Introduction Descriptive parameters of texture Equigranular texture Inequigranular texture Directive texture Intergrowth texture Exsolution texture Devitrification Conclusion

1. TEXTURE OF IGNEOUS
ROCKS -Komal Mohanty

2. CONTENT
 Introduction
 Descriptive parameters of texture
 Equigranular texture
 Inequigranular texture
 Directive texture
 Intergrowth texture
 Exsolution texture
 Devitrification
 Conclusion

3. INTRODUCTION
 Texture is the intimate mutual relations of the mineral constituents
and glassy matter in a rock made up of a uniform aggregate.
 It is best studied in thin sections under transmitted light microscope
but can also be interpreted megascopically.
 They indicate the geological processes that were in operation
during the cooling and solidification of igneous rocks.

4. DESCRIPTIVE PARAMETERS OF TEXTURE
1.Crystallinity:
It is the ratio between crystallised and non-crystallised matter.
There are generally 3 categories of crystallinity:
 Holocrystalline-When a rock is composed entirely of crystals. Example-Granite,
Gabbro
 Holohyaline-When a rock consists entirely of glass. Example-Obsidian, Pitchstone
 Merocrystalline/ Hypocrystalline-When a rock is composed partly of crystals and
partly of glass. Example-Rhyolite, Trachyte

5. Holocrystalline
Merocrystalline Holohyaline

6. 2.Granularity:
It refers to the absolute size of crystals. Igneous rocks are categorised into:
 Phenocrystalline-When the individual crystals are visible to naked eye.
 Aphanetic-If the individual crystals are not visible to naked eye or through a lens.
 Microcrystalline-When the crystals are distinguishable with aid of a petrological microscope.
 Cryptocrystalline-When the crystals are too small that can not be distinguished separately
 Coarse grained-The average crystal diameter is above 5mm. Example-Pegmatite
 Medium grained-The average crystal diameter is between 1 to 5mm. Example-Syenite
 Fine grained-The average crystal diameter is less than 1mm. Example-Basalt

7. FACTORS AFFECTING GRANULARITY
 Rate of cooling-Slow cooling favours growth of large crystals due to slow diffusion of
ions.
 Viscosity-: If the magma is viscous, it opposes ionic diffusion thereby, hindering
crystallisation. Thus, fine-grained or glassy rocks like rhyolite (obsidian) resulted from
siliceous viscous magma.
 Volatile content-: Presence of volatiles, particularly water, in magma reduces the viscosity,
but, promotes larger crystal growth.

8. 3.Shape of crystals:
The shape of crystals are described in terms of their development with reference to
the 3 dimensions of space.
 Equidimensional-When the development of crystal is more or less equally in all
directions.
 Tabular-Crystals are developed more in two spatial directions than the third.
 Prismatic-When crystals are comparatively more developed in one direction.
 Irregular-Crystal development with respect to different directions is not well
defined.
 Euhedral-When the crystal has well developed faces.
 Subhedral-When the crystal faces are partially developed.
 Anhedral-Absence of any crystal faces.
Anhedral garnet
Subhedral pyrite
Anhedral quartz

9. 4.Mutual relation of crystals
Based on the mutual relation of crystals the texture of igneous rocks are:
1. Equigranular texture
2. Inequigranular texture
3. Directive texture
4. Intergrowth texture
5. Exsolution texture

10. EQUIGRANULAR TEXTURE
When majority of the crystals in the igneous rock are of same relative size, it is called equigranular
texture.
 Panidiomorphic texture-When most of the crystals are euhedral. Example-Lamprophyre
 Hypidiomorphic texture-When majority of the crystals are subhedral. Example-Granite,
Gabbro
 Allotriomorphic texture-When most of the crystals are anhedral. Example-Aplites
 Microgranitic texture-It is characteristic of fine grained rocks where shape of grains is
subhedral to anhedral.
 Orthophyric texture-It occurs in fine grained rocks with euhedral grains.
 Felsitic texture-When the grains are microcrystalline or cryptocrystalline that is the the grains
are difficult to distinguish.

