The term ‘Mica’ covers a large group of rock-forming minerals Natural mica forms hexagonal crystals of varying size, the distinguishing feature of which is their ability to split readily into separate leaves.

Minerals:

Minerals of the mica-group are composed of the orthosilicates of aluminium, with potassium and hydrogen and generally magnesium, iron, sodium and rarely rubidium and caesium. The main mica minerals are the following:

1. Muscovite, also known as white-mica or potash-mica.

ADVERTISEMENTS:

2. Biotite, known as black-mica or magnesium-iron mica.

3. Phlogopite, i.e., Amber mica or magnesium-mica.

4. Lepidolite–also known as lithium-mica.

5. Zinnwaldite also called lithium-iron mica.

ADVERTISEMENTS:

6. Roscoelite, (vanadium-mica).

7. Fuschsite, i.e., chrome-mica.

Origin:

The micas, which are usually associated with acid igneous rocks, are formed towards the end of the process of crystalli­zation of rock-forming silicates. They are mostly formed as peg­matite deposits, consisting of very coarse grained igneous rocks occurring as dykes or veins.

ADVERTISEMENTS:

Mode of occurrence:

Mica-minerals occur in igneous, sedinientary and metamorphic rocks formed under different geological conditions. While muscovite occurs in pegmatites of acidic nature, phlogopite mica is restricted to basic-pegmatite. Lepidolite occurs in pegmatites associated with topaz. Commercial biotite is found to occur mostly in biotite-schists.

Distribution:

India is the most important mica-producing country in the world and it supplies about 80 % of the world requirements of block-mica. The occurrence of muscovite-mica is associated with the rocks of Archaean age. The three most important occurrences are

ADVERTISEMENTS:

(i) the Koderma Mica Belt in Bihar,

(ii) the Nellore Mica Belt in Andhra Pradesh,

(iii) the Rajasthan Mica Belt in Rajasthan.

Koderma-mica-belt:

ADVERTISEMENTS:

It is about 32 km wide and stretches from Gaya district through Hazaribagh and Monghyr to Bhagalpur district for about 145 km. In this mica belt, the deposits of mica are associated with the pegmatite veins which traverse through the schistose and gneissose country rocks. The pegmatite veins contain mica-deposits at places where they traverse through the mica-schists. The blocks of muscovite which occur within the Bihar-mica-belt are generally reddish in colour and are, therefore, known as ‘Ruby- mica’.

(ii) Nellore-mica-belt:

It has a length of about 100 km. between Gudur and Sangam. The country rocks are Archaean- mica-schist and Hornblende-Schist which are intruded by pegmatite veins. Here muscovites are light green in colour.

(iii) Rajasthan-mica-belt:

ADVERTISEMENTS:

It is a quite wide belt and produces 19% of the total Indian-production. Here the mica bearing peg­matites are intrusive mainly to rocks of the gneissic complex and also into Aravalli-schists.

Uses:

(i) As an insulating material in electrical industry.

(ii) Muscovite, phlogopite splitting are used in making of build-up-mica or micanite and other insulation product, for both heat insulation and also electrical insulation.

(iii) In powder-form it is used in lubricating oils and decorative wall papers.