Types of Textures

Types of Textures

These can be broadly divided into five categories:

. Equigranular textures

. Inequigranular textures

. Directive textures

. Intergrowth textures and

 . Intergranular textures.

(1)                   Equigranular Textures

Ø              All those textures in which majority of constituent crystals of a rock are broadly equal in size are described as equigranular textures.

Ø             In igneous rocks, these textures are shown by granites and felsites and hence are also often named as granitic and felsitic textures

Ø              In  the  granitic  texture,  the  constituents  are  either  all  coarse  grained  or  all

medium grained and the crystals show euhedral to subhedral outlines.

Ø                    In the felsitic texture, the rock is micro granular, the grains being mostly microscopic crystals but these invariably show perfect outlines.

Ø             Thus felsitic textures may be described as equigranular and panidiomrphic.

Orthophyric texture is another type of equigranular texture, which is in between

the granitic and felsitic textures. The individual grains are fine in size but not micregranular.

(2)                   Inequigranular Texture

Ø                Igneous textures in which the majority of constituent minerals show marked difference in their relative grain size are grouped as inequigranular texture.

Ø             Porphyritic and Poiklitic textures are important examples of such textures.

Ø             Porphyritic Texture is characterised by the presence of a few conspicuously large sized crystals

(the phenocrysts) which are embedded in a fine-grained ground mass or matrix.

Ø              The texture is sometimes further distinguished into mega-porphyritic and microporphyritic depending upon the size of the phenocrysts.

Difference in. molecular concentration

Ø When the magma is rich in molecules of particular mineral, the latter has better chance to grow into big crystals which may get embedded in the fine-grained mass resulting from the deficient components.

Change in physico-chemical conditions.

Ø                            Every magma is surrounded by a set of physico-chemical conditions like temperature, pressure and chemical composition, which influence the trend of crystallisation greatly.

Ø                                 Abrupt and discontinuous changes in these textures may result in the formation of the crystals of unequal dimensions.

Ø                           Thus, magma crystallizing at great depths may produce well-defined, large sized crystals.

Ø                                  When the same magma (carrying with it these large crystals) moves upward, the pressure and temperature acting on it are greatly reduced.

Ø                            Crystallisation in the upper levels of magma becomes very rapid resulting in a fine-grained matrix that contains the big sized crystals formed earlier.

Relative insolubility

Ø      During the process of crystallisation, their crystal grains get enlarged whereas crystals of other soluble constituents get mixed up again with the magma; thus, the relatively insoluble constituents form the phenocrysts

Ø And the soluble constituents make up the ground mass crystallizing towards the end.

(3)                   Directive Textures

  The textures that indicate the result of flow of magma during the formation of rocks

are known as directive textures.

Ø     These exhibit perfect or semi perfect parallelism of crystals or crystallites in the direction of the flow of magma.

Ø   Trachytic and Trachytoid textures are common examples.

Ø     The former is characteristic of certain felspathic lavas and is recognised by a parallel arrangement of felspar crystals; the latter is found in some syenites.

(4)                   Intergrowth Textures

Ø                 During the formation of the igneous rocks, sometimes two or more minerals may crystallize out simultaneously in a limited space so that the resulting crystals are mixed up or intergrown.

Ø               This type of mutual arrangement is expressed by the term intergrowth texture.

Ø               Graphic and granophyric textures are examples of the intergrowth textures.

Ø                    In graphic texture, the intergrowth is most conspicuous and regular between quartz and felspar crystals. In granophyric textures the intergrowth is rather irregular.

(5)                   Intergranular Textures

Ø     In certain igneous rocks crystals formed at earlier stages may get so arranged that polygonal or trigonal spaces are left in between them.

Ø          These spaces get filled subsequently during the process of rock formation by crystalline or glassy

masses of other minerals.