The Common Rock Forming Minerals

The most Common rock forming minerasl are listed below.

1- Silicates

  • The felspars

These are complex silicates of aluminium with potash, soda or lime and are most abundant in igneous rocks. Potash felspar, also known as orthoclase, is the most common type. It is white, grey or pink in colour with a glassy lustre. When weathered it leaves a hydrated silicate of aluminium known as kaolin or china clay. the soda and lime felspars are known as plagioclase. they also decompose and disintegrate in a similar way. [lagioclase felspars occur in most igneous rocks, especially the darker varieties rich in lime. the specific gravity of felspars is between 2.6 - 2.7 and its hardness on the scale of Mohs is 6.

  • Mica

this occurs as a white variety known as muscovite (a silicate of aluminium and potassium) and as a black variety known as black mica or biotite (a silicate of aluminium, iron and magnesium). the mica can be easily scratched by fingernail and has a well-developed cleavage. it is a common constituent of igneous rocks and crystalline schists. its specific gravity is = 2.7 to 3.1 and its hardness is = 2.5.

  • Hornblend and augite 

these minerals are complex silicates of calcium, magnesium and iron. they are both greenish-black in colour. Hornblende is also called amphibole. It is a rather dull, black mineral, forms six-sided crystals, is found in most igneous rocks and has a specific gravity of 3 to 3.5, and its hardness = 5 to 6. Augite (also called pyroxene) is black but more brilliant than hornblende, forms monoclinic crystals and is found in most of the basic igneous rocks. It often alters to chlorite. its specific gravity is = 3, 3.5 and its hardness is = 5 to 6.

  • Olivine (also called peridot)

it is a silicate of magnesium and iron found in basic igneous rocks such as dolerite, basalt and peridot. it is greenish and looks like quartz and its specific gravity is =3.2 to 4 and hardness is 6 to 7
the above minerals which contain iron and magnesium are also known as  ferromagnesian minerals and are generally found in abundance in the more basic igneous rocks (basalt, dolerite, etc.)

2- Oxides 

  • quartz (silica, SiO2)

it is an important constituent of the granite and other acid igneous rocks, and a chief constituent of sandstone rare backers in the broken form. Its specific gravity is =2.6 and its hardness is 7.

  • Magnetite (magnetic oxide of iron, Fe3O4) 

It is bluish-black in colour. earthy varieties are known as red ochre. the streak is always cherry red and its specific gravity is = 4.5 to 5.3 and its hardness is = 6.

  • limonite (hydrated ferric oxide, 2Fe2O3, 3H2O)

it is amorphous, brown or nearly black in colour and the streak is yellowish-brown its specific gravity is 3.6 - 4  and its hardness is 5.

  • Bauxite:

it is essentially a hydrated aluminium oxide (Al2O3, H2O), dry white in colour. it results from the decay and weathering of aluminium rocks, often igneous, under tropical conditions. its specific gravity is 3.5 and its hardness is 2.5.

3- carbonates 

the common carbonates are of calcium and iron. all carbonates effervesce when treated with dilute hydrochloric acid.

  • calcite (CaCO3)

it is an essential constituent of marble, chalk or limestone; it is with dilute hydrochloric acid. 

  • Siderite (FeCO3)

The colour is brown in various shades and the streak is white. its specific gravity is 3.7 to 3.9 and its hardness is 4

  • Dolomite

This is a carbonate of calcium and magnesium, Ca, Mg (CO3)2. Calcite and dolomite constitute the larger bulk of the limestones but are sometimes found as secondary minerals in the igneous rocks. (its specific gravity is = 3  and its hardness is = 4

4- Sulphides

The common sulphides are the sulphides of iron, lead, zinc and copper.

  • Iron pyrite (FeS2) 

Brass yellow in colour, this mineral is sometimes found interspered in a coal seam. e.g. in the pench Valley Coalfield. The streac is greenished or brounish black. Coal containing iron Pyrite is observed to be more liable to spontaneous heating than other coal. its specific gravity is = 4.1 and its hardness is =6 to 6.5.

  • Galena (Pbs)

it is grey in colour and has grey streak. its specific gravity is 7.2 to 7.7 and its hardness is = 2.5.

  • Sphalerite, Blende ZnS

It is also known as black jack. It is usually brown or black the streak being reddish brown, its specific gravity is 3.9 to 4.2 and its hardness is 3.5 to 4

  • Chalcopirite, copper pyrite (Cu2s, Fe2S3)

It is golden yellow in colour; its Streak is greenish black. its specific gravity is 4.1 to 4.3 and its hardness is 3.4 to 4

5- Sulphates 

  • Gypsum (Hydrated Calsium Sulphate, CaSO4, 2H20)

it forms colourless crystals, but some varieties may be white or grey. Plaster of pairs is made from gypsum by heating it or wxpel some of its water of crystallisation and the grinding it to a fine powder its specific gravity is 2.3 and its hardness =2.

6- Chlorides

  • Rock salt (known as common salt, NaCl) 

it is colourless or white when pure. its specific gravity is 2.2 and its hardness is 2.


Othe Common rock forming mineral are:

7- Kaoline (Also called china clay, Al2O3, 2SiO2, 2H2O)

It may be described as hydrated silicate of aluminium. it is a soft, white, clayey Material crumbling to powder when pressed between fingers. Felspars when decomposed yield china clay. 

8- fireclay

Any clay or shale that does not fuse below 1580 ℃ is called fireclay. the predominant constituent of these clays is the mineral Kaolinite with the formula Al2O3, 2SiO2, 2H2O. vesides aluminium and silica the fireclay also contained oxides or iron, calcium, magnesium, sodium and potassium.

9- Fluorspar (CaF2) :

It may be colourless, purple, green or yellow; lustre is vitreous but often transparent. It occurs in veins of metalliferous ores with galena (Sp. gr. = 3; H = 4).

10- Baryte (BaS04) ;

It may be white, yellow, red or blue with a vitreous lustre and is commonly found in mineral veins associated with the ores of lead and zinc. (Sp. gr. = 4.5 ; H = 3).

11- Uranium oxides :

Uranium is the metal used for production of nuclear energy. Two uranium isotopes U-235 and U-233 are fissionable materials. Uranium ore contains 0.03 to 0.1% of uranium. The mined ore is processed to give a marketable concentrate containing 75% U 3O8.

12- Rare Earth Metals :

This is a group of metals which includes metals like cerium (Ce), lanthanum, erbium, yittrium and others. Thorium is closely associated in nature with this group. Thorium is a metal related to titanium and has a sp. gr. of 11. Monazite is a phosphate of cerium metals, but is industrially important as a source of thorium compounds as it contains a small percentage of thorium oxide or thorium silicate. Monazite : (Sp. gr. = 5.27; H = 5.5).


The Common Rock Forming Minerals
The Common Rock Forming Minerals

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