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Rock Forming Minerals

Er. Parbhakar Dwivedi


The igneous rocks contain many minerals. The various natural minerals having range of different distinctive characteristics are available. But only few of them form the bulk of rocks. These minerals are called the rock-forming minerals.

(1) Minerals in the igneous rocks: Following are the commonly found mineral in the igneous rocks:

(i) Augite: This mineral resembles hornblende except that it is heavier than hornblende. It forms octagonal crystals. It changes to chlorite by hydration. Its density is 23 to 36 kN/m3 and hardness is 5 to 6. It is also called Pyroxene

(ii) Chlorite: It has green colour. It is mainly derived from the decomposition of augite, biotite and hornblende.

(iii) Felspar: It has many varieties, the common being that of orthoclase microcline and one or other Member of the plagioclase series. The orthoclase is whitish, greyish or pinkish in colour. It is a straight-splitting mineral it shows development of tabular crystals. It has usually well-defined faces the presence of decomposed rocks sometimes impart cloudy appearance to this mineral. The rocks disintegrate easily, if orthoclase is in abundance in their structure. The micro line has deep green or flesh-red colour. Is found along with orthoclase.

(iv) Hornblende: This mineral is heavy, strong and durable, but brittle. Its colour is dark-green or black.

It has glassy luster. It frequently occurs as six-sided crystals having the appearance of elongated hexagons. It Chan to chlorite when exposed to weather. Its density is 29 to 30 kN/m3 and hardness is 5 to 6. It is an essential component of many igneous rocks

(v) Mica: It is silicate of aluminium with potassium and occurs in thin transparent lamina or plates. The layers of mica shine with metallic luster. The hardness varies from 2 to 3. The two forms of mica commonly occurring in igneous rocks are muscovite and biotitic. They occur separately or simultaneously the muscovite is also known as the white mica or potash mica. It has light colour and it may be colorless when available in thin layers. Its density varies from 27.60 to 31 kN/m3.

The biotite is also known as the black mica. It has dark colour and metallic lustre. It iron content and hence, when exposed to weather, it changes to chlorite and loses its elasticity. Its density varies from 28 to 32 kN/m3.

(vi) Olivine: It has black, olive-green or yellow colour. It is colorless when found in thin sections. It frequently changes to the serpentine.

(vii) Plagioclase: This is a general name given to a series of feldspars which ranges from sodium aluminium silicate known as the albite to calcium aluminium silicate known as the as the annotate. The Characteristic of Plagioclase is multiple twinning. It is an oblique-splitting mineral.

(viii) Quartz: It is the crystalline form of silica. It is usually colorless. But it may be sometimes brownish, Pink or yellow in colour due to presence of metallic oxides in small quantities. It is weather proof andresists the attack of acids with the exception of hydrofluoric acid. Its density is 26.50 kN/m3 and its hardness is 7. It may be noted that common sand is a variety of quartz. The transparent crystal of quartz is known as the rock crystal. The compact translucent mass of quartz is called the hornstone.

(ix) Serpentine: This mineral resembles chlorite. It has pale-green or yellow colour. It presents a massive appearance.

(2) Minerals in the sedimentary rocks: The sedimentary rocks also contain many minerals. Following are the commonly found minerals in the sedimentary rocks:

(i) Calcite: This mineral rarely occurs in igneous rocks. But it is the chief constituent of many sedimentary rocks. The calcite is calcium carbonate CaCOs and it gives out carbon dioxide when attacked by mineral acids it is generally colorless. But the presence of impurities may give it yellow, brown or red colour. It is available in various shades. It is poorly soluble in water and it reacts vigorously with acids. Its density is 27 kN/m3 and its hardness is 3. It deteriorates in water containing carbon dioxide CO2 since calcium bicarbonate CA (HCO3)2 is formed and it is about 100 times more soluble in water than calcite.

(ii) Magnesite: This mineral occurs rarely in nature and its chemical composition is MgCOs. It is harder and less soluble than calcite.

(iii) Dolomite: It is available in various shades. Its crystals are brittle. It is however stronger and heavier than calcite. It is insoluble in ordinary water. In chemical composition, it is a bicarbonate of magnesium and calcium, MgCO3•CaCOs. It consists of 46% of MgC03 and 54% of CaCO3.

iv) Glauconite: This mineral has no definite crystal form. Its colour varies from yellow to green. It is practically insoluble in acids as well as ordinary water.

(v) Limonite: It has an earthy appearance. Its colour varies from yellow to reddish brown. It is insoluble in ordinary water, but it dissolves in acids.

(vi) Gypsum: It is the hydrated sulphate of calcium with chemical composition as CaSO4 2H2O. It is white when in pure state. Due to presence of impurities, it is tinted into grey, reddish, yellowish or black colours. Its density is 23 kN/m3 and its hardness is 2. It is a crystalline substance. Its solubility in water is very poor.

(vii) Anhydrate: This is an anhydrous variety of gypsum and its chemical composition is CaSO4. Its colour varies from reddish-white grey. Its density varies from 28 to 30 kN/m3 and its hardness varies from 3.00 to 3.50. If it remains in contact with water for a long time, it is converted into gypsum with a slight increase in volume.


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