IGNEOUS ROCKS

Texture Of Igneous Rocks

Texture of Igneous Rocks is defined as the mutual relationship of different mineralogical constituent in a rock. It is determined by the size, shape, and arrangement of mineral grain within the rock body.  It also depends upon degree of crystallization, if the rate of cooling is slower coarser is the grain.

Textures are of various types on the basis of:

Degree of Crystallization–   On this basis the textures are of following types:

  1. Holohyaline: This texture is seen in those rocks which are made entirely of glassy material i.e. there is no crystal visible. This texture is the result of rapid cooling (Quenching).
  2. Holocrystalline: This texture is seen in those rocks which are made entirely of crystals formed as the result of slow cooling. The crystals may or may not be visible without magnification.
  3. Hypocrystalline: This texture is seen in those rocks which are made up of both crystals and glass which indicate that initially their was a period of slow cooling followed by quenching.

Textures on the bases of Degree of Crystallization

 

Crystal Size– On this basis the textures are of following types:

Texture on the bases of Crystal Size

  1. Equigranular: All the crystals are approximately of the same size which indicates that the entire rock crystallized under the same temperature and pressure condition. It is further subdivided into Aphanitic and Phaneritic textures.
  • Aphanitic Texture– This is a uniformly fine grained texture in which the individual crystals are so small to be seen with the naked eye. E.g. – rocks formed in the volcanic extrusion. It is also further subdivided into two types of textures-
  • MICROCRYSTALLINE
  • CRYPTOCRYSTALLINE
  • Phaneritic Texture It is a uniformly coarse grained texture in which all the individual crystals are easily visible without magnification. These type of textures are the result of slow cooling. It is also further subdivided into three types of textures-
Fine Phaneritic Medium Phaneritic Coarse Phaneritic
Ø  Average crystal size is less than 1mm.

Ø  These are the result of slow cooling i.e. Hypabyssal Intrusion. (Sills and dikes)

Ø  Average crystal size ranges from 1mm to 5mm.

Ø  This texture is primarily seen in the plutonic rocks.

 

Ø  Average crystal size ranges from 5mm to 10mm.

Ø  These indicate deep intrusive cooling i.e. Plutonic Intrusion. (Batholiths).

 

 

  1. Inequigranular: Igneous rock showing variations in the size of mineral grains are said to have this texture. Inequigranular texture is of following types-
  • Porphyritic texture When an igneous rock contains large crystals of some minerals in a matrix which is much finer grained or even glassy, the texture is called “Porphyritic”. The large crystals are called ‘phenocrysts’ and the finer grained material is called ‘groundmass’.

This texture is found largely in volcanic and Hypabyssal rocks.

  • Poikilitic Texture– When in a rock smaller crystals are enclosed within larger crystals without common orientation, the texture is called ‘pokilitic Texture’. This texture is commonly found in syenites and monzonites where orthoclase forms the host mineral.

Inequigranular

Other textures are:

  • Ophitic Texture When the host mineral is identified as Augite and the inclusions are of plagioclase feldspar, the texture is known as Ophitic Texture.
  • Intergrowth Texture- The Intergrowth of Quartz and orthoclase take place when they crystallize simultaneously. This intergrowth frequently produces “Graphic Texture” when the numerous quartz crystals are embedded in the orthoclase.

 

Crystal Shape– On this basis the textures are of following types:

  1. Panidiomorphic Texture– When most of the grains are euhedral, the texture of rock is called ‘panidiomorphic’. This texture is usually found in lamprophyres.
  2. Hypidiomorphic Texture- When most of the grains are subhedral, the texture of rock is called ‘hypidiomorphic’. This is characterstic of many plutonic rocks such as granites and synites.
  3. Allotriomorphic Texture- When most of the grains are anhedral, the texture of rock is called ‘Allotriomorphic’.

Texture on the bases of Crystal Shape

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