Gypsum is the most common sulfate mineral, characterized by the chemical formula CaSO4 2H2O; it shows little variation from this composition.
Gypsum is one of the several evaporate minerals. This mineral group includes chlorides, carbonates, borates, nitrates, and sulfates. These minerals precipitate in seas, lakes, caves, and salt flats due to concentration of ions by evaporation. When heated or subjected to solutions with very large salinities, gypsum converts to bassanite (CaSO4 . H2O) or anhydrite (CaSO4). Under equilibrium conditions, this conversion to anhydrite is direct. The conversion occurs above 42°C in pure water.
Gypsum is used for making building plaster, wallboard tiles, as an absorbent for chemicals, as a paint pigment and extender, and for coating papers. Natural gypsum of California, containing 15 to 20% sulfur, is used for producing ammonium sulfate for fertilizer. Gypsum is also used to make sulfuric acid by heating to 1093°C in an air-limited furnace. The resultant calcium sulfide is reacted to yield lime and sulfuric acid. Raw gypsum is also used to mix with portland cement to retard the set. Compact massive types of the mineral are used as building stones. The color is naturally white, but it may be colored by impurities to gray, brown, or red. The specific gravity is 2.28 to 2.33, and the hardness 1.5 to 2. It dehydrates when heated to about 190°C, forming the hemi-hydrate 2CaSO4 H2O, which is the basis of most gypsum plasters. It is called calcined gypsum, or when used for making ornaments or casts is called plaster of Paris. When mixed with water, it again forms the hydrated sulfate that will solidify and set firmly owing to interlocking crystallization. Theoretically, 18% of water is needed for mixing, but actually more is necessary. Insufficient water causes cracking. Water solutions of synthetic resins are mixed with gypsum for casting strong, waterproof articles.
Much calcined gypsum, or plaster of Paris, is used as gypsum plaster for wall finish. For such use it may be mixed in lime water or glue water, and with sand. Because of its solubility in water it cannot be used for outside work.
The presence of halite (NaCl) or other sul-fates in the solution lowers this temperature, although metastable gypsum exists at higher temperatures.
Crystals of gypsum are commonly tabular, diamond-shaped, or lenticular; swallow-tailed twins are also common. The mineral is mono-clinic with symmetry 2/m. Gypsum is the index mineral chosen for hardness 2 on the Mohs scale. In addition to free crystals, the common forms of gypsum are satin spar (fibrous), alabaster (finely crystalline), and selenite (massive crystalline).
Gypsum is used for a variety of purposes, but chiefly in the manufacture of plaster of Paris, in the production of wallboard, in agriculture to loosen clay rich soils, and in the manufacture of fertilizer. Plaster of Paris is made by heating gypsum to 200°C in air.
Gypsum deposits play an important role in the petroleum industry. The organic material commonly associated with its formation is considered a source of hydrocarbon (oil and gas) generation. In addition, these deposits act as a seal for many petroleum reservoirs, preventing the escape of gas and oil.
