Gypsum is hydrous calcium sulphate having chemical composition CaSO4,2H2O (chemical composition of anhydrite is CaSO4). It has five varieties, namely:
- Alabaster: the massive fine grained granular compact type; the name is related to a place in ancient Egypt.
- Selenite: the transparent and crystalline type, name originating from Greek word ”selen” meaning moon.
- Satinspar: the fibrous type with silky lustre.
- Gypsite (also called gypseous clay); the porous earthy impure gypsum mixed with sand and clay.
- Rock gypsum: medium coarse grained gypsum forming extensive thick sedimentary beds.
Out of these, the first two, rock gypsum and alabaster, are of the most significant from an economic point of View and are most widely mined and used. Usually, people do not differentiate between these two and refer to them by the generalized term gypsum. But all the types are chemically alike and exhibit the same properties.
Gypsum is a very common mineral found in nature (very frequently, in association with anhydrite). Most of the gypsum deposits originated due to evaporation of salt-rich waters, seawater or lake brine or spring water or underground saline water rising to the surface by capillary force. Some deposits also originated due to hydration of anhydrite (CaSO4) and a few vein deposits, due to action of sulphuric acid on limestone.
Its availability in nature in the form of massive blocks (alabaster), in silky fibrous form (satinspar) and crystalline form (selenite) is taken advantage of in certain uses.
Gypsum is soft, its hardness being only 2 on Mohs scale (it may even be less for impure gypsum). It has moderate specific gravity of 2.2-2.4, but the bulk density of its fine powder is higher. Crystalline varieties exhibit subvitreous to pearly shining lustre while the massive rock gypsum and alabaster have a dull earthy appearance.
Pure gypsum is saturated with water and has 32.6% CaO, 46.5% SO; and 20.9% H2O. Chemically, gypsum is acidic because of the SO3 radical and reacts with HCI acid, and it dissolves in excess water (400-500 times). It has low oil absorption (less than 5%).
When partially calcined at 150-180 C under steam pressure, gypsum loses three fourths of its water to become calcium hemihydrate (CaSO4,0/5H2O) which is a loosely bonded granular compound crumbling easily to powders, but when mixed with water it returns to and sets as gypsum. As calcination temperature rises, it starts losing its remaining water, losing all of it and fusing at 950°C to becomes anhydrite (or dead-burnt gypsum) which is a harder, heavier and compact substance unable to set in water, and having hardness 3.5 on Mohs scale and specific gravity 3.0.
Pure gypsum is white, but it may also be red, yellow, etc. depending on presence of impurities.