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Minerals


The whole earth is made of rocks & minerals. Inside the earth there is a liquid core of molten rock and on the outside there is a hard crust.If you compare the earth to an egg, the shell on an egg is like the crust on the earth.

The crust is made up of rocks and minerals.Much of the crust is covered by water, sand, soil and ice. If you dig deep enough, you will always hit rocks. Below the loose layer of soil, sand & crumbled rocks found on Earth is bedrock, which is a solid rock.

The Crust makes up less than 1% of the Earth’s mass (0.4%) It is made of oxygen, magnesium aluminum, silicon calcium, sodium potassium, iron. There are 8 elements that make up 99% of the Earth’s crust. The continents are about 35 km thick and the ocean floors are about 7 lm thick.

The Mantle is the solid casing of the Earth and is about 2900 km thick. It makes up about 70% of the Earth’s mass (68.1%). It is made up of silicon, oxygen, aluminum and iron.

The Core is mainly made of iron and nickel and makes up about 30% of the Earth’s mass (31.5%). The Outer Core is 2200 km thick and is liquid and the Inner core is 1270 km thick and is solid.

About 98 per cent of the total crust of the earth is composed of eight elements like oxygen, silicon, aluminium, iron, calcium, sodium, potassium and magnesium, and the rest is constituted by titanium, hydrogen, phosphorous, manganese, sulphur, carbon, nickel and other elements.

The Major Elements of the Earth’s Crust (Elements By Weight(%)) are like
1. Oxygen 46.60
2. Silicon 27.72

3. Aluminium 8.13
4. Iron 5.00
5. Calcium 3.63
6. Sodium 2.83
7. Potassium 2.59
8. Magnesium 2.09
9. Others 1.41


The elements in the earth’s crust are rarely found exclusively but are usually combined with other elements to make various substances. These substances are recognised as minerals.

A mineral is a naturally occurring inorganic substance, having an orderly atomic structure and a definite chemical composition and physical properties. A mineral is composed of two or more elements. But, sometimes single element minerals like sulphur, copper, silver, gold, graphite etc. are found.

What is a Mineral?

The following definitions on what constitutes a mineral were taken from several different sources and are arranged by year:

  • "A mineral is an element or chemical compound that is normally crystalline and that has been formed as a result of geological processes" (Nickel, E. H., 1995).
     
  • "Minerals are naturally-occurring inorganic substances with a definite and predictable chemical composition and physical properties." (O' Donoghue, 1990).
     
  • "A mineral is a naturally occurring homogeneous solid, inorganically formed, with a definite chemical composition and an ordered atomic arrangement" (Mason, et al, 1968).
     
  • "These... minerals ...can be distinguished from one another by individual characteristics that arise directly from the kinds of atoms they contain and the arrangements these atoms make inside them" (Sinkankas, 1966).
     
  • "A mineral is a body produced by the processes of inorganic nature, having usually a definite chemical composition and, if formed under favorable conditions, a certain characteristic atomic structure which is expressed in its crystalline form and other physical properties" (Dana & Ford, 1932).
     
  • "Every distinct chemical compound occurring in inorganic nature, having a definite molecular structure or system of crystallization and well-defined physical properties, constitutes a mineral species" (Brush & Penfield, 1898).

There are 103 known chemical elements. Minerals are sorted into 8 groups.
Some common examples have been listed for each.

  • Native Elements ~ copper, silver, gold, nickel-iron, graphite, diamond
  • Sulfides ~ sphalerite, chalcopyrite, galena, pyrite
  • Halides ~ halite, fluorite
  • Oxides & Hydroxides ~ corundum, hematite
  • Nitrates, Carbonates, Borates ~ calcite, dolomite, malachite
  • Sulfates, Chromates, Molybdates, Tungstates ~ celestite, barite, gypsum
  • Phosphates, Arsenates, Vanadates ~ apatite, turquoise
  • Silicates ~ quartz, almandine garnet, topaz, jadeite, talc, biotite mica 

Though the number of elements making up the lithosphere are limited they are
combined in many different ways to make up many varieties of minerals. There are at least 2,000 minerals that have been named and identified in the earth crust; but almost all the commonly occurring ones are related to six major mineral groups that are known as major rock forming minerals.

The basic source of all minerals 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. 



PHYSICAL CHARACTERISTICS


(i) External crystal form — determined by internal arrangement of the molecules — cubes, octahedrons, hexagonal prisms, etc.

(ii) Cleavage — tendency to break in given directions producing relatively plane surfaces — result of internal arrangement of the molecules — may cleave in one or more directions and at any angle to each other.

(iii) Fracture — internal molecular arrangement so complex there are no planes of molecules; the crystal will break in an irregular manner, not along planes of cleavage.

(iv) Lustre — appearance of a material without regard to colour; each mineral has a distinctive lustre like metallic, silky, glossy etc.

(v) 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.

(vi) Streak — 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 green streak, fluorite is purple or green but gives a white streak.

(vii) 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.

(viii) Structure — particular arrangement of the individual crystals; fine, medium or coarse grained; fibrous — separable, divergent, radiating.

(ix) 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. 

Compared to this for example, a fingernail is 2.5 and glass or knife blade is 5.5.

(x) Specific gravity — the ratio between the weight of a given object and the weight of an equal volume of water; object weighed in air and then weighed in water and divide weight in air by the difference of the two weights. 





SOME MAJOR MINERALS AND THEIR CHARACTERISTICS


Feldspar

Silicon and oxygen are common elements in all types of feldspar and sodium, potassium, calcium, aluminium etc. are found in specific feldspar variety. Half of the earth’s crust is composed of feldspar. 

It has light cream to salmon pink colour. 

It is used in ceramics and glass making.


Quartz 


It is one of the most important components of sand and granite. It consists of silica. It is a hard mineral virtually insoluble in water. 

It is white or colourless and used in radio and radar.

It is one of the most important components of granite.



Pyroxene 


Pyroxene consists of calcium, aluminum, magnesium, iron and silica. Pyroxene forms 10 per cent of the earth’s crust. 

It is commonly found in meteorites. 

It is in green or black colour.



Amphibole

Aluminium, calcium, silica, iron, magnesium are the major elements of amphiboles. They form 7 per cent of the earth’s crust. 

It is in green or black colour and is used in asbestos industry. 

Hornblende is another form of amphiboles.



Mica

It comprises of potassium, aluminium, magnesium, iron, silica etc. It forms 4 per cent of the earth’s crust. 

It is commonly found in igneous and metamorphic rocks. 

It is used in electrical instruments.



Olivine

Magnesium, iron and silica are major elements of olivine. 

It is used in jewellery. 

It is usually a greenish crystal, often found in basaltic rocks.

Besides these main minerals, other minerals like chlorite, calcite, magnetite, haematite, bauxite and barite are also present in some quantities in the rocks.



Types of Minerals


Metallic Minerals

These minerals contain metal content and can be sub-divided into three types:

(i) Precious metals : gold, silver, platinum etc.

(ii) Ferrous metals : iron and other metals often mixed with iron to form various kinds of steel.

(iii) Non-ferrous metals : include metals like copper, lead, zinc, tin, aluminium etc.


Non-Metallic Minerals

These minerals do not contain metal content. Sulphur, phosphates and nitrates are examples of non-metallic minerals. 

Cement is a mixture of non-metallic minerals.


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