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In this article we will discuss about the texture and classification of igneous rocks.
Texture of Igneous Rocks:
Factors Explaining Texture of Igneous Rocks:
The following three factors will primarily define the type of texture in a given igneous rock:
(a) Degree of Crystallization:
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In an igneous rock, all the constituent minerals may be present in distinctly crystallized forms and easily recognized by unaided eye, or, they may be poorly crystallized or be even glassy or non- crystallized form.
The resulting rock textures are then described as:
(i) Holocrystalline:
When all the constituent minerals are distinctly crystallized;
(ii) Holohyaline:
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When all the constituents are very fine in size and glassy or non-crystalline in nature.
The term merocrystalline is commonly used to express the intermediate type, i.e. when some minerals are crystallized and others are of glassy character in the same rock.
Rocks with holocrystalline texture are also termed as phaneric and the holohyaline rocks are referred as aphinitic. The term microcrystalline is used for the textures in which the minerals are perceivably crystallized but in extremely fine grain.
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(b) Granularity:
This defines the grain size of the various components of a rock. These are the average dimensions of different constituent minerals which are taken into account to describe the grain size of the rock as a whole.
Thus the rock texture is described as:
(i) Coarse-Grained:
When the average grain size is above 5 mm; the constituent minerals are then easily identified with naked eye.
(ii) Medium-Grained:
When the average grain size lies between 5 mm and 1 mm. Use of magnifying lens often becomes necessary for identifying all the constituent mineral components.
(iii) Fine-Grained:
When the average grain size is less than 1 mm. In such rocks, identification of the constituent mineral grains is possible only with the help of microscope for which very thin rock sections have to be prepared for microscopic studies.
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(c) Fabric:
This is a composite term expressing the relative grain size of different mineral constituents in a rock as well as the degree of perfection in the form of the crystals of the individual minerals.
The texture is termed as equigranular when all the component minerals are of approximately equal dimensions and as inequigranular when some minerals in the rock are exceptionally larger or smaller than the other.
Similarly, the shape or form of the crystals, which is best seen only in thin sections under microscope, may be described as perfect, semi perfect or totally irregular. The textural terms to describe these shapes are, respectively, euhedral, subhedral and anhedral.
An igneous rock may contain crystals of any one type in a predominating proportion; hence its fabric will be defined by one of the following three terms related to fabric:
(i) Panidiomorphic:
When majority of the components are in fully developed shapes;
(ii) Hypidiomorphic:
The rock contains crystals of all the categories- euhedral, subhedral or anhedral;
(iii) Allotriomorphic:
When most of the crystals are of anhedral or irregular shapes.
Types of Textures:
After having known various factors that define the texture types, it will be easy to understand the important textures as exhibited by the igneous rocks.
These can be broadly divided into five categories:
I. Equigranular textures
II. Inequigranular textures
III. Directive textures
IV. Intergrowth textures and
V. Intergranular textures.
Following is a brief account of these textures of igneous rocks:
I. Equigranular Textures:
All those textures in which majority of constituent crystals of a rock are broadly equal in size are described as equigranular textures. In igneous rocks, these textures are shown by granites and felsites and hence are also often named as granitic and felsitic textures.
In the granitic texture, the constituents are either all coarse grained or all medium grained and the crystals show euhedral to subhedral outlines.
In the felsitic texture, the rock is microgranular, the grains being mostly microscopic crystals but these invariably show perfect outlines. Thus felsitic textures may be described as equigranular and panidiomorphic.
Orthophyric texture is another type of equigranular texture, which is in between the granitic and felsitic textures. The individual grains are fine in size but not microgranular.
II. Inequigranular Texture:
Igneous textures in which the majority of constituent minerals show marked difference in their relative grain size are grouped as inequigranular texture.
Porphyritic and Poiklitic textures are important examples of such textures.
The Porphyritic Texture is characterised by the presence of a few conspicuously large sized crystals (the phenocrysts) which are embedded in a fine-grained ground mass or matrix. The texture is sometimes further distinguished into mega-porphyritic and microporphyritic depending upon the size of the phenocrysts.
