There are many rock-forming materials. Some common rock-forming minerals along with their properties are given below:-
Silica
It is a compound of silicon and oxygen (SiO2). It contains no water. It occurs as silicate in chemical combination with other minerals. Quartz is pure silica. It is colourless when pure but may be white, grey, or pink if certain impurities are present. Its hardness by Mohr’s scale and its specific gravity is about 2.65.
Hornblende
It contains complex silicates of calcium, sodium magnesium, iron and aluminium. It may be black, green, white or dark brown in colour. Its hardness by Mohr’s scale is 5-6. Its specific gravity varies between 2.9 to 3.4. It is a common crystalline mineral and of great importance as a constituent of rocks.
Augite
It resembles hornblende except that it is heavier than hornblende. It forms octagonal crystals. It changes to chlorite by hydration.
Felspar
It is a group of minerals and all members of the group are aluminium silicates and have varying amounts of other elements such as potassium, sodium and calcium. It may be white or pink in colour. Its specific gravity varies between 2.6 to 2.8. Its hardness by Mohr’s scale is 7. It is crystalline in structure. These are the most common of all minerals and are used for pottery, glass, tiles, vitreous enamels. It is also used as an abrasive agent in soaps and other cleaning compounds.
Chlorite
It has green colour. It is mainly derived from the decomposition of augite, biotite and hornblende.
Mica
It is a group of minerals which are potassium aluminium silicates and give off water when heated. It has the property of peeling into their sheets. It may be colorless brown, black or yellowish. Its hardness by Mohr’s scale is 2-3. It is crystalline in structure. Thick crystals of mica are called books are they resemble a volume having many pages on leaves. It is a strategic mineral used for heat and electric insulation.
Olivine
It is black, olive green or yellow in colour. It is colourless when available in thin layers. It frequently changes to serpentine.
Plagicolase
This is a general name given to a series of felspars which range from sodium, and aluminium silicates known as albite to calcium aluminium silicate known as anortile. The main characteristic is multiple twinning. It is an oblique-splitting mineral.
Calcite
It is calcium carbonate. It may be white or colourless. Its specific gravity is 2-7. Its hardness by Mohr’s scale is 3.5. It is crystalline in structure. It fizzers in acids. It has more than 300 forms but it always breaks into fragments of one shape, the rhomb. It is used as a flux in melting certain ores and in glass making.
Quartz
It is the crystal form of silica. It is usually colourless. But it may be brown pink or yellow in colour due to the process of metallic oxides in small quantities. It is weatherproof and can resist the attack of acids except of hydrofluoric acid. Its hardness by Mohr’s scale is 7. Common sand is a variety of quartz.
Serpentine
This mineral resembles chlorite. It has pale green or yellow colour. It gives a massive appearance. These rock minerals are mostly found in igneous rocks.
Dolomine
It contains calcium and magnesium carbonates, (CaMgCO2). It may be white, grey, pink or brown. It hardness by Mohr’s scale is 4. Its specific gravity is between 2.8 to 2.9. It has massive granular structure. Being a carbonate, it fizzes in acids but less vigorously than calcite. Beds of dolomite may be used as building stones. It is also used for making refractories, cement and lime.
Magnesite
It is MgCO3. It is harder and less soluble than calcite. It is rarely found in nature.
Glanconite
It has no definite crystal form. Its colour varies from yellow to green. It is insoluble in acids as well as ordinary water.
Limonite
It has an earthy appearance. Its colour varies from yellow to reddish brown. It is insoluble in ordinary water but dissolves in acids.
Gypsum
It is CaSO4, 2H2O. It is white in pure state. It is tinted grey, red or yellow due to impurities. It hardness by Mohr’s scale is 2. It is a crystalline substance. Its solubility in water is very poor.
Anhydrate
It is CaSO4. Its hardness by Mohr’s scale varies from 3-3.5. 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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