Minerals

MINERAL

A mineral is a naturally occurring homogenous solid inorganically formed with a definite chemical composition and an orderly atomic arrangement.

Mineral:

More than 90% of all of the minerals in the Earth’s Crust are made up of compounds containing Silicon and Oxygen, the two most abundant elements on Earth. There are over 2000 minerals on Earth, but only 100 are commonly found. 30 minerals make up the majority of the rocks on Earth.

A mineral, by definition, must satisfy five conditions:

It must be naturally occurring.
It must be inorganic.
It must be a solid element or compound.
It must have a definite composition.
It must have a regular internal crystal structure

Occurrence

Of the 2000 known minerals, only few occur in abundance in the Earth crust.

The relative abundance of important rock forming minerals is given below:

Minerals Important constituents Percent distribution

Primary minerals

Ferro magnesium minerals

Ortho-ino silicates 16.8

Olivine Fe, Mg

Pyroxenes Ca, Na, Fe, Mg

Amphiboles Ca, Na, Fe, Mg, Al, OH

Phyllo Silicates 3.6

Biotite K, Fe, Mg, Al, OH

Muscovite K, Al, OH

Non-Ferro Magnesium minerals

Tecto Silicates 61.0

Feldspars

Anorthite Ca, Al

Albite Na, Al

Orthoclase K, Al

Quartz

Secondary minerals

Clay minerals Na, K, Ca 11.6

Others Mg, Fe, Al, OH 6.0

Formation of minerals:

When the molten magma solidifies, the different elements present in it freely arrange themselves in accordance with attractive forces and geometric form.
The earth’s crust contains dominant amount of oxygen (46.60%) followed by silicon (27.72%).

In order to achieve neutrality between the negatively charged oxygen and the positively charged silicon, there would be a greater tendency for silicon and oxygen to combine to form the basic compound, called the silicon – oxygen tetrahedron (SiO4)4-.
This explains the dominance (>90%) of silicate minerals in the earth’s crust.
Geometrically, it is possible to arrange only 4 oxygen anions around a central silicon cation so that all are touching each other. This is the arrangement of a tetrahedron.
The amount of charge carried by silicon ion is 4+ and by oxygen is 2-. In order to attain neutrality, one silicon (4+) ion would combine with two oxygen ion (2 x 2-) to form SiO2 but geometrically stable structure is formed when 1 silicon combines with 4 oxygen ions to form tetrahedron (SiO4)4- which carries a net negative charge of 4-.