Porphyritic texture may be caused by any one or more of the following factors:
(i) Difference in Molecular Concentration:
When the magma is rich in molecules of a particular mineral, the latter has better chance to grow into big crystals which may get embedded in the fine-grained mass resulting from the deficient components.
(ii) Change in Physico-Chemical Conditions:
Every magma is surrounded by a set of physico-chemical conditions like temperature, pressure and chemical composition, which influence the trend of crystallisation greatly. Abrupt and discontinuous changes in these textures may result in the formation of the crystals of unequal dimensions.
Thus, magma crystallizing at great depths may produce well-defined, large sized crystals. When the same magma (carrying with it these large crystals) moves upward, the pressure and temperature acting on it are greatly reduced. Crystallisation in the upper levels of magma becomes very rapid resulting in a fine-grained matrix that contains the big sized crystals formed earlier.
(iii) Relative Insolubility:
Some mineral constituents may be relatively insoluble in the magma than the others. During the process of crystallisation, their crystal grains get enlarged whereas crystals of other soluble constituents get mixed up again with the magma; thus, the relatively insoluble constituents form the phenocrysts and the soluble constituents make up the ground mass crystallizing towards the end.
Poiklitic Texture is characterised with the presence of fine-grained crystals within the body of large sized crystals.
In other words, it is just the reverse of the porphyritic texture. Rocks like syenite, monzonites, peridotites etc. commonly show this texture. When the host mineral is identified as augite and the inclusions are of plagioclase felspars, the poiklitic texture is further distinguished as ophitic.
The development of ophitic texture in rocks is yet incompletely understood. It may be due to operation of factors mentioned above (for porphyritic texture) but in a different manner. In this case, the molecular concentration of host minerals should be greater and these should crystallise under favourable conditions only towards the end of the process so that these are capable of enclosing many other crystals formed at earlier stages in the process.
III. Directive Textures:
Those textures that indicate the result of flow of magma during the formation of rocks are known as directive textures. These exhibit perfect or semi-perfect parallelism of crystals or crystallites in the direction of the flow of magma. Trachytic and Trachytoid textures are common examples. The former is characteristic of certain felspathic lavas and is recognised by a parallel arrangement of felspar crystals; the latter is found in some syenites.
IV. Intergrowth Textures:
During the formation of the igneous rocks, sometimes two or more minerals may crystallize out simultaneously in a limited space so that the resulting crystals are mixed up or intergrown. This type of mutual arrangement is expressed by the term intergrowth texture.
Graphic and granophyric textures are examples of the intergrowth textures. In graphic texture, the intergrowth is most conspicuous and regular between quartz and felspar crystals. In granophyric textures the intergrowth is rather irregular.
V. Intergranular Textures:
In certain igneous rocks crystals formed at earlier stages may get so arranged that polygonal or trigonal spaces are left in between them. These spaces get filled subsequently during the process of rock formation by crystalline or glassy masses of other minerals. The texture so produced is called an intergranular texture. Sometimes the texture is specifically termed intersertal if the material filling the spaces is glassy in nature.
Classification of Igneous Rocks:
The subject of classification of igneous rocks is perhaps one of the most thoroughly discussed problems of petrology and yet without any universally accepted solution. At the same time, a few methods, as mentioned below, have been variously used successfully for classifying these rocks for different purposes.
Among these, classification systems based on following four factors will only be mentioned in outline:
(i) The chemical classification of the rock;
(ii) The mineralogical classification of the rock;
(iii) The textural classification of the rock;
(iv) The tabular classification of the rock.
(i) The Chemical Classification of the Rock:
Many attempts have been made to classify igneous rocks on the basis of their chemical compositions. All such systems are founded on the assumption that the chemical composition of the ultimate igneous rocks is definitely related to the original chemical composition of the magma.
