 2. GRANULARITY
2. Granularity: Based on grain size, tecture
can be named as under:
 1. Phaneric: Grains are visible with the
nacked eyes. They are three types:
 A. Course , B. Medium, C. Fine
2. Aphanetic: Grains are not visible
megascopically. They are two types:
 A. Microcrystalline:
 All sizes recognizable only with a microscope,
and therefore embracing most of those that are
megascopically cryptocrystalline.
 B. Cryptocrystalline:
 All sizes too small to be seen with the unaided
eye. Aphanitic is a purely negative term which
does not state whether the rock is crystalline or
glassy, and is very convenient when it is
desirable to express simply the fact that the
matter is not determinable megascopically.
Cryptocrystalline states that the rock is
crystalline, and that the crystals are too small to
be seen megascopically.

5. 2. GRANULARITY
• Rock : Gabbro
Igneous intrusive rocks have evident crystals
[the Greek word phaneros means visible
or evident] that can be discerned
without the aid of microscope.
Phaneritic – With smaller crystals

6. 3. SHAPE OF GRAIN
3. Shape of Grains: Based on shape textures are named as under.
 Euhedral: Well developed crystals
 Subhedral: Half faces of the crystals developed.
 Anhedral: Rock totally composed of glass.

7. 4. Mutual relations: Means the relation between grains and
background matrix.
Based on this the texture can be classified into following
types:
 Equigranular Texture: Texture is known as equigranular
when all the grains of a rock is composed of equal size of
grains.
 Rocks with equigranular texture have mineral grains that are
generally the same size. Diameters of component minerals
are comparable.
4. MUTUAL RELATIONS

8. Equigranular Texture
 They can be subdivided into three
types:
A. Panidiomorphic:
When all grains of a rock are
euhedral in shape.
B. Hypidiomorphic:
When all grains of a rock are
subhedral in shape.
C. Allotrimorphic:
When all grains of a rock is
made up of anhedral grains.

9. INEQUIGRANULAR TEXTURE
 Inequigranular Texture: Texture is known
as inequigranular when all the grains of a rock is composed
of unequal sizes. They can be divided in to following types:
1. Porphyritic Texture:
When Large crystal (Phenocryst) surrounded by
groundmass of small crystals.
Origin : The origin of Porphyritic Texture may be
explained by following theories:
I) Two Generation theory
ii) Molecular concentration theory
iii) Relative solubility theory
Granodiorite in which K-
feldspar crystals are the
phenocrysts

10. INEQUIGRANULAR TEXTURE
2. Poikilitic Texture: When small
crystals of different mineral are
embedded in a large crystal.
Example:
Syenite, Monzonite
3.Ophitic Texture:
It is a special type of poikilitic tex.
Between Augite and Felspar in
which small laths of plagioclase
present in big crystal of Augite
(Pyroxene)
A single pyroxene envelops
several well- developed
plagioclase laths.
Poikilitic texture. Orthopyroxene
oikocryst that encloses rounded
chadacrysts of olivine

11. Intergranular texture
 Intergranular intersertal texture: In
this texture, grains of plagioclase are
arranged in triangular shape .
 In this texture angular interstices
between plagioclase grains are
occupied by grains of
ferromagnesium minerals such as
olivine, pyroxene, or iron titanium
oxides. Then it is called Felsophyric.
And when interspace is filled with
glassy matter then the texture is called
Intersertal or Vitrophyric:
Tiny, equant clinopyroxene grains
interstitial to plagioclase laths.

12. INTERGROWTH TEXTURE:
Graphic texture: Intergrowth texture: Between
quartz and felspar.
a single crystal of cuneiform quartz (darker)
intergrown with alkali feldspar (lighter).
Graphic Texture:
Mutually interpenetrating
crystals produce irregularly
shaped forms, which in
some cases yield straight-
edged patterns as in graphic
granite, where quartz and
feldspar-are intergrown

13. Directional texture
• Trachytic texture:
• Texture wherein plagioclase
grains show a preferred orientation
due to flowage, and the interstices
between plagioclase grains are
occupied by glass or
cryptocrystalline material. It shows
the direction of flow of lava.
Trachytic texture in which microphenocrysts
of plagioclase are aligned due to flow. Note
flow around phenocryst (P).
Trachyte, Germany. Width 1 mm. From MacKenzie
et al. (1982). © John Winter and Prentice Hall

14. References
• Principles of Petrology- G.W. Tyrrell
• Text Book of Geology_ G.B. Mahapatra
• Text Book of Geology- P.K. Mukherjee
• Geology Principles - Rajeeva Guhey
and Practical Manual