Gypsum
calcium sulphate crystal. lized with two molecules of water Ca- So4 2H2O. It is often found by the decomposition of pyrites when lime is present. Gypsum calcined is called plaster of Paris, and is used for taking casts of statues. Gypsum is used as a manure; it facilitates the decomposition of rocks containing alkaline silicates.
Technically gypsum is regarded as a mineral deposit, in some places constituting rock masses. It may be of any age. Near Paris it is Middle Eocene. Frapoli says that some gypsums were originally carbonates instead of sulphates of lime, and that they underwent metamorphism by the action of volcanic sulphurous or sulphurohydrous vapors. The production of gypsum in the United States in 1919 was about 3,430,000 short tons, valued at about $16,000,000.