Texture Of Igneous Rocks

Texture-

The texture of a rock is the size, shape, and arrangement of the grains (for sedimentary rocks) or crystals (for igneous and metamorphic rocks).

It is basically divided into 4 parts:

1. Crystallinity (Degree of Crystallization)

2. Granularity (Actual Size Of Grain)

3. Shapes Of Crystals

4. Mutual Arrangement Of Grain

1. Crystallinity- Ratio between Crystallised and Non Crystallised materials.

a) Holocrystalline- Rock composed entirely of crystal.

b) Hemicrystalline- Rock composed of crystal and glass.

c) Holohyalline- Rock composed entirely of glass.

2. Granularity- Actual size of grain.

a) Phaneritic/Phanerocrystalline- When we can identify the crystals by naked eye, formed during plutonic environment.

Coarse Grained- >5 mm

Medium Grained- 1 to 5 mm

Fine Grained- <1 mm

b) Aphanetic- When we can't identify crystals by naked eye.

Microcrystalline- Rocks that contains small crystals, visible only through microscope.

Cryptocrystalline- When crystals are too small to be distinguished,even under the microscope.

3. Crystal Shape-

Euhedral- Crystal boundary is perfect grown.

Subhedral- Crystal boundary is partially develop.

Anhedral- Crystal have no boundary.

Tabular- Crystal grain grow in 2 opposite direction and 3rd dimension remain undeveloped.

Prismatic- Crystal outline is developed in one direction compare to other direction.

Equidimension- Crystals are equally developed in all directions.

4. Mutual Arrangement Of Grain-

a) Non-Directive- Grain do not show parallel orientation or arrangement of grain.

(1) Equi-granular- Minerals are even grained in hand specimen.

Panidiomorphic- Majority of crystals are euhedral.

Hypidiomorphic- Majority of crystals are subhedral.

Allotriomorphic- Majority of crystals are anhedral.

(2) Inequigranular- When grain size are not equal.

Porphyritic- Large crystals are enclosed by small ones.

Large crystals are called Phenocryst and small are called groundmass or matrix.

Poikilitic- Small crystals are enclosed by large ones.

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