presentation about igneous texture

1. Indira Gandhi National Tribal University
Amarkantak ,M.P
a presentation on
TEXTURE OF IGNEOUS ROCKS
SUBMITTED BY
NARESH DAS
M.SC GEOLOGY
DEPT. OF GEOLOGY
ENROLLMENT NO-
2101222010
SUBMITTED TO
DR. MERAJ ALAM
SAFI
DEPT. OF
GEOLOGY

2. TEXTURE OF IGNEOUS ROCKS
DEFINATION
 Texture is the size ,shape and arrangement of mineral
mineral grain in a rock
It best studied under thin section(0.03mm)
Why Read about Textures?
The texture of igneous rock is a result of various
processes that controlled the rock’s genesis and along with
along with mineralogy and chemical composition ,provides
,provides information that we may use to interpret the rocks
rocks origin and history . It is thus important for us to be

3. TEXTURE OF IGNEOUS ROCK DIVIDED
INTO TWO PRINCIPAL CATAGORIES
1.PRIMARY TEXTURE :-
It forms during the cooling ,solidification and
crystallization of magma and lava i.e. Texture form
during formation of igneous rock and result from
interaction between mineral and melt
2. SECONDARY TEXTURE :-
secondary texture result after the formation of
rock which were completely solidified

4. PRIMARY TEXTURE
Primary texture form by 3 principal
process
1. Initial nucleation of crystal ,the rate at
which enough of chemical constituent of crystal can
come together in one place without dissolving
2. Subsequent crystal growth, the rate at
which new constituents can arrive at surface of the
growing crystal. This depends largely on the diffusion
rate of molecule concern
3. Diffusion of chemical species, (and
heat) through the surrounding to and from the surface
of a growing crystal ,Or the rate at which atoms or
molecules can move (diffuse) through the liquid
BUT the main factor that demines the igneous texture is
1. Rate of cooling dT/dt
Other factors include
TIME

5. Crystal formation: magma enough of the chemical
constituents that will make up the crystal must be at the same
place at the same time to form a nucleus of the crystal. Once
nucleus forms, the chemical constituents must diffuse through
the liquid to arrive at the surface of the growing crystal. The
crystal can then grow until it runs into other crystals or the
supply of chemical constituents is cut off. These rates will
depend on the temperature of the system
1.NUCLEATION
✓ It is the critical first initial step in the development of a crystal. The nucleation is the first
step in the formation of primary texture.
✓nucleus crystal have very high surface area / volume ratio ,thus high no of unbalanced ion
at the surface which result high unstabilty of initial crystal due to large surface energy .under
such condition crystallization would be possible , but the prerequisite nucleation isn’t. This
required greater degree of under cooling (cooling of melt below the true crystallization
temperature of mineral)
✓A crystal with simple structure tends to nucleate more easily than those with more
complex structures.

6. ii. Crystal Growth:
✓ The crystal growth involves the addition of ion on an existing
crystals because of crystal growth the size of crystal increase as
crystallization proceed.
✓In simple structure with high symmetry, faces with high density of
latice points tend to form more prominent faces.
✓fast growing faces tend to be those with smaller inter planar lattice
spacing ( and higher surface energies).
✓but faces with low surface energy will generally be most stable and
manifest themselves growing freely in liquid.
iii. Diffusion (Shape of crystal developed)
✓ The diffusion involves interaction between early formed crystal
and residual liquid or melt. Because of diffusion the crystal shape
will develop along with formation of new crystal mineral.

7. The diffusion rate of chemical species is also
dependent on the temperature.
 At higher temperature with a magma of low viscosity the
rate of diffusion will be high and therefore the texture will be
euhedral to sub-hederal.
At surface condition where temperature is low and viscosity
is high, the rate of diffusion is less than crystal growth and
therefore the texture will be anhedral.
With addition of H₂O(water) the viscosity of the magma
decreases and therefore the diffusion rate increases and
hence the texture will be euhedral.
Note: As degree of under-cooling increases (from deep
seated condition to surface condition) the mineral grain
changes to well-developed crystal to acicular habit then to
dendritic then to spherulitic form.
When rate of diffusion is less than the rate of crystal
growth i.e. crystal growth rate exceed wrt nucleation and
diffusion, therefore in this case the crystal take on an
increasing radiating form or a tree like branching called
dendritic texture.

