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Here is a list of eleven most important igneous rocks: 1. Granites 2. Diorite 3. Andesite 4. Syenites 5. Gabbro 6. Dolerites 7. Basalts 8. Pegmatites 9. Aplites 10. Lamprophyres 11. Peridotites.
1. Granites:
Granites may be defined as plutonic light coloured igneous rocks. These are among the most common igneous rocks. The word granite is derived from Latin word granum meaning a grain and obviously refers to the equigranular texture of the rock.
Composition:
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Two most common and essential mineral constituents of granite are- Quartz and Felspar. Quartz is always recognized by its glassy lustre, high hardness (H = 7), and cleavageless transparent white appearance. Felspars making granites may be of two varieties- the potash felspars, commonly orthoclase and the soda-bearing felspars like albite and oligoclase. Felspar microcline may also be present in some granites.
Among the accessory minerals in granites, micas deserve first mention. Both varieties (muscovite or white mica and biotite or black mica) are present in small proportions in most granites. Amphiboles like hornblende and pyroxene like augite and hypersthene are also often present as accessory minerals. Among other accessory minerals found in granites may be mentioned magnetite (oxide of iron), apatite, garnet and tourmaline.
Texture:
Granites are generally coarse to medium grained, holocrystalline (phaneric) and equigranular rocks. Granitic, graphic, porphyritic and intergrowth textures are the most common types of textures met with in granites of different varieties. As regards structures, granites occur in large massive bodies, often as batholiths, stocks and bosses beside in usual intrusive bodies like sills and dykes.
Types:
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Many types of granites are distinguished on the basis of relative abundance in them of some particular accessory mineral. For instance, when white mica, muscovite is present as a prominent accessory mineral, the granite may be distinguished as muscovite-granite. Similarly, when it is the black mica or biotite, which is a prominent accessory mineral, the granite may be called a biotite-granite.
When both the biotite and muscovite are present in almost equal amounts as accessory minerals, the type may be named as muscovite-biotite granite. Other common types of granites distinguished on the same pattern are hornblende-granite, augite-granite, tourmaline- granite, and so on. Sometime granite may also be named on the basis of its texture such as graphic granite, porphyritic granite and so on.
Occurrence:
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Granites are the most widely distributed igneous rocks in the crust of the earth. They occur chiefly as deep-seated intrusive bodies like sills, bosses, stocks and batholiths. Their occurrence on the surface of the earth is attributed to prolonged weathering and erosion of the overlying strata through historical times running over millions of years.
Megasacopic Identification:
Granites may be identified in hand specimens by their:
(i) Light-coloured (leucocratic) appearance, such as grey, pink, brownish and yellowish. Some of the shades may take brilliant polish to make it eminently suitable as a decorative building stone.
(ii) Coarse to medium-grained texture; fine-grained granites are rare specimens.
(iii) Abundance of quartz and felspar orthoclase as essential minerals.
Use:
Granites find extensive use in architectural and massive construction where they are found in abundance. These rocks have been used extensively in monuments and memorials, as columns and steps and as flooring in buildings.
Origin:
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The origin of granites is a controversial problem in petrology. Many minor granitic bodies occurring as sills and similar masses are clearly of igneous plutonic origin. Their formation from parent magma through the normal process of cooling and crystallisation is easily accepted.
But exceptionally large bodies like batholiths and stocks and bosses running over hundreds of square kilometers close to or on the surface are not accepted by many as simple igneous intrusions mainly because of their extensive dimensions. These large granitic masses are believed by many to have been formed from preexisting rocks through the process of granitization, which is in-situ conversion of those rocks into masses of granites without involving intrusion phase.
Variations:
As we pass from granites to diorites through granodiorites , following variations appear in the composition of these rocks:
(i) The relative proportion of quartz (SiO2) falls gradually so that in diorites it is reduced to a subordinate amount;
(ii) Felspar orthoclase, which is a dominant mineral in granites, is reduced in relative amount and replaced by felspar plagioclase in granodiorites. In diorites, it is felspar plagioclase that makes the bulk of felspar constituent. A number of rock types get distinguished on the basis of this variation. For example, adamellite is a variety of granodiorites that contains felspar orthoclase and plagioclase in equal proportion.
