What is texture? What is the importance of texture? What types of texture show by the igneous rocks?

   The rocks are formed by cooling and solidification of magma, such types of rocks known as igneous rock. Approximately 90% of the earth’s crust is composed of igneous rocks but their great abundance is hidden on the surface of the earth by a relatively thin layer of sedimentary and metamorphic rocks.

      Chemically an igneous rock classified on the basis of silica percentage-

1.      Acidic igneous rock: if igneous rocks contain silica more than 65% then the rocks are called acidic igneous rock. For example-  Granite

2.      Intermediate: if igneous rocks contain silica between 55% and 65% then the rocks are called intermediate. For example- Diorite

3.      Basic rock: if igneous rocks contain silica between 45% and 55% then the rocks are called basic rock. For example- basalt

4.      Ultrabasic rock: if igneous rocks contain silica less than 45% then the rocks are called ultrabasic rock. For example- Peridotite

                     On the basis of occurrence of igneous rocks are divided into two major types-

1)      Extrusive rocks: the igneous rocks which are formed due to the solidification of lava called extrusive rocks. It is also known as volcanic rocks. Volcanic rocks show vesicular structure. Sometimes these rocks show flow structure which is the result of movement in viscous lava.

For example- Basalt

2)      Intrusive rocks: The igneous rocks are formed by the crystallization of magma beneath the surface of the earth. Depending on the depth of formation, the intrusive rocks are divided into two types:

a)      Plutonic rocks: intrusive igneous rocks which are formed at a great depth below the earth's surface known as plutonic rocks. For example - granite

b)      Hypabyssal rock: intrusive igneous rocks which are formed near the earth's surface known as hypabyssal rock.  For example- Dolerite

                   On basis of minerals commonly occurring in an igneous rock may be broadly classified into two types-

Felsic mineral: These minerals are light in color, low melting point, and have low specific gravity. For example- quartz, Feldspar group of minerals

 

Mafic mineral: these minerals are dark in color, high melting point, and have high specific gravity.

   

 

a)      Felsic rock: The rocks which are formed by the accumulation of Felsic minerals known as Felsic rocks. These rocks are also known as acidic rocks. For example- Granite.

b)      Mafic rock: The rocks which are formed by the accumulation of mafic minerals known as mafic rocks. These rocks are also known as basic rocks. For example- Basalt.

 

On the basis of color, igneous rocks are classified into three types-

1)      Leucocratic: the igneous rocks which are light in color due to the presents of Felsic minerals known as leucocratic rocks.

2)      Mesocratic: the igneous rocks which are intermediate in color due to the presents of Felsic and Mafic both minerals known as Mesocratic rocks.

3)      Melanocratic rock: the igneous rocks which are dark in color due to the presents of Mafic minerals known as Melanocratic rocks.

 

Texture: texture means the size, shape, and arrangements of mineral grains in a rock. The grain size of igneous rocks depends on the rate of cooling of magma. In general, slower is the rate of cooling, the coarser is the grain of rock. In the study of texture four points are considerable:

1)      Degree of crystallization: on the basis of the degree of crystallization, texture of igneous rock can be divided into three group

a)      Holocrystalline: when a rock is made up entirely of crystals, then the rock is said to have Holocrystalline texture. For example granite.

b)      Holohyalline:  when a rock is composed entirely of glassy material then the rock is said to have Holohyalline texture. For example obsidian

c)      Merocrystalline: when a rock is composed partly of crystals and partly of glassy material then the rock is said to have merocrystalline texture. For example

2)      Size of grains: On the basis of the size of grains, the texture of igneous rock can be divided into five group

a) Coarse-grained texture: if the grain size of an igneous rock is more than 5 millimeters in diameter, then the texture is called coarse-grained texture.

b) medium-grained texture: if the grain size of an igneous rock is between 1mm to 5mm then the texture is called medium-grained texture.

c) Fine-grained texture: if the grain size of an igneous rock is less than 1mm then the texture is called fine-grained texture.

d)      Microcrystalline texture: if the grain size of an igneous rock is too small that can be only distinguished under the microscope, not by the naked eye, then the rock is said to have microcrystalline texture.

e)      Cryptocrystalline texture: in cryptocrystalline texture, the individual crystals are very small. They are not visible under the microscope but their presence can be felt as they react to the polarized light

3)      On the basis of the shapes of crystals the igneous rocks are divided into three groups:-

a)      Euhedral: the shape of the grain of an igneous rock is said to have Euhedral texture if they show well-developed crystal faces.

b)      Subhedral: the shape of the grain of an igneous rock is said to have a Subhedral texture if they show partly developed crystal faces.

c)      Anhedral:  the grain shape of an igneous rock is said to have an Anhedral texture if the crystal faces are absent.

4)      Mutual relations of grains:

a)      Equigranular: igneous rocks are said to have an Equigranular texture if they contain the more or less equal sizes of grains. The equigranular texture is further divided into six types:

·    Panidiomorphic: when most of the grains are Euhedral, the texture of the rock is called Panidiomorphic.

·   Hypidiomorphic: when most of the grains are Subhedral, the texture of the rock is called Hypidiomorphic.

·   Allotriomorphic: when most of the grains are Anhedral, the texture of the rock is called Allotriomorphic.

·   Micro granular: when the grains of the microcrystalline igneous rocks are Anhedral or Subhedral then the texture is called Micro granular.

·   Orthophyric: the texture of the igneous rocks is said to have an Orthophyric texture if they possess a fine-grained Panidiomorphic texture.

·   Felsitic: if an igneous rock contains a uniform mass of cryptocrystalline matter, then the rock is said to have a felsitic texture.

b)      Inequigranular: igneous rocks are said to have an Inequigranular texture if they contain unequal sizes of grains. It is also divided into 4 types:

·         Porphyritic: the texture of an igneous rock is said to have a porphyritic texture if the large crystals are surrounded by fine-grained minerals. The large crystals are called phenocryst and the fine grains are called matrices.

·         Poikilitic: the texture of an igneous rock is said to be Poikilitic if the fine grains are surrounded by large crystals.

·         Ophitic: Ophitic texture is a special type of Poikilitic texture in which the bigger crystals of Augite enclose the smaller laths of plagioclase. If the smaller laths of plagioclase are partly enclosed in the larger crystals of Augite, the texture is called sub-Ophitic.

·         Intergranular and Intersertal Texture: In many types of basalt plagioclase laths occur in such a way that they form a network with triangular or polygonal interspaces. These interspaces are filled with minute grains of Augite, olivine, and iron oxide. Such texture is called Intergranular texture.

 

If these interfaces are filled with glassy or fine-grained chlorite or serpentinous (The serpentine group describes a group of common rock-forming hydrous magnesium iron phyllosilicate ((Mg, Fe)₃ Si₂ O₅ (OH)₄) minerals) materials, then the texture is called Intersertal texture.

c)      Directive texture:

                  The texture produced as a result of the flow of lavas during their consolidation is called “directive texture”. The chief directive- textures are as follows:

1)      Trachytic texture: Certain volcanic rock, such as trachyte, contain Augite lath arranged in lines parallel to the direction of flow of lava, such texture is called the “Trachytic texture”.

2)      Hyalopilitic texture: In a volcanic rock, if feldspar laths are found intermixed with glass, the texture is called “Hyalopilitic texture”.