Home » Mineral Directory

Gypsum

A widespread colorless, white, or yellowish mineral, CaSO₄·2H₂O, used in the manufacture of plaster of Paris, various plaster products, and fertilizers.

Gypsum is a kind of minerals composed of calcium sulfate (calcium, sulfur, and oxygen) with two molecules of water, calcium sulphate di-hydrate. It is the most common sulfate mineral, occurring in many places in a variety of forms. A transparent crystalline variety is selenite. A massive gypsum of delicate color and texture, readily worked into ornamental vases, boxes, and the like, is called alabaster. A lustrous gypsum with fibrous structure, called satin spar, is used in jewelry and for other ornaments, but it is soft and easily marred.

Plaster of Paris, a fine white powder, is produced by heating gypsum to expel the water. If this powder is moistened and then allowed to dry, it becomes hard, or sets. Its major use is in the manufacture of gypsum lath and wall board, and for casts and molds. It is widely used for staff, the material of which temporary exposition buildings are made. Uncalcined gypsum is added to Portland cement as a retarder.

Material Notes:
Alabaster is a fine grained, translucent gypsum. It has three cleavage directions. Occurrence: Formed by evaporation; found in sediments and desert floor. Common in efflorescence.

Crystallization Pressure for C/CS = 2 is 282 atm at 0°C.

Crystallization Pressure for C/CS = 2 is 334 atm at 50°C.

Crystallization Pressure for C/CS = 10 is 938 atm at 0°C.

Crystallization Pressure for C/CS = 10 is 1110 atm at 50°C.

Crystallization Pressure for C/CS = 50 is 1595 atm at 0°C.

Crystallization Pressure for C/CS = 50 is 1900 atm at 50°C.

Gypsum is an abundant natural mineral. It originates from the drying out of ancient seas and is quarried (or mined if deep) in many parts of the world. Gypsum does not occur in New Zealand.

Gypsum enjoys a growing application in agriculture and horticulture. It is used as a 'clean green' soil conditioner and also as a fertiliser. Gypsum has an advantage over certain other minerals of being pH neutral. Although most gypsum is used in the building industry, an increasing proportion is now being used in agriculture and horticulture. Here, it is very useful for improving the textural and drainage properties of heavy (clay) soils. Gypsum is also an excellent calcium and sulphur fertiliser. Its special benefits are that horticultural gypsum is fast acting and pH neutral (contrasting with other calcium fertilisers that are slower and either raise or lower soil pH).

Gypsum is an industrially important mineral. It is the primary ingredient of plaster-of-Paris (finely ground Gypsum) and is also used in the production of cement. It is also the main component of sheet-rock. It is used as a flux for creating earthenware, and can be used as a fertilizer. The variety Alabaster is is carved for ornamental use, such as artistic sculptures and pottery. It is porous and is therefore easily dyed.

In a small number of Gypsum specimens, water gets trapped inside a crystal in a hollow channel while the crystal forms. When such a crystal is rotated, a water "bubble" moves around inside it toward the lowest point in the channel. Such specimens are considered a mineralogical oddity, and are very much sought by collectors.

Gypsum sometimes forms in sandy areas, and crystals may trap sand inside when forming, causing a specimen to become brown and opaque. These sand inclusions sometimes form hourglass formations in a crystal. They are also present in the well-known "Desert Rose", which is rosette shaped gypsum with sand inclusions. The term "Desert Rose" also applies to rosette shaped barite with sand inclusions, and the two should not be confused.

[ Close ]