2. Diorite:
It is an intermediate type of igneous rock of plutonic origin with silica percentage generally lying between 52-66 per cent.
Composition:
Diorites are typically rich in felspar plagioclase of sodic group (e.g. Albite). Besides plagioclase and alkali felspars, diorites also contain accessory minerals like hornblende, biotite and some pyroxenes. Quartz is not common but may be present in some varieties that are then specially named as quartz-diorites.
Texture:
In texture, diorites show quite close resemblance to granites and other plutonic rocks. They are coarse to medium grained and holocrystalline.
Occurrence:
Diorites commonly occur as small intrusive bodies like dikes, sills, stocks and other irregular intrusive masses. They also get formed at the margins of bigger igneous masses.
Hypabyssal and Volcanic Equivalents:
Rocks named as aplites and granophyres are the hypabyssal equivalents and those named as rhyolites are the volcanic equivalents of granites. There is not much difference in the chemical and mineralogical composition of granites and their hypabyssal and volcanic equivalents; the main difference lies in textural and structural forms necessitated due to mode of formation.
Thus, while granite is coarse to medium-grained in texture, its hypabyssal equivalent rhyolite is invariably fine-grained in texture. Volcanic equivalent of diorite is termed andesite that is also quite abundant in occurrence.
3. Andesite:
These are volcanic rocks in which plagioclase felspars (sodic and sub-calcic varieties like albite, andesine and labradorite) are the predominant constituents making the potash felspar only a subordinate member.
Composition:
Besides plagioclase and potash felspars, andesites may contain small amount of quartz as well as biotite, hornblende, augite, olivine and hypersthene from the dark minerals giving them an overall grayish or darker appearance.
Occurrence:
Andesites are known to be quite abundant volcanic rocks, next only to basalts and may occur as crystallized lava flows of extensive dimensions. Petrologists are sharply divided over the origin of andesites.
Some believe them to be the products of normal crystallisation from a mafic magma whereas others think that some andesites may be the products from mixed magmas or magmas enriched with fragments from the wall rocks. The second view is supported by the presence of some foreign materials in andesites.
4. Syenites:
Syenites are defined as igneous, plutonic, even-grained rocks in which alkali-felspars (including orthoclase and albite) are the chief constituent minerals. They may contain, besides these essential constituents, dark minerals like biotite, hornblende, augite and some accessories.
Composition:
As said above, syenites chiefly contain felspars of which many types may occur simultaneously in the same rock or in different varieties. The most common felspars of syenites are orthoclase and albite; microcline, oligoclase and anorthite are also present in them in subordinate amounts. In some syenites, the felspathoids (nepheline, leucite) also make appearance.
Common accessory minerals occurring in syenites are apatite, zircon, and sphene. Quartz so common in granites is altogether absent or is only a minor accessory in syenites.
Texture:
Syenites show textures broadly similar to those of granites, that is, they are coarse to medium-grained, holocrystalline in nature and exhibiting graphic, inter-grown or porphyritic relationship among its constituents.
Types:
A good number of types of syenites have been recognized on the basis of presence of particular accessory minerals.
Few well-known types of syenites are as follows:
(i) Nordmarkite:
A syenite that contains some amount of quartz in them.
(ii) Monzonite:
Monzonite, in which plagioclase felspars become almost equal to the potash felspars as essential minerals.
(iii) Larvikite:
It is also sometimes known as blue-granite; it is, however, actually a syenite that contains felspar labradorite as a predominant constituent.
(iv) Nepheline (or Alkali) Syenites:
These are a group of syenite rocks in which nepheline (a typical felspathoid) becomes an important constituent. Quartz is typically absent in nepheline syenites. Theralite is a special type of nepheline syenite containing felspar labradorite. The syenite is known as teschenite if instead of nepheline, analcite is the felspathic mineral. Theralites and teschenites are, of course, rare in occurrence.
The Alkali Syenites have attracted special attention of petrologists because of their extreme variation in mode of occurrence and mineralogical composition. Divergent views have been expressed regarding their origin. A common association of nepheline syenites with normal syenites and limestones in various occurrences has led many to suggest that these rocks have been formed from silicic magma that has been desilicified because of reactions with the associated limestone.
