TEXTURES OF IGNEOUS ROCKS
Ø The term texture is defined as the mutual relationship of different mineralogical constituents in a rock. It is determined by the size, shape and arrangement of these constituents within the body of the rock.
Factors Explaining Texture
The following three factors will primarily define the type of texture in a given igneous
Degree of Crystallization
Ø 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.
Ø The resulting rock textures are then described as:
(i) Holocrystalline: When all the constituent minerals are distinctly crystallized;
(ii) Holohyaline: When all the constituents are very fine in size and glassy or non crystalline in nature.
Ø 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.
Ø 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.
Ø The grain size of the various components of a rock 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 ail 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
Ø 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.
Ø 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.
Ø 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.
Ø An igneous rock may contain crystals of anyone type in a predominating proportion;
hence its fabric will be defined by one of the following three terms related to fabric:
(i) Panidiomrphi: 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
These can be broadly divided into five categories:
. Equigranular textures
. Inequigranular textures
. Directive textures
. Intergrowth textures and
. Intergranular textures.
(1) Equigranular Textures
Ø All those textures in which majority of constituent crystals of a rock are broadly equal in size are described as equigranular textures.
Ø In igneous rocks, these textures are shown by granites and felsites and hence are also often named as granitic and felsitic textures
Ø In the granitic texture, the constituents are either all coarse grained or all
medium grained and the crystals show euhedral to subhedral outlines.
Ø In the felsitic texture, the rock is micro granular, the grains being mostly microscopic crystals but these invariably show perfect outlines.
Ø Thus felsitic textures may be described as equigranular and panidiomrphic.
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 micregranular.
(2) Inequigranular Texture
Ø Igneous textures in which the majority of constituent minerals show marked difference in their relative grain size are grouped as inequigranular texture.
Ø Porphyritic and Poiklitic textures are important examples of such textures.
Ø 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.
Ø The texture is sometimes further distinguished into mega-porphyritic and microporphyritic depending upon the size of the phenocrysts.
Difference in. molecular concentration
Ø When the magma is rich in molecules of 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.
Change in physico-chemical conditions.
Ø Every magma is surrounded by a set of physico-chemical conditions like temperature, pressure and chemical composition, which influence the trend of crystallisation greatly.
Ø Abrupt and discontinuous changes in these textures may result in the formation of the crystals of unequal dimensions.
Ø Thus, magma crystallizing at great depths may produce well-defined, large sized crystals.
Ø When the same magma (carrying with it these large crystals) moves upward, the pressure and temperature acting on it are greatly reduced.
Ø Crystallisation in the upper levels of magma becomes very rapid resulting in a fine-grained matrix that contains the big sized crystals formed earlier.
Ø 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
Ø And the soluble constituents make up the ground mass crystallizing towards the end.
(3) Directive Textures
The textures that indicate the result of flow of magma during the formation of rocks
are known as directive textures.
Ø These exhibit perfect or semi perfect parallelism of crystals or crystallites in the direction of the flow of magma.
Ø Trachytic and Trachytoid textures are common examples.
Ø 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.
(4) Intergrowth Textures
Ø 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.
Ø This type of mutual arrangement is expressed by the term intergrowth texture.
Ø Graphic and granophyric textures are examples of the intergrowth textures.
Ø In graphic texture, the intergrowth is most conspicuous and regular between quartz and felspar crystals. In granophyric textures the intergrowth is rather irregular.
(5) Intergranular Textures
Ø In certain igneous rocks crystals formed at earlier stages may get so arranged that polygonal or trigonal spaces are left in between them.
Ø 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.