All Igneous rock textures with examples in easy and simple way to understand and increase microscopic studies skills and the way to easily identify igneous rocks under polarized microscope.

1. Igneous Rock
Textures
By: Ahmed Essam

2. What are
Igneous Rocks?
▪ Igneous rocks is the primary type of rocks that formed by direct
solidification or crystalization due to the cooling down of molten
magma or lava at high temperatures ranges from 650 to 1300 o c.
▪ Plutonic or Intrusive Igneous Rocks: that types of igneous rocks that
formed beneath the surface of the earth due to direct cryatalization from
molten magma at high pressure and temperature which characterized by
presence and formation of quite large crystals that can be clearly
observed by naked eyes.
- The formation of crystals is due to and also indicates
slow cooling of molten magma which may takes
milions of years depending on the rate of cooling
and some other factors.
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3. What are
Igneous Rocks?
▪ Volcanic or Extrusive Igneous Rocks: that type of igneous rocks that on
earth's surface from vocanoes or fissure eruptions or at very shallow
depths beneath the surface of the earth which formed due to direct
solidification of lava and characterized by very fine small crystals
which cannot be oberved by naked eyes or crystals may not be formed
and just glassy texture can be found.
- Rapid cooling of molten lava is the main factor of
forming very fine crystals or even no crystals at all
(glassy texture).
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4. Bowen's
Reaction
Series
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5. Igneous Rocks
Classification
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6. Textures And
It's Importance
In Studying
Igneous Rocks
▪ What are Textures? Textures are the relative and geometric
relationships between rock-forming minerals crystals or even glassy
which require microscopic studies while it cannot be observed in hand
specimens.
▪ Importance of Textures in studying Igneous rock:
1. Indicates the predominant conditions during
magmatic crystalization.
2. Gives information about relative history of
minerals crystalization.
3. Gives inform ation about form ation an d
crystalization stages of rocks.
4. Essential in Rock identification and nomenclature.
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7. How Igneous
Rock Textures
Are Described?
A. Degree of Crystallization: it also called Crystallinity which measured
or determined as the ratio of crystalline minerals relative to glassy materials.
1- Holocrystalline: the rock consists mainly of crystalline
material or minerals.
2- Holohyaline: the rock consists mainly of glassy materials.
3- Hemi-crystalline: the rock consists of both crystalline and
glassy materials.
B. Granularity: descibed as crystal dimensions measurements which
represents the actual size of crystals.
1- Phaneritic: rocks that consist of crystal up to 1mm which can be
seen without the need of microscope.
2- Aphanitic: rocks that consist of micro-crystals or cryptocrystalline
materials which cannot be seen without microscope.
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8. How Igneous
Rock Textures
Are Described?
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9. How Igneous
Rock Textures
Are Described?
▪ Phaneritic Igneous Rock:
▪ Aphanitic Igneous Rock:
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10. Types Of
Igneous Rock
Textures
A
Equigranular
Texture
▪ Allotriomorphic Texture: a relationship between equigranular
Anhedral crystals and characterizes " Aplite rock ".
▪ Hypidiomorphic Texture: a relationship between equigranular
Subhedral crystals and characterizes " Granite rocks ".
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11. Types Of
Igneous Rock
Textures
A
Equigranular
Texture
▪ Panidiomorhic Texture: a relationship between equigranular Euhedral
crystals and characterizes plutonic rocks especially " Lamprophyre ".
▪ Mico-granitic Texture: a relationship between equigranular fine
Anhedral or Subhedral crystals as in Rhyolite.
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12. Types Of
Igneous Rock
Textures
A
Equigranular
Texture
▪ Orthophyric Texture: a relationship between equigranular fine Euhedral
crystals.
▪ Felsitic Texture: a relationship between equigranular cryptocrystalline
crystals and characterizes " Felsite rock ".
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13. Types Of
Igneous Rock
Textures
B
Inequigranular
Texture:
I.
Porphyritic
▪ Porphyritic Texture: a relationship between large crystals called
Phenocryst and ground mass, according to the nature of ground mass,
porphyritic texture can be classified into 4 main types and 2 other main
types based on Phenocryst which are usually characterize Extrusive
rocks.
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14. Types Of
Igneous Rock
Textures
B
Inequigranular
Texture:
I.
Porphyritic
▪ Granular Porphyritic: ground mass consists of fine crystals less than
1mm.
▪ Micro-porphyritic: ground mass consists of micro-crystals.
▪ Felsophyric: ground mass consists of cryptocrystalline materials.