8. Example
Spinifix Texture: This texture
formed in ultramafic lava such as
Precambrian Komatite, in this the
olivine crystal developed irregular
elongated shape upto 1m long.
Skeletal Texture: When corner
and edges of crystal radiate more
prominent than crystal surface,
resulting texture will called
skeletal texture.
Swallow Tail Texture: The corner
of plagioclase crystal tends to
grow straighter, the characteristic
texture will called shallow tail
texture
Large, downward-radiating
spinifex textures in komatiitic.
Komati River Valley, South Africa.
From Large igneous provinces.
Sketch of spike projecting from crystal
corner, through layer of liquid rich in
impurity components into pure liquid,
initiating dendritic growth habit. Modified
from Vernon, R. H. (2018).

9. Rate of Nucleation , crystal
Growth and Diffusion :
There are three principle processes involves
in the formation primary texture. Along with
these three principle processes the role of
cooling also affect the primary texture.
 At deep seated condition where temperature
is high and cooling rate is slow also called
equilibrium cooling.
 In this situation(Ta) the rate of crystal
growth will exceed wrt rate of nucleation and
therefore the deep seated plutonic rocks are
coarse grained .
Note: At high temperature when cooling rate is
slow, the rate of crystal growth greater than rate
of nucleation.

10. At condition where temperature is moderate to
low and cooling rate is fast also called under
cooling (at temperature Tb in given diagram), here
the rate of nucleation is greater than the rate of
crystal growth therefore texture will be fine grain
which is common among volcanic igneous rock.
 Under very fast rate of under cooling)(Temp-Tc)
here neither nucleation nor crystal growth
therefore crystal will be glassy texture.
Two stages cooling can create a bimodal
distribution of grain size. The slow cooling
followed by rapid cooling, during slow cooling the
crystal growth rate exceed the nucleation rate
therefore coarse grained texture and in later stage
during fast cooling (under cooling) the nucleation
rate exceed the crystal growth rate and therefore
the texture will be fine gained. Hence overall
texture will be inequigranular.

11. INEQUIGRANULAR TEXTURE
Porphyritic Textures
Poiklitic Textures
Larger grains
surrounded by smaller
grains
•Larger grains are
called phenocryst
•Smaller grains are
called groundmass
•Igneous rock showing
this texture called
porphyre
•Ex- Basalt,Granite
Smaller grains are
sorrounded by
larger grains
•Larger garains are
called chadacryst
and smaller grains
are called
oikocrysts
•The texture
found in syenite
and monzonite

12. General Classification Of Texture
1.On the basis of degree of crystallization.
1. Holocrystalline: When rock is made up
of entirely of crystals.
2. Hemi/merocrystalline: When rock is
made up of partly of crystals & partly
glass.
3. Holohyalline: When rock is made up of
entirely of glass material.
II. On the basis of grain size;
Slow cooling gives time to grow their
crystal sizes greater than 5mm. In rapid
cooling mineral crystallizes quickly
(Smaller crystal size less than 1mm). In
some lava, cooling is so fast that they fails
to crystallize and "glassy texture" result.
holohyalline
Holocrystalline

13.  Phaneric igneous Rock:
Mineral grain seen with naked eye.
1.Coarse grained texture: Grain > 5mm diameter
Example: Mostly intrusive igneous rock
2. Medium grained texture: Between 1 to 5 mm.
3. Fine grain: < 1mm,
Ex: Extrusive igneous rock.
 Aphanitic: Cannot seen with naked eyes.
1. microcrystalline: Mineral grain distigushed under
microscope
2. Cryptocrystaline: individual crystal do not visibe
under microscope but their presence can be felt as
they react with other mineral
3. Hemicrystalline
4. Holohyaline
III. On the basis of grain shape
1. Euhedral
2. Sub-hedral
3. Anhedral
phaneric
Aphanitic

14. compositional Zoning Texture:
*Normal Zoning:
In normal zoning the core will be more anorthite rich and rim will be more albite rich.
The texture is formed in igneous process.
*Reverse zoning:
The core is more albite rich and rim is more anorthite rich. It is formed in metamorphic
process.
*Oscillatory Zoning:
It is most common type of zoning in plagioclase because a regular decrease in anorthite
content rarely dominates the full crystallization period.:-This zoning is results of
diffusion dependent depletion and re-enrichment of the liquid zone:-adjacent to the
growing crystal in an undisturbed magma chamber. A depletion component could be
the anorthite molecule or constituents such as H2O that can lower the melting point
and thus shift the equilibrium composition of the plagioclase.
*Cumulate Texture:
In the early stage of crystallization of ultramafic minerals which are relatively higher
density, will settle at the bottom of chamber because of gravity settling in the form of
cumulate, the characteristic texture formed is called cumulate texture.
*types of cummulate texture
1. Accumulated -2. Orthocumulated 3. Adcumulate 4. Poikilitic (Heteradcumulate)

