Gypsum is a hydrated calcium sulphate, which crystallizes in the monoclinic system. The mineral is having the chemical composition as CaS04,.2H20, where CaS04 constitutes about 79″1% of the mineral and water is about 20’9%. The material should not be called as gypsum, if it contains less than 64’5% of CaS04, 2HsO by weight. It belongs to the class of mineral deposit called ‘Evaporites’.
There are five varieties of gypsum as
(a) Pure gypsum or salenite, which is crystalline in character and is transparent.
(b) Albaster, a dense, massive, granular, transluscent variety.
(c) Satin-spar, a fibrous variety having silky lusture.
(d) Gypsite, an earthy, soft, impure variety containing abundant small gypsum crystals scattered through clayey or sandy-soil.
(e) Rock-gypsum, a coarse granular, compact, massive variety, which usually occurs interbedded with sedimentary rocks, and is usually impure.
It is believed that gypsum is formed by the action of sulphuric acid produced by the oxidation of pyritic matter, on limestone and marl. For this reason, well developed crystals and plates of gypsum are found associated with clay and limestone. It may be deposited because of evaporation of water in saline inland basins or lagoons, when 37 % of water has been evaporated.
According to the ‘solar diagram’ of Kurnakov, the order of crystallization is (i) Gypsum, (ii) Halite, (iii) Epsomite, (iv) Hexa- hydrite, (v) Carnalite and (vi) Bischofite.
It is suggested that anhydrite is deposited from water at a higher temperature than gypsum.
Mode of occurrence:
The important commercial deposits of gypsum are those of rock-gypsum which occurs as beds with sedimentary rocks and are deposited from the solution by the evaporation of sea-water. Gypsite and albaster occur as beds or lenses and are formed by the evaporation of sea-water. ‘Salenite’ and ‘satin- spar’ occurs as beds and lenses and are due to crystallization from solution.
Distribution in India:
The most important sources of gypsum are in the state of Rajasthan. They are confined to the Tertiary rock-formations of Jodhpur region.
Beds of gypsum, half to two metres thick occur at several places around the Great Indian Desert of Rajasthan, particularly in the districts of Bikaner, (Jamser deposit), Jodhpur (Nagaur deposit), Barmar and Jaisalmer districts.
In Tiruchirapolly district, where gypsum occurs as thin irregular veins in the clays and limestones of the Uttatur and Trichinopoly stages of the cretaceous system.
In the district of Barmula and Doda, rich deposits occur as lenticular bands in the pre-cambrian Salakhal schist or with nummulitic-limestones of Eocene age.
(d) Himachal Pradesh:
Deposits, associated with Krol-lime- stone ar.d dolomite and also with the Subathu series; reported from Chamba, Mahasu and Sirmur district.
Besides the above, gypsum deposits also found to occur in the states of Gujarat, Uttar Pradesh, West Bengal, Madhya Pradesh, Andhra Pradesh etc.
It is used as a building material for the manufacture of cement; for the production of various types of plasters, in pottery and statutory industries.
It is used as a fertilizer and also used in the paint, rubber and paper-industries, as well as in the manufacture of ‘Plaster of Paris’.