CLASSIFICATION OF IGNEOUS ROCKS

 Chemical classification

a chemical classification based on composition as ascertained by chemical analysis.

In 1903 four American petrologists, Cross, Iddings, Pirsson, and Washington, (CIPW) advanced a classification which is in reality based on chemical composition.  Analyses are first calculated into a set of standard minerals (the norm) under fixed rules which are based on the known laws of mineral formation in magmas. Certain important rock-forming minerals, the amphiboles, pyroxenes, and micas, are not utilised in the norm, because of their complex chemical composition. Their components are distributed between the norm minerals, which are based on magmatic molecules of fixed composition. The norm is divided into a salic a and a Femic group, the most important constituents of which are the following :-

Salic Minerals.                         Femic Minerals.
Quartz                                                Diopside
Orthoclase                                          Hypersthene
Albite                                                 Olivine
Anorthite                                            Acmite (Aegirine)
Leucite                                               Magnetite
Nepheline                                           Ilmenite
Corundum                                          Haematite
Zircon                                                 Apatite

The conception of the norm is found to be of the greatest value in the comparison of igneous rocks.

 

Another quasi-chemical mode of classifying igneous rocks is by the use of the saturation principle, as expounded by Shand and Holmes.  

The minerals which are capable of existing in igneous rocks in the presence of free silica are said to be saturated, they are minerals of high silication.

On the other hand, certain minerals of low silication, especially olivine and the felspathoids minerals (including analcite), are very rarely found in association with quartz, and may be termed unsaturated. These facts may form the basis of a classification as follows: -

I. Oversaturated rocks, containing free silica of magmatic origin.
II. Saturated rocks, containing only saturated minerals.
III. Undersaturated rocks, containing unsaturated minerals.

Or

Shands and Holmes classified the igneous rocks as

(i) Supersaturated rocks. Also known as oversaturated rocks. Here, the excess of silica crystallizes 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.

 

Mineralogical Classification

In order to classify igneous rocks for research or teaching purpose, it is convenient to consider mineralogical differences between different rocks. It is easier than chemical classification because minerals can be easily identified with naked eyes or microscope and their relative amount can be conveniently estimated with a fair amount of accuracy. This classification considers minerals as either essential, accessory or secondary. The first two are of igneous origin while the secondary minerals could be either of metamorphic or sedimentary origin. The essential minerals are abundant and therefore, are necessary to the diagnosis of the rock type.

Accessory minerals on the other hand, are those in small amounts, and whose presence is not mandatory for the classification of that rock. For example, apatite and iron oxides in igneous rocks. Igneous minerals are also Subdivided into felsic and mafic types. The term felsic is derived from the word’s feldspar, feldspathoid and silica. While 'mafic' has been coined from ferro-magnesian (Fe-Mg) minerals. Few of these minerals are as follows:

Felsic – Quartz, Feldspar, Feldspathoids

Mafic – Mics, Pyroxenes, Amphiboles, Olivine, Iron Oxides

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.

 

Classification on the basis of colour Index

The colour of a rock has been quantified by a value known as the colour index, which is simply the volume percentage of dark minerals.

	Rock Division	Colour Index	Example
1	Leucocratic (Light colour mineral)	1-30	Granite
2	Mesocratic (Grey colour minerals)	31-60	Gabbro
3	Melanocratic (Dark colour minerals)	61-100	Peridotite

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.

 

TABULAR CLASSIFICATION-Taking into consideration all the principles of classification i.e. mineral composition, silica saturation or contains as well as depth of formation. Tabular classification of rock is evolved as this classification is set in a Tabular form it is term as tabular classification.

In this classification the vertical divisions are made on the basis of three felsic mineral groups which are Quartz, Feldspar & Feldspathoid. It is well known that quartz do not occur along with feldspathoid and by taking this into consideration five divisions are made which are characterised by mineral association as follows –

I)                Quartz

II)              Quartz + Feldspar

III)           Feldspar

IV)           Feldspar + Feldspathoid

V)             Feldspathoid  

Later on, to these six subdivisions is added which is mafic mineral predominant & felsic minerals are absent.

Division I & II belongs to oversaturated class of Shand’s and Holm’s classification.

Division III belongs to Saturated class

While Division IV, V, VI belongs to undersaturated class of shands & Holm’s classification. 