The most commonly followed system of chemical classification is due to four American petrologists Cross, Iddington, Pirsson and Washington, referred in abbreviated form as C.I.P.W classification. In this classification, a standard set of minerals called norm is selected. Chemical composition of a given rock is ultimately expressed in terms of these normative minerals. The relative abundance of different groups of normative minerals is taken as a basis for dividing the rocks into classes, subclasses and orders etc.
The salient features of this classification are summarized as below:
(a) The Norm:
It is a set of standard minerals of calculated chemical composition. The norm minerals are divided into two groups namely, salic and femic minerals.
The salic minerals are- Quartz, felspars orthoclase, albite and anorthite; leucite; nephelite; corundum; zircon and halite etc.
The femic minerals are- Acmite; diopside; hypersthene; olivine; magnetite; chromite; hematite; pyrite; rutile, etc.
(b) Nomenclature:
In the C.I.P.W. classification, the igneous rocks are divided into five classes, which have been named according to salic/femic mineral ratio. The convention is to use the prefixes per and do to the normative group dominating in a given rock signifying domination of high or moderate order respectively. No prefix is used when the two groups are equally represented.
The five classes are:
Subclasses, orders and sub orders are further distinguished on the basis of individual predominance of different normative minerals.
(a) Calculations:
These form the most important part of the classification programme and involve frequent use of tables of molecular weights, percentage weight (of oxides) and molecular values etc.
The ultimate result (of defining a class etc.) is achieved in following steps:
(i) The rock is chemically analyzed and the data obtained is converted and expressed in percentage terms of the respective oxides, (which is done by consulting tables).
(ii) The percentage weight of each of the oxide is divided by its molecular weight to obtain what is called, molecular number of that oxide. For example, in a given rock, silica, SiO2 is 72.67 per cent, its molecular number will be 72.67/60 = 1.21. While calculating the molecular number, some negligible oxides are either neglected or added to some other related oxides.
(iii) The molecular numbers and the various oxides are then arranged in an orderly manner.
(iv) From the molecular numbers normative minerals are defined using set of rules and tables. Further the percentage weights of these minerals as also their relative abundance and broad salic/femic ratio is also established at this stage from which class of the rock is determined. For instance if on the basis of chemical analysis and above process, it is observed that in a given rock, 70 % minerals are of salic group and 30% of femic group, the ratio obtained is 2.33 which places the rock into Dosalic group.
Limitations:
The chemical system of classification, as is obvious, is very elaborate and often less accurate. Rocks of identical chemical composition may be made up entirely or partly of different set of minerals. Moreover, the system does not reflect satisfactorily the cooling history of the rock. Further most of the normative or so-called standard minerals are least important as rock forming minerals. Still the system has its own importance.
(ii) The Mineralogical Classification of the Rock:
The mineralogical composition of an igneous rock is actually an expression of the chemical composition of the parent magma and cooling history of the rock. This can also form, therefore, a sound basis for classifying the igneous rocks.
Moreover, the mineralogical composition of most of the igneous rocks can be determined fairly accurately megascopically or at best by making thin sections and studying them under microscope. Hence, mineralogical classification is regarded as an easy and more accurate method.
It has been observed over wide areas that only a few minerals make bulk of igneous rocks.
These are called rock-forming minerals and are divided into two main classes:
1. The Felsic minerals and
2. The mafic minerals.
1. The Felsic Minerals:
These are generally light in colour and lower in density and include: quartz, felspar group and felspathoid group of minerals;
2. The Mafic Minerals:
These are dark in colour, comparatively heavier in density and contain ferro-magnesian minerals such as amphiboles, pyroxenes, micas, olivine, oxides of iron etc.
The relative abundance of minerals of these groups is made the basis for defining the rock divisions, groups, and series.
It is a convention to broadly assess the sum of the coloured minerals in a rock and express it in percentage terms of total mineralogical composition, which gives the colour index of the mineral.
The three rock divisions recognised on the basis of colour index are given below:
Some authors (e.g. Shand) distinguish a special type of igneous rocks excessively rich in mafic minerals with a colour index above 90 as Hypermelanic.
