Minerals and Rocks

The basic source of all minerals and rocks found on the earth surface is the hot magma in the interior of the earth. When magma cools, crystals of minerals appear and a systematic series of minerals are formed in sequence to solidify so as to form rocks. Minerals such as coal, petroleum and natural gas are organic substances found in solid, liquid and gaseous forms respectively.

Table of Contents

1. Minerals and Rocks
2. Physical characteristics of minerals
3. Some major minerals and their characteristics
4. Rocks and their classification
5. Conclusion

What are minerals and rocks?

A mineral, basically, is a naturally occurring substance that can be either organic or inorganic. It has an orderly atomic structure along with definite chemical composition and physical properties. A mineral is made up of two or more elements. Although single element minerals like sulphur, copper, silver, gold, graphite etc. are found.

Rock is a naturally occurring solid mass or aggregate of minerals. It is categorized by the minerals & chemical composition and the way in which it is formed. The science dealing with the study of rocks is called Petrology.

Physical characteristics of minerals

1. External crystal form – determined by the internal arrangement of the molecules – cubes, octahedrons, hexagonal prisms, etc.
2. Cleavage – tendency to break in given directions producing relatively plane surfaces – the result of the internal arrangement of the molecules — may cleave in one or more directions and at any angle to each other.
3. Fracture – an internal molecular arrangement so complex there are no planes of molecules; the crystal will break in an irregular manner, not along planes of cleavage.
4. Lustre – the appearance of material without regard to colour; each mineral has a distinctive lustre like metallic, silky, glossy etc.
5. Colour – some minerals have characteristic colour determined by their molecular structure – malachite, azurite, chalcopyrite etc., and some minerals are coloured by impurities. For example, because of impurities quartz may be white, green, red, yellow etc.
6. Streak – the colour of the ground powder of any mineral. It may be of the same colour as the mineral or may differ — malachite is green and gives a green streak, fluorite is purple or green but gives a white streak.
7. Transparency – transparent: light rays pass through so that objects can be seen plainly; translucent- light rays pass through but will get diffused so that objects cannot be seen; opaque – light will not pass at all.
8. Structure – particular arrangement of the individual crystals; fine, medium or coarse-grained; fibrous – separable, divergent, radiating.
9. Hardness – relative resistance being scratched; ten minerals are selected to measure the degree of hardness from 1-10. They are: 1. talc; 2. gypsum; 3. calcite; 4. fluorite; 5. apatite; 6. feldspar; 7. quartz; 8. topaz; 9. corundum; 10. diamond.
10. Specific gravity – the ratio between the weight of a given object and the weight of an equal volume of water; objects weighed in air and then weighed in water and divide weight in the air by the difference of the two weights.

Major minerals and their characteristics

Some major minerals and their characteristics

Feldspar

• Half of the earth’s crust is made up of alkali feldspar (12%) and feldspar (39%).
• It has a light cream to salmon pink colour.
• Silicon and oxygen are major elements of all types of feldspar.
• Feldspar is a common raw material used in glass, ceramics and to some extent used in paints, plastics and rubbers.

Feldspar

Quartz

• A hard mineral is virtually insoluble in water.
• It is usually white or colourless.
• It consists of silica and is one of the most important components of sand and granite.
• Used in manufacturing of radar, radio, oscillators for watches, and video games.

Quartz

Pyroxene

• Pyroxene forms 10% of the earth’s crust and is usually found in meteorites.
• Pyroxene minerals are minor to secondary additives of a few rocks which are used as beaten stone and measurement stone
• It is usually found in green or black colour.
• A few pyroxene minerals are used as gem materials.

Pyroxene

Amphibole

• Amphibole forms 7% of the earth’s crust and is generally green or black in colour.
• Aluminium, magnesium calcium, silicon and iron are significant elements of it
• It is mainly used in asbestos industries.
• Hornblende is another form of amphibole.

Amphibole

Mica

• It consists of potassium, aluminium, magnesium, iron, silica etc and forms 4% of the earth’s crust.
• It is commonly found in igneous and metamorphic rocks and used in electrical instruments.
• It is commonly found in igneous and metamorphic rocks.
• Mica is widely used in electronic instruments.

Mica

Olivine

• It consists of magnesium, iron and silica.
• It is a greenish crystal, often found in basaltic rocks.
• Other minerals such as chlorite, calcite, magnetite, haematite, bauxite, and barite are present in small amounts in the rocks in addition to these primary minerals.
• Olivine is used commonly in jewellery.

Olivine

Metallic Minerals

These minerals contain metal content and can be subdivided into three types:

1. Precious metals: gold, silver, platinum etc.
2. Ferrous metals: contains iron mixed with other minerals
3. Non-ferrous metals: non-ferrous metallic minerals do not have any trace of iron in them.

Non-Metallic Minerals

These minerals do not contain metal content. Sulphur, phosphates and nitrates are some examples. Cement is a mixture of non-metallic minerals.

Rocks and their classification

Rocks and their classification

Rocks differ in their properties, the size of particles and mode of formation and accordingly can be classified into three families

Igneous rock

Igneous rocks are called primary rocks or parent rocks as they are formed by highly heated molten fluid magma and lava. Most of the igneous rocks are hard and resistant and hence used for road constructions and polished as monuments and gravestones. Granite, gabbro, pegmatite, basalt, volcanic breccias are some examples of it.

Sedimentary rocks

Sedimentary rocks are developed by the accumulation of sediments over time. These sediments include eroded debris from any previously existing rock, including igneous, metamorphic, and ancient sedimentary rocks. The particles that form sedimentary rocks may be brought by winds, streams, glaciers and even animals.

Metamorphic Rocks

Metamorphic rocks are formed due to the transformation of pre-existing rocks through the process of metamorphism in which recrystallisation and reorganization of minerals occur within a rock. Metamorphism occurs due to mainly lava inflow, geodynamic forces or mountain building. Slate and marble are common examples of metamorphic rock.

Conclusion

A mineral is a natural or inorganic substance, having an orderly atomic shape and a particular chemical composition and bodily properties. The rocks can be made up of one mineral or diverse minerals. Rocks no longer have a particular composition of mineral materials and can be hard or soft.

For example, granite is hard, soapstone is soft. Rocks may also have various colours. Some rocks are darkish and a few are mild coloured. For example, Gabbro is black and quartzite may be milky white.