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Gypsum
Gypsum is hydrous
calcium sulphate crystallising in the monoclinic system. Crystals are common. Twin crystals
are also common. Its hardness is 2 and the specific gravity is 2.3. Its lustre is either
pearly, vitreous, silky or earthy. It is colourless or white, grey, pink, brown or black
in colour. It produces a white streak. The mineral dissolves readily in hydrochloric acid.
Gypsum is used in the manufacture of cement and plaster of Paris. In
agriculture it is used as a fertiliser.
In Tamilnadu, gypsum occurs in Tiruchirapalli, Tirunelveli, Coimbatore
and Chingelput districts of Tamilnadu.
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Galena
Galena,
commonly known as lead glance, is a sulphide of lead crystallising in the cubic system. Cubic
crystals are common. It is a heavy mineral having a specific gravity of 7.5. Its hardness
varies from 2.5 to 2.8. Galena has a perfect cubic cleavage. Its colour and streak are lead
grey and the lustre is metallic.
Galena is the chief ore of lead.
The most important source of galena in India is from Zawar in
Udaipur, Rajasthan.
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Mica
Mica is a peculiar group of minerals having certain physical and chemical characteristics in
common. They have a perfect basal cleavage yielding extremely thin, tough, flexible sheets.
The important varieties of micas such as Biotite, Vermiculite, Phlogopite, Muscovite and Ruby
mica are displayed in the gallery.
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Biotite
Biotite is a dark
coloured mica, black to green in colour. In thin flakes it is green or brown. Its hardness
varies from 2.5 to 3 and the specific gravity ranges from 2.7 to 3.
Biotite occurs in
igneous and metamorphic rocks. |
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Mineralogy