Shand and others developed a method of classifying igneous rocks into three divisions on the basis of predominant chemical composition as expressed by the relative abundance of the rock forming minerals. The results are expressed in terms of saturation with SILICA.
Three classes are recognized on this basis:
(a) Oversaturated (Acidic) Rocks:
The rocks in which silica percentage is greater than 66 and is indicated by abundance of minerals of primary/pure silica composition as Quartz, SiO2.
(b) Saturated (Basic) Rocks:
These are characterized by silica percentage between 45-66; free silica lies between 0-10 percent. Sometimes the term Intermediate rocks are also used for silica percentage between 52-66 per cent.
(c) Undersaturated (Ultrabasic) Rocks:
These are silica-deficient igneous rocks, the silica percentage being less than 45 per cent. The unsaturated minerals like leucite, nepheline, sodalite, olivine etc. make these rocks.
(iii) The Textural Classification of the Rock:
The property of texture is indicative of the conditions under which rocks have formed. It has also been used for classifying rocks into three divisions.
(a) Phanerites:
These are coarse-grained (average grain size greater than 5mm) in which all the constituent minerals can be identified megascopically, that is, with unaided eyes.
(b) Aphanites:
These igneous rocks are composed chiefly of fine-grained (grain size less than 1 mm) constituents. Microscopic examination of thin sections of these rocks becomes essential to determine their mineralogical composition. Sometimes an aphinitic rock may show a few well-developed coarse crystals present as phenocrysts. It is then distinguished as porphyritic aphanite.
(c) Glasses:
Igneous rocks in which all the minerals are present in practically uncrystallised form or glass due to very rapid cooling are grouped as glasses. These may be defined as rocks of zero grain size. The material of the rock had no time to differentiate into individual grains or crystals. Glasses are sometimes referred as supercooled liquids.
The textural classification is in a general way, a megascopic or field classification. It is difficult to further classify the rocks on the basis of texture.
Igneous rocks are also divided into three divisions on the basis of their mode of formation as Plutonic, Volcanic and Hypabyssal rocks.
The plutonic rocks are formed from magma at greater depths under conditions very favourable to formation of good crystals. Hence these are coarse to medium grained in texture. Granites and syenites are common examples.
The volcanic rocks are formed from lava erupted at the surface under subareal or subaqeous (under water, as at ocean floors). They are generally fine grained in texture. Basalts are best examples.
The hypabyssal rocks are of intermediate type, formed neither at great depths nor on the surface. These are formed at shallower depths, generally less than 2 km below the surface where conditions of crystallisation favour porphyritic texture. Porphyries of various compositions are generally hypabyssal in origin.
(iv) The Tabular Classification of the Rock:
A field geologist and a civil engineer or mining engineer are concerned more with a practical rather than theoretical classification system. For them a broad compromise between the most useful and important systems of classification would be most satisfying.
An attempt has been made in the following table to present a tabular classification with above end in view:
The essential features of the tabular or field classification are as follows:
(a) The igneous rocks are first divided into three main subdivisions based on their mode of formation as indicated by their textural and structural features – plutonic, hypabyssal and volcanic.
(b) Textural divisions are then recognized in all the three subdivisions as follows:
i. Plutonic Rocks – Phanerites [P], Microphanerites [M]
ii. Hypabyssal Rocks – Porphyries [PO]; Microporphyries [M] and Glasses [G]
iii. Volcanic Rocks – Aphanites [A] and Glasses [G]
(c) Each class of the above rocks is then subdivided into rock subclasses according to the relative abundance of following rock forming mineral assemblages:
(i) Quartz and felspars
(ii) Felspars;
(iii) Felspars and felspathoids
(iv) Ferro-magnesian minerals.
(d) Further subdivision of some of these classes is made on the basis of predominance of particular type of a rock-forming mineral.
(e) The relative abundance of felsic and mafic minerals serves as a basis for broadly defining the saturation of the rock with silica.