11. INEQUIGRANULAR TEXTURE
When the size of crystals vary between wide limits that is the difference between
the grain size of individual crystals is very pronounced, it is called inequigranular
texture.
1. Porphyritic texture-When the early formed large crystals, phenocrysts are
surrounded by later formed fine grained groundmass.
 Vitrophyric-When the groundmass is glassy.
 Felsophyric-When the groundmass is cryptocrystalline.
 Megaporphyritic-When the grains are visible to naked eye.
 Microporphyritic-When the grains are visible only under microscope.
 Glomeroporphyritic-The phenocryst is formed by an aggregate of crystals
rather than a single individual grain.
Porphyritic Granite

12. 2. Poikilitic texture-Smaller grains(chadacrysts) are surrounded by larger
grains(oikocrysts)
 Ophitic texture-It is a special type of poikilitic texture where anhedral Augite
surrounds laths of Plagioclase.
 Sub-ophitic texture-When the augite partially encloses plagioclase.
 Granulo-ophitic-Here the grains of plagioclase are surrounded by a large patch of
pyroxene which is made up of a number of pyroxene grains.
 Hyalophitic texture-It is same as ophitic texture but the plagioclase laths are
completely surrounded by glass.
3. Intergranular texture-When the triangular or polygonal interspaces between the
crystals are entirely filled with granules of other minerals like olivine or augite.
4. Intersertal texture-When the polygonal spaces between the grains are filled with
glass.
5. Seriate texture-When the size of crystals vary gradually from smallest to largest.
Ophitic texture

13. DIRECTIVE TEXTURE/FLOW TEXTURE
This type of texture is produced by flow in magma during crystallisation of minerals. During the
time of crystallisation when the magma undergoes flow movement, the crystallising minerals tend
to arrange in directive bands thus producing flow texture.
 Trachytic texture-This is typical of volcanic rocks like trachyte. The calcic plagioclase and
glassy material show parallel to sub parallel alignment due to magma flow.
 Hyalopilitic texture-It develops when the plagioclase laths are intergrown with glass to
produce a felted mass.
 Eutaxitic texture-Layered or banded texture in explosive volcanic rocks. It is caused by the
compaction and parallel arrangement of glass shards and pumice fragments.

14. Trachytic
Hyalopilitic Eutaxitic

15. INTERGROWTH TEXTURE
Intergrowth texture results from the intergrowth of grains of two different minerals due
to simultaneous crystallisation of of two minerals of the magma at a particular
temperature.
 Graphic texture-This texture occurs from the intergrowth between quartz and
orthoclase. When this texture is observed in hand specimens, it is called graphic
texture and when it is in a microscopic level called as micrographic and the rock is
called granophyre. Quartz is disposed in the form of prismatic wedge-shaped areas
intersecting at an angle of about 60 degrees. This texture is common in granites.
 Myrmekite texture-It is formed from the intergrowth between quartz and
plagioclase(usually oligoclase).
 Corona texture-It results from the reaction between crystallised mineral with
magma forming a zone of reaction products surrounding the mineral called reaction
rim.
Graphic texture(blebs of
grey quartz in the host of
white orthoclase)

16. EXSOLUTION TEXTURES
Sometimes mineral composition that are stable at high temperature become unstable when
temperature decreases and exsolve(unmix) to form two different crystalline minerals such that
the grain contains patches or stringers of two minerals.
 Perthitic texture-Intergrowth between orthoclase(host) and albite that is patches of albite
are found within orthoclase.
 Antiperthitic texture-Intergrowth between orthoclase and plagioclase(host) that is patches
of orthoclase occur within plagioclase.
Perthite and Antiperthite

17. DEVITRIFICATION
 If a highly viscous magma is rapidly supercooled it does not get enough time for
crystallisation thus forming glass. Being unstable at this condition they change in course of
time to fibrous crystals called crystallites. This process of conversion of glass into crystals is
called devitrification.
 It is a slow process at ordinary temperature but increasing heat, pressure and circulating fluids
can speed up the process.
 Glasses older than pre-carboniferous is practically absent, it is inferred that the ancient glasses
must have been converted to crystallites.
 Perlitic cracks-They refer to curved or spherical cracks found in glassy or devitrified igneous
rocks, formed by contraction during rapid cooling of the magma.

18.  Spherulitic texture-More rapid cooling and crystallisation of viscous magma yields spherulitic
texture. Spherulitic aggregates are radiating arrays of fibrous or needle-like crystals. This texture
is common in glassy felsic volcanic rocks.
 Spinifex texture-This texture is characteristic of komatite (an ultramafic rock of volcanic origin).
Spinifex texture is defined as randomly oriented, extremely fine-grained, slender hollow crystals
or acicular olivine phenocryst formed by rapid cooling or quenching of ultramafic lavas.
Perlitic cracks Spherulitic texture Spinifex texture

19. CONCLUSION
 The study of texture is significant in determining the mode of origin of the igneous rock.
 It is used in rock classification on the basis of size and shape of mineral grains.

20. REFERENCE
 Igneous Petrology, M.K. Bose
 Principles of Igneous and Metamorphic Petrology, J.D. Winter
 A Text Book of Geology, G.B. Mahapatra
 Wikipedia
 www.ignou.ac.in