Brief notes on the Texture of Igneous Rocks

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Textures of igneous rocks describe the actual relations between crystals or that between the crystals and the glassy material present within igneous rocks. Rocks have been formed under diverse physicochemical environment, and textural studies indicate the cooling history of the magma. Texture of igneous rocks is a function of three important factors:

(1) degree of crystallisation, i.e., crystallinity.

(2) size of the grains (or crystals), i.e., granularity.

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(3) fabric, which includes :

(i) shape of crystals.

(ii) mutual relations of grains or of crystals and glassy matter.

1. Crystallinity:

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(a) Holocrystalline:

When an igneous rock is made up of mineral grains only.

(b) Hemi-crystalline:

When a rock contains both crystalline as well as glassy matter in variable proportions.

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(c) Holohyaline:

When the igneous rock consists wholly of glass.

The degree of crystallisation depends on the following factors:

(i) Rate of cooling.

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(ii) Viscosity of magma (i.e., composition of the magma and the presence of volatile components).

(iii) Depth of cooling.

(iv) Volume of the magma.

2. Granularity:

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It refers to the grain size of the crystals present in the igneous rocks. These are

(a) Phaneric:

When individual crystals are visible to the naked eyes and are

(i) Coarse grain:

When the grain size is 5 mm or above.

(ii) Medium grain:

Grain size is 1 mm to 5 mm.

(iii) Fine grain:

Grains are smaller than 1 mm in diameter.

(b) Aphanitic:

When individual grains cannot be distinguished, with unaided vision and are

(i) Micro-crystalline:

When individual crystals are distinguish­able only under microscope.

(ii) Mero-crystalline:

Intermediate in range.

(iii) Crypo-crystalline:

When individual crystals are too small to be separately distinguished, even under the microscope.

(iv) Glassy:

When there is no crystallisation at all.

In general, pegmatitic and plutonic rocks are coarse grained, hypabyssal rocks are medium grained and volcanic rocks are either fine grained or glassy.

In most natural glasses there are found number of minute bodies of various shapes, which represent the beginnings of crystals. They are

(a) Crystallites:

These are embryo crystals.

(b) Microlites:

Minute crystals, which may exhibit the crystal outline appropriate to their mineralogical nature.

Terms like globulites, margarites, longulites, trichites, scopulites are used to describe various forms of microlites.

Devitrification:

It is the process of conversion of glassy material to crystallised state. Perlitic cracks in crystals are evioences of their original glassy condition.

3. Fabric:

(i) Shape of the grains:

It refers to the degree of develop­ment of crystal faces and are:

(a) Euhedral:

When the mineral grains are found to have developed a perfect crystal outline. These are known as idiomorphic or automorphic crystals.

(b) Sub-hedral:

When the crystal outlines have partially developed. These are also known as hypidiomorphic or hypautomorphic crystals.

(c) Anhedral:

When the crystal faces are absent.

With reference to the three dimensions in space, crystals are classified as

(a) Equidimensional:

Crystals found to have developed equally along all directions in space.

(b) Tabular:

When better developed in two directions.

(c) Prismatic:

When better developed in one direction only.

(ii) Mutual relationship:

It refers to the relative size, shape and dimensions of crystals and their relation to one another. This may be:

(a) Equigranular, and

(b) Inequigranular.

(a) Equigranular:

(i) Auotriomorphic. In this case all the crystals are anhedral Synonymous terms are xenomorphic, aplitic, mosaic, sugary, sacchraoidal.

(ii) Hypidiomorphic:

Here all the crystals are subhedral.

(iii) PaaidioiBorphic:

Here all the crystals are euhedral. This is the characteristic texture of lamprophyres.

In case of aphanitic rocks, they are

(a) Micro-granitic:

Consisting of anhedral and subhedral grains.

(b) Orthopyric:

Crystals are microscopically euhedral.

(c) Felsitic:

Grains are too small to be identified in thin sections.

(b) Inequigranular:

In this case the grain size shows a marked difference from grain to grain. This is also known as seriate texture, and is of the following types:

I. (i) Porphyritic texture:

In this case larger crystals are enveloped in a groundmass which may be micro-granular, mero- crystalline or even glassy. This texture is characteristic of volcanic and hypabyssa rocks. Its origin may be attributed to:

(a) Change in physico-chemical condition.

(b) Molecular concentration.

(c) Insolubility.

(ii) Glomero-porphyritic:

Here phenocrysts gather at one spot.

(iii) Vitrophvric:

When the groundmass is glassy in a porphy­ritic texture, it is called vitrophyric texture.

(iv) Felsophyric:

Here groundmass is crypto crystalline in nature.

II. (i) Poikilitic texture:

In this case, smaller crystals are closed in the larger ones without common orientation. The eaclosing crystal is known as ‘oikocrysts’ and the enclosed ones are called ‘qtiadacrysts.’

(ii) Ophitic texture:

It is a type of poikilitic texture in which euhedral plagioclase crystals are partially or completely surrounded by anhedral augite crystals. This is the characteristic texture of dolerites.

(iii) Sub-ophitic texture:

With equal size of feldspar grains and augite, the enclosure is partial and it is known as sub-ophitic.

(iv) Granub-ophitic:

Here grains of plagioclase are enclosed within a large patch of pyroxene, and the latter in turn is made up of a number of pyroxene grains.

(v) Hyalo-ophitic:

When diversely oriented grains of plagio­clase occur within the glassy groundmass.

(vi) Ophimottling:

Ophitic texture confined to separate areas.

III. Intersertal and intergranular texture:

When the plagio­clase laths are arranged in a tringular fashion and the polygonal interspace left between the crystals is having glassy infillings, it is known as ‘interstrtal’. However, if the interspace is filled in with mineral grains, it is called ‘intergranular’. These are commonly found in basalts.

IV. Directive textures. These are produced by flow in magma during their crystallisation.

(i) Trachytic texture:

Produced by subparallel arrangement of plagioclase along the direction of flow of lava. Without evident flow, it is known as Fdsitic texture.

Hyalopilitic. Also known as felty, which is due to appre­ciable admixture of glass and feldspar needles within the body of a rock.

Intergrowth:

(a) Graphic intergrowth between orthoclase and quartz.

(b) Perthitic intergrowth between albite and orthoclase.

(c) Myrmekite growth between quartz and feldspar.

(d) Granophyric is the graphic texture found in hypabyssal and volcanic rocks of granitic composition.

As described above, these are important and common textures of igneous rocks.

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