15. 1. Accumulated:
In this type of texture, the composition of early formed crystal and the composition of
interstitial liquid are same therefore there will not interaction between early formed
crystal & interstitial liquid therefore size & shape of the early formed crystal will remain
preserved.
2. Orthocumulate:
If composition of early formed crystal and interstitial liquid are different then there will
be some modest additional overgrowth on early formed crystal. Hence shape and size of
early formed crystal changes.
3. Adcumulate Texture:
If the interstitial liquid can escape & exchange material from liquid of the main chamber
by diffusion and/or convection, the early formed cumulate mineral may continue to grow
as the rejected component in the interstitial liquid has exchange with main magma
chamber.
4. Poikilitic Texture:
In this texture, all the interstitial liquid in the later stage of crystallizes to form a bigger
crystal which enclosed the early formed crystal mineral.

16. Epitaxis texture:
Preferred nucleation of one mineral on a pre existing
mineral
Similarity of the crystal structures of the mineral substrate
and new phase is a prerequisite for epitaxial growth
E.g., growth of sillimanite on biotite or muscovite
Rapakivi texture:
Plagioclase overgrowths on orthoclase (K-feldspar) Occurs
in some granites
Spherulitic texture: In silicic volcanic rocks in which
needles of quartz and alkali feldspar grow radially from a
common center
Variolitic texture: Radiating plagioclase laths in some
basalts are probably the result of nucleation of later crystals
on the first nuclei to form during devitrification of glass
comb texture: A texture in which individual crystals have
their long axis perpendicular to the walls of a vein
NUCLEATION AT PREFERERED SITE

17. Intergrowth Texture: Two crystals of different composition
crystallize simultaneously, the resulting texture formed called
intergrowth texture.
1. Graphic texture: The intergrowth between quartz & K-feldspar
Example: Granophyric igneous rock
2 . Myrmeketic texture: The intergrowth between quartz &
plagioclase feldspar. that shows small wormlike bodies of quartz
enclosed in plagioclase. This texture is found in granites
Note: The graphic and myrmekitic texture formed during eutectic
crystallization at eutectic point.
3. Perthite Texture: The intergrowth between k-feldspar and albite
in which the patches of albite form in k-feldspar groundmass.
4. Anti-perthite texture: The intergrowth between k-feldspar & albite
in which the patches of k-feldspar formed in the albite
groundmass.
a single crystal of cuneiform quartz
(darker) intergrown with alkali
feldspar (lighter)

18. VOLCANIC TEXTURE
Ophitic texture - laths of plagioclase in a coarse grained matrix of pyroxene crystals,
wherein the plagioclase is totally surrounded by pyroxene grains.
Subophitic texture - similar to ophitic texture wherein the plagioclase grains are not
completely enclosed in a matrix of pyroxene grains.
Hyalo-ophitic texture - a texture similar to ophitic texture except that glass completely
surrounds the plagioclase laths.
Hyalo-pilitic texture - a texture wherein microlites of plagioclase are more abundant than
groundmass, and the groundmass consists of glass which occupies the tiny interstices
between plagioclase grains
Intergranular texture - a texture in which the angular interstices between plagioclase
grains are occupied by grains of ferromagnesium minerals such as olivine, pyroxene, or iron
titanium oxides.
Intersertal texture - a texture similar to intergranular texture except that the interstices
between plagioclase grains are occupied by glass or cryptocrystalline material

19. Other textures that may be evident on microscopic
examination of igneous rocks are as follows:
Trachytic texture - a texture wherein plagioclase grains
show a preferred orientation due to flowage, and the
interstices between plagioclase grains are occupied by
glass or cryptocrystalline material.
Coronas or reaction rims - often times reaction rims
or coronas surround individual crystals as a result of the
crystal becoming unstable and reacting with its
surrounding crystals or melt. If such rims are present on
crystals they should be noted in the textural
description.
.
Moth eaten texture (also called sieve texture)- This
sometimes occurs in plagioclase wherein individual
plagioclase grains show an abundance of glassy
inclusions.

20. references
 Winter, John D. : Principles of igneous and
metamorphic petrology
 YouTube videos
 google