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Gypsum is a soft sulfate mineral composed of calcium sulfate dihydrate, with the chemical formula CaSO 4 ·2H 2 O. [4] It is widely mined and is used as a fertilizer and as the main constituent in many forms of plaster , drywall and blackboard or sidewalk chalk .
This reaction is exothermic and is responsible for the ease with which gypsum can be cast into various shapes including sheets (for drywall), sticks (for blackboard chalk), and molds (to immobilize broken bones, or for metal casting). Mixed with polymers, it has been used as a bone repair cement.
Solutions containing magnesium sulfate are generally more aggressive, for the same concentration. This is because magnesium also takes part in the reactions, replacing calcium in the solid phases with the formation of brucite (magnesium hydroxide) and magnesium silicate hydrates. The displaced calcium precipitates mainly as gypsum.
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Phosphogypsum is a by-product from the production of phosphoric acid by treating phosphate ore with sulfuric acid according to the following reaction: Ca 5 (PO 4) 3 X + 5 H 2 SO 4 + 10 H 2 O → 3 H 3 PO 4 + 5 (CaSO 4 · 2 H 2 O) + HX where X may include OH, F, Cl, or Br
Plaster was first used as a building material and for decoration in the Middle East at least 7,000 years ago. In Egypt, gypsum was burned in open fires, crushed into powder, and mixed with water to create plaster, used as a mortar between the blocks of pyramids and to provide a smooth wall facing.
Iron sulfide concretions, such as the Kansas Pop rocks, consisting of either pyrite and marcasite, are nonmagnetic. [79] On the other hand, iron sulfide concretions, which either are composed of or contain either pyrrhotite or smythite, will be magnetic to varying degrees. [80]
The process by which this synthetic gypsum is created is also known as forced oxidation: 2 CaSO 3 + H 2 O + O 2 → 2 CaSO 4 ·2H 2 O. A natural alkaline usable to absorb SO 2 is seawater. The SO 2 is absorbed in the water, and when oxygen is added reacts to form sulfate ions SO 2− 4 and free H +.