texture

TEXTURE : -

1      It is intimate, mutual relationship between minerals. Sometimes the glassy matter is also present in the rock.

2      Igneous rocks shows that they are vide variety of Texture due in mode of formation.

Physical condition : -

a.  Pressure under which it cools.

b.  Presence or absence of magma tic gases.

c.  Rate of cooling.

d.  Viscosity of magma.

Factors :-

a.)     Rate of crystallization or degree of crystallization. It is ratio of crystalline matter and non crystalline matter. Depending upon rate of crystallization.

Three types of Texture :

a) Holocrystalline            b)  Hemi crystalline          c)  Holohalyine 

Absolute grain size i.e. granularity.

Depending upon grain size.

a)       Phaneritic Textures – Crystals to be seen by necked eye (megascopically identified )Depending upon grain size diameter following divisions : 1)  Coarse grain (>75 mm)  2)  Medium grain (5to1 mm)  3) Fine grain Texture ( < 1 mm )

b)       Aphanitic Texture : -

Grains are identified only under microscope

c)       Shape of crystals

i.     Euhedral  - well developed crystal

ii.   Sub hedral – development of crystal faces is incomplete.

iii. Unhedral – not well development irregular

i.     Equidimentional - growth has been taken place in all 3 direction.

ii.   Tabular or elongated – growth is more in 2 direction as compared to third.

iii. Prismatic - growth is more in one direction only.

iv.  Irregular -  No growth.

Mutual Relation :-

Mutual relationship includes the arrangement of diff minerals or minerals and glossy matter. Depending upon mutual relation.

a)   Equigranular Texture ( Granitic Texture )

b)   Inequigranular Texture ( porphyritic Texture )

c)    Intergrowth Texture ( Graphic Texture )

Texture of plutonic igneous Rocks : -

Plutonic rocks cools under condition  of slow cooling. High pressure, presence of gases. These condition are favourable for crystallization. All crystal that are formed under these conditions attain same size and shape typically.

Equigranular Granitic coarse grained Texture.

Inquigranular porphyritic Texture : - This text results when a magma cools slowly first and faster afterword due to change in depth of magma & rate of cooling i.e. physicochemical conditions.

Graphic texture : - Two minerals quartz and orthoclase crystallized simultaneously from magma to form intergrowth texture.

Textures of volcanic igneous Rocks : -

Volcanic rock cools under conditions to fast cooling, low pressure , absence of magmatic gases.

1.       Volcanic rocks typically aphanitic ( glassy )

2.       Porphyritic Texture – The magma before being outpoured as lava cools for some time below the surface under plutonic conditions and some crystals develop then magma comes in surface along with crystals and cools fast glassy matter formation of porphyritic Texture with plagioclase phenocryst and aphanitic ground mass.