▪ vitrophyric: ground mass is glassy.
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15. Types Of
Igneous Rock
Textures
B
Inequigranular
Texture:
I.
Porphyritic
▪ Mega-porphyritic: Phenocrysts can be easily seen in hand specimen.
▪ Glomero-porphyritic: Phenocrysts exist in groups and may differ in
composition.
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16. Types Of
Igneous Rock
Textures
B
Inequigranular
Texture:
II.
Poikilitic
▪ Poikilitic Texture: it's a relationship between large mineral crystal and
other small crystals of minerals which the large one contains small
crystals of a mineral or more than one mineral inside it. Depending on
the geometric relation between those crystals poikilitic texture can be
divided into 4 main types of textures.
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17. Types Of
Igneous Rock
Textures
B
Inequigranular
Texture:
II.
Poikilitic
▪ Ophitic Texture: the whole small crystal all exists inside the large one.
this texture characterizes intrusive basic igneous rocks which usually
represents a relationship between Pyroxene as the large crystal and
Plagioclase as small ones as in " Gabbro rocks ".
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18. Types Of
Igneous Rock
Textures
B
Inequigranular
Texture:
II.
Poikilitic
▪ Sub-ophitic Texture: same as and usually associated with Ophitic
texture except large crystals (usually Pyroxene) are partially contain the
small crystals (usually Plagioclase).
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19. Types Of
Igneous Rock
Textures
B
Inequigranular
Texture:
II.
Poikilitic
▪ Diabasic Texture: a geometric relationship between prismatic crystals
of plagioclase which usually forms relatively triangular shapes where
small crystals of pyroxene and iron oxides accumulate. This type of
texture characterizes " Diabase rock ".
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20. Types Of
Igneous Rock
Textures
B
Inequigranular
Texture:
II.
Poikilitic
▪ Intersertal Texture: it's a geometric relationship between prismatic
crystals of Plagioclase which forms random shapes usually filled with
Cryptocrystalline or glassy materials or fine crystals of secondary
minerals like chlorite and usually characterizes volcanic rocks.
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21. Types Of
Igneous Rock
Textures
C
Directive
Texture
▪ Trachytic Texture: in this texture small crystall of ground mass
arranged parallel to each other indicate the direction of flow which
characterizes Extrusive igneous rocks especially " Trachyte rock ".
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22. Types Of
Igneous Rock
Textures
C
Directive
Texture
▪ Trachytoid Texture: this type of texture characterizes " Syenite rock "
as Trachytic texture Alkali feldspar takes the pattern of parallel
arrangement.
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23. Types Of
Igneous Rock
Textures
C
Directive
Texture
▪ Hyalopilitic Texture: in this texture small or thin crystals arranged
parallel to each other which exists into glassy ground mass and
characterizes volcanic rocks especially " Basalt rock ".
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24. Types Of
Igneous Rock
Textures
C
Directive
Texture
▪ Pilotaxitic Texture: in this texture Plagioclase crystals arranged parallel
to each other and characterizes volcanic rocks especially " Andesite
rock ".
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25. Types Of
Igneous Rock
Textures
D
Intergrowth
Texture
▪ Micro-graphic Texture: in this texture Quartz exists into Alkali
Feldspar forms arranged prismatic or triangular shapes due to
contemporaneous growth of both minerals.
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26. Types Of
Igneous Rock
Textures
D
Intergrowth
Texture
▪ Granophyric Texture: in this texture Quartz exists into Alkali Feldspar
form irregular random shapes usually drop-like due to
contemporaneous growth of both minerals.
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27. Types Of
Igneous Rock
Textures
D
Intergrowth
Texture
▪ Spherulitic Texture: in this texture Quartz and Alkali Feldspar
combined into thin radial fibers relationship sharing the same center
and can be easily detected under crossed light.
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28. Types Of
Igneous Rock
Textures
E
Reaction
Texture
▪ Corona Texture: this texture characterizes "Dunite and Peridotite rocks"
which is a relationship between Olivine and Pyroxene (Pyroxene forms
a rim surrounds Olivine crystals which exist as inclusion into Pyroxene).
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29. Types Of
Igneous Rock
Textures
E
Reaction
Texture
▪ Myrmekitic Texture: this texture formed during the overgrowth of
Plagioclase over K-feldspar which requires less silica than K-feldspar,
silica are released forming Quartz rods or worms-like set in Plagioclase
crystals.
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30. Thanks
E-mail: geo.ahmedessam@gmail.com
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