Textures of igneous rocks

Textures of igneous rocks

1. Md. YousufGazi, Lecturer, Department of Geology, University of Dhaka ([email protected])
Md. YousufGazi
Lecturer
Department of Geology
University of Dhaka

2. WhatisTexture?
Textureistheintimatemutualrelationshipofthe
constituentsofrock.Thatis,itdealswithsize,shape,
natureandarrangementsofigneousrocks.Textureisbest
studiedinthinsectionundermicroscoperatherthanin
theoutcrop.
Required considerable factors for the description of texture
1. Degree of Crystallization or Crystallinity
2. Absolute size of the crystal, the grain or granularity
3. Fabric
Md. YousufGazi, Lecturer, Department of Geology, University of Dhaka ([email protected])

3. Crystal
Whenthemoleculesinarockhavesettled
downintoastablearrangementandwitha
recognizablepatterniscalledcrystal.
Md. YousufGazi, Lecturer, Department of Geology, University of Dhaka ([email protected])
Glass
Itisdeformedstateofcrystallization,regardedas
anamorphoussolid.Inaglass,themoleculeshave
settleddownintoastablearrangementwithout
anyrecognizablepattern.

4. MajorFactorsforthedeterminationofCrystallinity
Rateofcoolingofmagma
Viscosityofmagma
Rapidcoolingandhighviscosityfavor
theformationofGlass
Slowcoolingandlowviscositypromote
theformationofcrystal
Md. YousufGazi, Lecturer, Department of Geology, University of Dhaka ([email protected])

5. A. Crystallinity
Crystallinityismeasuredbytheratiosubsistingbetweencrystallizedandnoncrystallizedmatter.
Types of Crystallinity
1.Holocrystalline
2.Holohyaline
3.Hemicrystalline/ Hypocrystalline/Merocrystalline
Md. YousufGazi, Lecturer, Department of Geology, University of Dhaka ([email protected])

6. 1. Holocrystalline
Arockcomposedentirelyofcrystalsissaidtobe
Holocrystalline.Holocrystallinetextureischaracterized
bydeep-seatedorintrusiveigneousrocks.Example:
Granite,Gabbro,Syenite.
Md. YousufGazi, Lecturer, Department of Geology, University of Dhaka ([email protected])

7. 2. Holohyaline
Therockwhichisentirelycomposedofglassymaterials
orglassisknownasHolohyaline.Theyoccurmost
oftenmarginalfaciesofrockbodiesbutmayoccur
aslavas(Obsidian)orasdykesandsills(Pitchstone).
Md. YousufGazi, Lecturer, Department of Geology, University of Dhaka ([email protected])

8. 3. Hemi/Hypo/Merocrystalline
Whentherockiscomposedpartlyofcrystalsandpartlyof
glass,thetermMero/Hypo/Hemicrystallineisused.
Example:Rhyolite,Tracayte.
Md. YousufGazi, Lecturer, Department of Geology, University of Dhaka ([email protected])

9. B. Granularity
Granularitydealswiththeabsolutesizeofthecrystalsin
igneousrocksrangesfromalmostsubmicroscopic
dimensionstocrystalmeasurableinyards.
Md. YousufGazi, Lecturer, Department of Geology, University of Dhaka ([email protected])
Types of Granularity
a.Phanericor Phanerocrystalline
b.Aphanitic

10. a. Phaneric or Phanerocrystalline
Ifthecrystalsarevisibletothenakedeyeorby
ordinaryhandlens,therockissaidtobePhaneric
orPhanerocrystalline.
Phaneric or Phanerocrystalline is further
classified by
I.Coarse Grain: > 5 mm
II.Medium Grain: 5 mm to 1 mm
III.Fine Grain: < 1 mm
Md. YousufGazi, Lecturer, Department of Geology, University of Dhaka ([email protected])

11. b. Aphanitic
Ifthecrystalsarenotvisibletothe
nakedeyeorbyordinaryhandlens,the
rockisknowntobeAphanitic.
It is classified by
I.Microcrystalline:individualcrystalsare
distinguishablewiththehelpofpetrographic
microscope.
II.Cryptocrystalline:individualcrystalsarenot
distinguishablewiththehelpofpetrographic
microscope.
Md. YousufGazi, Lecturer, Department of Geology, University of Dhaka ([email protected])

12. C. Fabric
Thefabricorpatternoftherocksdependonthe
shapes,relativesizeandarrangementofthe
crystals.
a.Shape of the crystal
b.Mutual relationship of crystal
Md. YousufGazi, Lecturer, Department of Geology, University of Dhaka ([email protected])

13. III.Subhedral:Crystalsarepartlyboundedbyfaces
a. Shape of the crystal
Crystalsformsaredescribedwithreferencesof
thedevelopmentoftheirfaces.
I. Euhedral: Crystals are bounded by faces
II. Anhedral: Crystals are not bounded by faces
Md. YousufGazi, Lecturer, Department of Geology, University of Dhaka ([email protected])

14. b. Mutual Relationship of crystal
Thefabricofarockisinfluencednotonlyby
theshapeofthecrystals,buttheirrelativesize
andtheirmutualarrangement.
Textures dependent on mutual relations
classified as
i.Equigranular
ii.Inequigranular
iii.Intergrowth
iv.Directive Texture
v.Intergranular
Md. YousufGazi, Lecturer, Department of Geology, University of Dhaka ([email protected])

15. i.Equigranular Texture: Grains are more
or less same size
Panidiomorphic: Most of the crystals
are euhedral
Allodiomorphic: Most of the crystals
are anhedral
Hypidiomorphic: Most of the crystals
are subhedral
Md. YousufGazi, Lecturer, Department of Geology, University of Dhaka ([email protected])

16. ii.InequigranularTexture:Grainsareofdifferentsizes
PorphyriticTexture:
Whenthelargercrystalsareembedded/envelopedbysmaller
crystalsorgroundmass.
OphiticTexture:
Itisspecialvarietyofpoikilitictextureinwhichplatesofaugite
enclosenumerousthinlathsofplagioclage.
PoikiliticTexture:
Whenthesmallercrystalsareenclosedwithinthelargerones.
Md. YousufGazi, Lecturer, Department of Geology, University of Dhaka ([email protected])

17. iii. Intergrowth Texture
When two minerals crystallize simultaneous, they may result
intergrowth texture.
Graphic intergrowth: intergrowth between
orthoclase(Feldsper) and Quartz.
Md. YousufGazi, Lecturer, Department of Geology, University of Dhaka ([email protected])
Perthiticintergrowth: intergrowth between
Orthoclase and Albite.
Myrmekiteintergrowth: intergrowth between
Plagioclase and Quartz.

18. iv. Directive Texture
Sincemagmaisafluid,itcanflow.Ifelongate
crystals,suchasfeldspararepresentinthe
magmawhilstitisflowing,thesecrystals
maywellalignthemselvesinthedirectionof
flow.
Md. YousufGazi, Lecturer, Department of Geology, University of Dhaka ([email protected])

19. v.IntergranularTexture
Whentriangularorpolygonalinterspacesbetween
thecrystalsareentirelyfilledwithgranulesofother
minerals,thetextureissaidtobeIntergranular.
Generally,Basaltshowsthistypeoftextureandthe
filledgranulesareolivine,augite.
Md. YousufGazi, Lecturer, Department of Geology, University of Dhaka ([email protected])

20. Md. YousufGazi, Lecturer, Department of Geology, University of Dhaka ([email protected])