Volcanic Equivalents:
Trachyte is the name given to the volcanic equivalent of syenite. This rock is fine grained and shows trachytic texture. It contains alkali felspars as chief constituents. Phonolite is volcanic equivalent of nepheline syenite.
5. Gabbro:
These are coarse-grained plutonic rocks of basic character. Plagioclase felspars of lime-soda composition (e.g. labradorite and anorthite) are the chief constituents of gabbros. Besides these, the mafic minerals like augite, hornblende, olivine, biotite and iron oxides are also common as accessory minerals.
Texture:
Gabbro shows variable texture; generally coarse to medium grained; reaction rim structure is seen in some gabbros.
Types:
Following are some of common types of gabbroic rocks:
(i) Norite:
Contains orthorhombic pyroxenes like hypersthene and enstatite in addition to labradorite.
(ii) Gabbro (Type Rock):
It contains monoclinic pyroxenes (e.g. augite) as the dominant mafic mineral besides the typical felspars.
(iii) Anorthosite:
It is a typical monomineralic rock (made up only of one mineral) containing generally felspar labradorite.
(iv) Eucrite:
It is a gabbroic rock in which felspar bytwonite or anorthite dominates. Pyroxenes are also abundant in them.
(v) Essexite:
Essexite is characterized by the presence of some nepheline in addition to felspars and olivine. Pyroxenes are absent.
(vi) Troctolite:
Troctolite is that gabbroic rock which contains mainly felspars and olivine. Pyroxenes are absent.
(vii) Dunite:
This gabbroic rock is characterized with the typical absence of felspars and dominance of olivine and pyroxenes.
Hypabyssal and Volcanic Equivalents; Dolerites and Basalts are typical hypabyssal and volcanic equivalents respectively of gabbro type rocks.
6. Dolerites:
These are igneous rocks of typically hypabyssal origin having formed as shallow sills and dykes. They may be regarded as equivalents of gabbros of plutonic origin and basalts of volcanic origin. The term diabase is also used for dolerite.
Composition:
Dolerites are predominantly made up of calcic plagioclase (e.g. anorthite and labradorite). Dark minerals like augite, olivine and iron oxide etc. are also present in good proportion in dolerites along with the plagioclase minerals.
Texture:
Dolerites are mostly medium to fine grained rocks. Ophitic and porphyritic textures are quite common in many dolerites.
Occurrence:
Sills and dykes of doleritic composition have been recorded at many places associated with magmatic activity. In the Singhbhum region of south Bihar, India, many doleritic dykes traverse the Singhbhum granites.
7. Basalts:
Basalts are volcanic igneous rocks formed by rapid cooling from lava flows from volcanoes either over the surface or under water on oceanic floors. They are basic in character.
Composition:
Basalts are commonly made up of calcic plagioclase felspars (anorthite and labradorite) and a number of ferro-magnesian minerals like augite, hornblende, hypersthene, olivine, biotite and iron oxides etc. In fact many types of basalts are distinguished on the basis of the type and proportion of ferro-magnesian minerals in them. Thus, for instance, Basanite is an olivine-rich basalt and Tepherite is an olivine-free type basalt. The olivine-free basalts, that are quite abundant in occurrence, are sometimes named collectively as Tholeiites.
Occurrence:
Basaltic rocks form extensive lava flows on the continents and also on the oceanic floors in almost all the regions of the world. In India, the Deccan Traps, which are of basaltic and related rocks, are spread over more than four hundred thousand square kilometers in Maharashtra, Gujarat, Madhya Pradesh and adjoining parts of Indian Peninsula.
8. Pegmatites:
These are exceptionally coarse-grained igneous rocks formed from hydrothermal solutions emanating from magmas that get cooled and crystallized in cavities and cracks around magmatic intrusions. These rocks are searched for their containing big-sized crystals of minerals. Some of these crystals may be gems and other precious minerals.
Composition:
Pegmatites exhibit great variation in their mineral composition. The granite – pegmatites contain alkali felspars (like orthoclase and microcline) and quartz as the dominant minerals. A variety of other minerals like tourmaline, micas (muscovite and biotite, lithium mica), topaz, flluorite, spodumene, beryl, cassiterite, wolframite, columbite and tantalite etc. occur in different pegmatites.
