Igenous Rock classification

Igneous rocks show a wide variation in mineral composition and texture due to conditions of crystallization. The most useful classification is based on mineralogy and texture.

Relative silica content – presence of quartz indicates excess of silica and felspathoids indicate deficiency of it.

Kinds of Feldspar – determination of relative amount of alkali and plagioclase feldspar.

Mafic minerals – the amount and type of mafic minerals present in the igneous rock.

Texture – the size, shape and arrangement of mineral grains in a rock.

 Textures

The grain sizes of an igneous rock depend on the rate of cooling of magma.

Slower the rate of cooling, coarser is the grain of a rock.

Texture is based on the following points:

Degree of crystallization.

Size of grain.

Shape of the crystal.

Mutual relation between mineral grains.

Degree of Crystallization

Holo-crystalline Texture – rock made up entirely of  crystal.                        

Holohyaline Texture – rock is composed entirely of glassy materials.

Merocrystalline Texture – rock compose partly of crystal and partly of glassy.

Size of Grain

Phaneric – Igneous rocks whose constituent mineral grain can be seen with naked eyes. It can be subdivided into two;

Coarse Grained Texture – diameter of the grain is more than 5 mm. Most of the intrusive igneous rocks are coarse grain.

Medium Grained Texture – grain size are in between 1 – 5 mm.

Fine Grained Texture – the grains are like granulated sugar with diameter less than 1 mm. Most of the extrusive igneous rocks are fine grain.

Aphanitic – Igneous rock whose constituent mineral grain cannot be seen with naked eyes.

Microcrystalline Texture – mineral grain can be distinguished under microscope.

Cryptocrystalline Texture – individual grain is very small to be seen through microscope but their presence can be felt as they react with polarized light.

Shape of Crystal

Euhedral – well developed crystal shape.

Subhedral – partly developed

Anhedral – absence of crystal faces.

Mutual Relations of Grains

Equigranular Texture – mineral grains with more or less same size.

Panidiomorphic Texture –mineral grains are euhedral, Ex. Lamprophyres

Hypidiomorphic Texture – when most of the crystals are Subhedral, this texture is characterized by plutonic rock like granites and syenites.

Allotriomorphic Texture – crystals shape are anhedral, Ex. Aplites.

Microgranular Texture – microcrystalline igneous rocks also have equigranular texture. The crystal of these fine grains is either anhedral or Subhedral.

Orthophyres Texture – some highly felspathic rock such as orthophyres and plagiophyres possess a fine grained panidiomorphic texture.

Felsitic Texture – an igneous rock containing uniform mass of cryptocrystalline matter.

Inequigranular Texture – igneous rock showing variation in size of the grains.

Porphyrite Texture

When rocks contain large crystals of some minerals set in matrix which is much finer grain or even glassy, the texture is called porphyrite.

Larger crystals are called phenocrysts and finer are called groundmass.

Igneous rock showing porphyrite texture is called porphyries, Ex. Granite porphyry, Diorite porphyry and Rhyolite porphyry.

This texture develops when some crystal grows to a considerable size before the main mass of the magma consolidate to finer and uniform grade materials. It is found in volcanic and hypabyssal rock.

Poikilitic Texture

Smaller crystals are enclosed within the larger crystal without common orientation.

It is found in syenites and monzonites, where orthoclase is the host minerals.

Ophitic Texture

Special type of poikilitic texture in which the larger crystals of augite enclose smaller laths of plagioclase.

If the plagioclase laths are partly enclose in the larger grains of augite then it is called sud-ophitic. It is characterize by dolerite.

Intergranular and Intersertal Texture

Basalt plagioclase lath occurs in such a way that they form network with triangular or polygon interspaces. These interspaces are filled with minute grains of augite, olivine and iron oxide. Such texture is called intergranular texture and if glassy or fine grained of chloritic or serpentinous materials occurs in the interspaces then it is called intersertal texture.

Directive Texture

The texture form as a result of flow of lava during their consolation

Trachytic Texture          

Certain volcanic rock like Trachyte contains felspar laths which are arranged in the line parallel to the direction of flow of lava.                                                                                                                                                                                           

Hyalopilitic Texture

In volcanic rock if felspar laths are found intermixed with glass.

Intergrowth Texture

The intergrowth of quartz and orthoclase may take place when they crystallize simultaneously. The intergrowth frequently produces graphic texture in which skeleton of quartz crystals are embedded in orthoclase. The two intergrowth minerals have same optical orientation over large areas.