Short essay on the Classification of Igneous Rocks

Igneous rocks show great variations both in chemical and mineralogical composition as well as textural characteristics. There is no general agreement among the penologists as to the classification of igneous rocks. Different classifications have been proposed or the basis of:

1. Mineralogical composition.

2. Chemical composition.

3. Textural characteristics.

4. Genesis.

5. Association.

1. On the basis of mineralogical composition:

Since the relative amounts of the various minerals in a rock can be measured or esti­mated with a fair degree of accuracy, this criteria is given more emphasis for the classification of igneous rocks.

The minerals occurring in igneous rocks may be classed as

(a) Essential, (b) Accessory, and (c) Secondary.

The first two are the products of magmatic crystallisation whereas the third one is formed by the alteration of the primary minerals, i.e. (the 1st two), through the processes of weathering, metamorphism etc.

(a) Essential minerals:

These are the major constituents of the rock which help in the diagnosis of the rocks types. The disappear­ance of these minerals would cause the relegation of the rock to another type.

(b) Accessory minerals:

These are the minor constituents of a rock and their presence has no bearing on the nomenclature of the lock.

The minerals commonly occurring in igneous rocks may be broadly classified into felsic and mafic varieties. Felsic is a term derived from feldspar, felspathoid and silica and mafic is similarly derived from ferromagnesian minerals, like biotite, pyroxene, amphiboles etc.

Felsic minerals are light in colour, low melting point, low specific gravity, comparatively of late crystallisation.

The mafic minerals are dark coloured, higher specific gravity, higher melting point and are early crystallised minerals.

Terms like leucocratic and melanocratic refer to light coloured and dark coloured minerals respectively. Usually the felsic minerals; are leucocratic and the rocks containing more of mafic minerals are. melanocratic. The rocks which are pitch dark in colour are termed. as ‘hypermelanocratic. Mesocratic rocks are intermediate in colour.

2. Chemical composition:

The most important chemical classi­fication have been proposed by four American penologists, Cross, Iddings, Pirsson and Washington. Here the chemical analyses are calculated into a set of hypothetical minerals called ‘standard’ or. ‘norm’, which are divided into two groups :

On this basis the rocks are classified as follows:

1. Salic to femic minerals into five classes.

2. Quartz to feldspar and of accessory minerals to other femic minerals into orders.

3. Salic alkalies to femic alkalies to famiclimes into rangs.

In their chemical composition, the igneous rocks vary between wide limits. Thus rocks like granite may contain about 70 to 8u%, of silica and very little quantity of iron, magnesia and lime while at the other extreme end, there are rocks like peridotitn, dunite, etc. which often contain about 35 to 40% of silica and larger quantities of iron, magnesia and lime. Thus igneous rocks may be classified as

(a) Acid (b) Intermediate

(c) Basic (d) Ultrabasic rocks.

(a) Add igneous rocks:

These rocks have more than 65% the silica content, e.g., Granite, Grano-diorites.

(b) Intermediate rocks:

These are having 55 to 65% of silica, syenite and diorites.

(c) Bask rocks:

Here the silica content is between 44 to 55%, e.g., basic.

(d) Ultrabasic rocks:

In this case, silica content is less than 44%. Rocks like anorthosite belong to this category. Ultrabasic rocks having higher magnesian contents are known as ultramafics.

Shands and Holmes classified the igneous rocks as

(i) Supersaturated rocks:

Also known as oversaturated rocks. Here, the excess of silica crystallize as quartz.

(ii) Saturated rocks:

They have just sufficient silica to form the stable silicate minerals but no free quartz.

(iii) Under-saturated rocks:

They contain insufficient silica and minerals like olivine, nepheline, leucite etc. are therefore present.

3. Textural classification:

It is mostly based on the cooling history of the magma and the modes of occurrence of these rocks. Accordingly there are (a) Plutonic (b) Hypabyssal, and (c) Volcanic rocks.

The plutonic rocks are formed under deep seated conditions, where the temperature and pressure are very high and the rate of cooling is very slow. Hence their texture is holocrystalline and coarse.

The hypabyssal group includes the rocks of dykes, sills and small laccoliths etc., which occupies intermediate position in the crust between the plutonic and volcanic rocks. Their texture is usually merocrystalline.

The volcanic rocks on the other hand are formed on the surface <of the earth and due to rapid rate of cooling their texture becomes holohyaline and fine grained.

Recent classification:

It takes into account, factors like minera­logical composition, chemical and textural characteristics etc. and is (represented in. a tabular form. The important features are as follows:

(i) Presence or absence of essential feldspar-classes 1st and 2nd.

(ii) Class-I:

Group-A-Acid Igneous Rocks. Group-B-Intermediate and Basic Rocks. Group-C-Alkaline Igneous Rocks. G roup – D-Ultrabasic and Ultramafics.

(iii) In Group-‘B’, there are two sub-groups, one is characteres by essential alkali feldspar and the other by essential plagioclase.

In Group-‘D’, there are two sub-groups on the basil of presence or absence of felspathoid.

(iv) Class-II:

There are two sub-groups on the basis of presence or absence of felspathoids.

Thus the igneous rocks have been classified. A list of the various important igneous rocks and their plutonic, hypabyssal and volcanic equivalents, are as follows:

(i) Granite:

Alkalifelspar predominant-Rhyolite.

(ii) Adamellite:

Plagioclase: Alkalifelspar-Rhyodacite.

(iii) Granodiorite:

Plagioclase predominant-Dacite.

(iv) Syenite:

Alkalifelspar dominant-Trachyte.

(v) Monzonite:

Alkalifelspar: Plagioclase-Trachyandesite.

(vi) Diorite:

Plagioclase dominant-Andesite.

(vii) Gabbro:

Plagioclase of labradorite tyjje-Doterite (Hypa­byssal)-Basalt (Volcanic).

(viii) Nepheline syenite:

Felspar+Felspathoid-Phonolite.

(ix) Anorthosite:

Only labradorite type of felspar-Limburgite.

(x) Theralite:

Ultramafic with felspathoid-Tephrite.

(xi) Ijolite:

Ultramafic with felspathod-felspathoid base.