Crystals of some minerals in exceptionally big sizes have been found from pegmatites at many places. A beryl crystal weighing 18 tons and measuring 3 meters in length and 1.3 meters in diameter has been recovered from Maine mines in U.S.A. Similar beryl crystals, weighing up to 20 tons have been recovered from Pegmatites in Rajasthan, India.
Texture and Structure:
Pegmatites do not show any special textures and structures except that they are invariably coarse grained and mostly inequigranular. In many pegmatites, the so- called complex pegmatites, a zonal structure is commonly observed. In such cases, different minerals of pegmatite occur in different zones starting from the periphery and proceeding towards the centre.
In a five-zoned pegmatite, for instance, the outermost zone is made up of muscovite and felspar, the second zone is of quartz and felspar, third zone of microcline and fourth of quartz. The central zone is ploymineralic containing albite and spodumene besides quartz and mica.
Origin:
Petrologically, pegmatites of complex composition are known to occur. As such no single mode of origin can be assigned to them.
At present two modes of origin are broadly- suggested:
First:
Pegmatites have been formed from magmatic melts towards the end of the process of crystallisation. The hydrothermal factions left behind at this stage are capable of taking in solution all metallic and non-metallic components by virtue of their temperature, pressure and chemical reactivity. Most of the granite- and syenite-pegmatites are believed to have been formed through this mode.
Second:
Pegmatites have formed due to replacement reactions between the hydrothermal solutions and the country rock through which these liquids happen to pass. Hydrothermal liquids at elevated temperatures are considered quite effective in replacing original minerals by new minerals.
Occurrence:
Pegmatites occur in a variety of forms as dykes, veins, lenses and patches of irregular masses.
Use:
Pegmatites are the source of many precious stones, gems, ores of rare-earths and heavy metals besides the industry grade muscovite mica.
9. Aplites:
These are igneous rocks of plutonic origin but characterized with a fine-grained, essentially equigranular, allotriomorphic texture.
Essential minerals of the aplites are the same as that of granites, that is, felspars and quartz. They commonly occur as dykes and are formed from magmas that have different gaseous content compared to magmas from which granites are formed.
10. Lamprophyres:
These form a group of igneous rocks that typically occur as dykes and sills.
Their important characters are:
i. Texture:
Panidimorphic (in which most of crystals show perfect outline), fine grained and holocrystalline.
ii. Composition:
Lamprophyres show a great variation in their mineralogical composition. Mostly they are rich in ferro-magnesian silicates. Important minerals forming lamprophyres are- biotite, augite and other pyroxenes, hornblende and other amphiboles, felspars and olivine.
iii. Types:
Many types of lamprophyres are distinguished on the basis of the type of felspar and the dark minerals occurring in them. Thus, Minette is a lamprophyre containing felspar orthoclase and the black mica, biotite; Vogesite is another variety having felspar orthoclase and augite or hornblende.
11. Peridotites:
The term peridotite is commonly used to express the ultra-mafic igneous rocks that are highly rich in a ferro-magnesian mineral OLIVINE, which has a composition of (Mg, Fe) SiO4.
The chief characteristics of peridotites are:
(i) Low silica index; such rocks invariably contain less than 45% silica.
(ii) High colour index; rich as they are in dark minerals, the colour index of peridotites is always above 70, generally in the range of 90-100.
Texture:
Peridotites are generally massive and coarse grained in texture.
Varieties:
A number of types of peridotites are distinguished on the basis of the accessory minerals, e.g. hornblende-peridotite, pyroxene-peridotite etc. Kimberlite is a peridotite in which olivine is altered to serpentine.
Other rocks related to peridotites are Dunite, which is composed wholly of olivine, Pyroxenite, containing wholly of pyroxenes, and Serpentinites and bronzitites etc.
It is important to note that peridotites have no volcanic equivalents.
Occurrence:
Peridotites generally form sills and dykes of moderate size.
Origin:
A number of modes of origin have been suggested for peridotites. Hess believes them to be the products of primary peridotitic magma, a view that is very strongly objected by many others. Another view holds them having been formed from a primary basic (basaltic) magma from which olivine and other mafic minerals were separated by some process. A third possibility suggested regards the development of peridotite bodies simply as a result of hydrothermal (pneumatolytic) transport of their material and its subsequent reaction with rocks of appropriate composition.