Horizontally classification is made into plutonic, hypabyssal and volcanic groups. In the plutonic group a further subdivision is made according to whether the rocks consist predominately of felsic minerals or of mafic minerals, or show equal proportion of felsic and mafic minerals and the rocks are placed depending upon the percentage of felsic and mafic minerals.

All though this system of classification is easier but a complicated one, therefore it is not considered as a perfect system. Further it is assuming that tabular classification is simply a method to arrange & describe the common igneous rock in simplest manner.

Mineralogical character of Acidic Igneous rock

In general, the chemical composition of rock is expressed in terms of oxides like SiO₂, Al₂O₃, Fe₂O₃, FeO, MgO, CaO etc. among these different oxides silicon dioxides (SiO2) is always predominant in igneous rocks. As the Percentage of silica is responsible for the formation of different minerals and their association. It is very useful for the classification of Igneous rocks into Acid / acidic, basic, ultrabasic etc. The mineralogical classification of acid igneous rocks are as follows.

·     The silica content is more than 66%

·    These rocks are rich in silica, alumina & Alkali but are poor in calcium, magnesium & Iron

·    Usually they are composed of quartz, Feldspar (Alkali Feldspar) and Muscovite. Mica which represent the late stage of crystallization of magma.

·  Acid igneous rock are Leucocratic i.e. light coloured because they are rich in plae coloured mineral.

·  Mafic minerals occur only as accessory minerals.

·     They have free primary quartz and are always devoid of unsaturated minerals like Olivine, Felspathoids etc,

·       E.g. Granite, Pegmatite, Rhyolite

 

 Mineralogical character of Basic Igneous rock

·       The basic igneous rock has 45 – 55 % silica.

·       In this rock mafic (dark colour) minerals occurs as an essential mineral.

·       Quartz and Olivine are generally absent or one of them may occur in small quantity and feldspar are always plagioclase type.

·       In basic igneous rock silica and alkali are less while Calcium, Magnesium & Iron are more in composition as compared to acidic rocks.

·       The basic igneous rocks are typical in melanocratic i.e. dark colour (e.g. basalt & gabbro)

 

Mineralogical character of Ultrabasic Igneous rock.

·       In ultrabasic igneous rock free quartz is always absent.

·       Undersaturated & mafic minerals occurs as essential minerals.

·       The ultrabasic igneous rocks are feldspar and quartz poor and mostly composed of olivine, pyroxene & amphibole minerals, biotite, chromite and iron oxide occurs as accessory minerals.

·       They are melanocratic (dark coloured) and are usually formed as a different product of early formed minerals like Olivine, Pyroxene & Calcium rick Plagioclase.

·       E.g. Dunite, Peridotite, Pyroxinite

Mineralogical character of Alkali Igneous rock.

·       This group of igneous rocks is characterised by the presence of higher ratios of alkali to silica.

·       This rock contains a large range of mineral species.

·       These rocks are characterised by the presence of Alkali feldspar. Feldspathoid & Na or K rick Ferromagnesium minerals.

·       They are rich in Orthoclase or sanidine or sodic plagioclase and few ferromagnesium minerals like pyroxene, Amphibole, or biotite.

·       E.g. Alkali basalt, Trachyte, Syenite

Characters of some common igneous rocks and their Indian occurrences

1.  Granite - In the Eastern-Ghats, Rajasthan, in the Himalayan ranges, these rocks are in abundance. They mostly contain, orthoclase, quartz, muscovite and small amounts of hornblende.

 

2. Basalt - As deccan traps, occur in abundance in Gujarat, Madhya Pradesh, Maharashtra, A.P. etc. Their mineralogic contents are usually plagioclase, augite sometimes a little olivine.

 

3.  Pegmatites - In Bihar, Andhra Pradesh and Rajasthan. Felspar, (Quartz and mica are the usual minerals.)

 

4.  Lamprophyres - These are dyke-rocks and are believed to have arisen from the same general body of magma. Mostly occur in the coal-fields where they are more prominent. In case of mica lamprophyres— biotite is the dominant mineral. In Hornblende Lamprophyres, hornblende is dominant. In Augite lamprophyres augite is dominant. But their occurrence is rare.

 

5.  Anorthosite - In Eastern Ghats, they are found, in Bankura of West Bengal, Sitapundi Complex of Tamilnadu, Banpur, Angul of Orissa. Dominantly composed of labradorite feldspar.