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 will be the rock grain.

Texture is based on:

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 three;

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. It is of following type.

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.

Granodiorite, with potassium feldspar phenocrysts much 
larger than the other minerals in the rock matrix

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. It is developed when some crystal grow toconsiderable size before main mass of the magma consolidate into finer and uniform grade materials. It is common in volcanic and hypabyssal rocks. Ex. Granite porphyry, Diorite porphyry and Rhyolite porphyry.

Poikilitic Texture - If smaller crystals are enclosed within the larger crystals without any particular orientation. It is found in monzonites and syenites where orthoclase form host minerals.

Ophitic texture - special type of poikilitic texture which bigger crystal of augite enclose smaller laths of plagioclase. If it is partially enclosed then it is called sub-ophitic texture. It occurs in dolerites.

Intergranular & Intersertial Texture - In many basalt plagioclase lath occurs in such a way that they form triangular network or polygonal interspace, this interspace are filled with minute grain of augite, olivine and iron oxide. such texture are called intergranular texture.

If glassy or fine grained chloritic or serpentinous materials then the texture is called intersertial. 

Directive texture - Texture form due flow lava during their consolidation and classified as follows
Trachytic texture - certain volcanic rock like trachyte contain felspar laths arranged in lines parallel to the direction of flow of lava. Such texture are said to be trachytic.
Hyalopilitic - in volcanic rock if felspar laths are found intermixed with glass the texture is called hyalopilitic.

Intergrowth texture - the intergrowth of quartz and orthoclase may take place when they crystallized simultaneously & frequently produces graphic texturein which skeleton of quartz crystal are embedded in the orthoclase. Two intergrowth minerals are having same optical orientation over large area. A variety of granite called graphic granite shows graphic texture.