Gypsum

Gypsum crystals have a variety of habits, but most are tabular, less commonly they are prismatic or acicular. Large euhedral crystals are known as selenite. Granular or foliated masses are quite common. Satin spar consists of parallel aggregates of fibrous gypsum and may fill veins. Simple contact twins are common and may form "swallow tail" twins. Prismatic crystals may be curved, and tabular crystals and cleavage sheets may be bent in certain directions with relative ease. Granular-massive rock gypsum is known as alabaster if white or light colored.

Gypsum is a very common mineral in marine evaporite deposits and may be associated with halite, sylvite, calcite, dolomite, and anhydrite, as well as clay and silicate detrital grains. Gypsum in some evaporite deposits is produced by hydrating primary anhydrite. This requires a volume increase, so original planar anhydrite beds may become crumpled or disrupted as a result of the mineralogical change. Gypsum may also be produced as a precipitate from saline lakes, as an effiorescence on desert soils, or may precipitate around fumeroles or volcanic vents. Infrequently it is found in the near-surface, oxidized zone of hydrothermal sulfide deposits.

Gypsum is one of the earlier minerals to be exploited by people; its use goes back at least 5000 years. About 70% of the gypsum now mined goes to manufacture gypsum wallboard, also known as drywall. Gypsum wallboard is used to cover interior walls of most houses, apartments, and offices in North America and in many other parts of the world. To make wallboard, gypsum is calcined (heated) to drive off part of the water and is then ground to form a material called stucco (similar material is sold as plaster of Paris). Stucco is mixed with water, reinforcing fibers, and other additives to form a thick slurry that is extruded and wrapped with heavy paper to make wallboard. This material sets or hardens by recrystallizing to form gypsum. When cured it forms a stiff panel that is attached to interior wall framing with screws or nails. Joints between panels are finished with strips of mesh or heavy paper set in a plaster-like compound.

Gypsum also is used in portland cement to control the setting rate, and as a soil amendment to improve soil structure and workability, provide sulfur and calcium to plants, and control the availability of other soil nutrients. Calcined gypsum also has medical applications. It is used as a dietary supplement to provide needed sulfur both for people and animals, and is used to make casts to support broken bones, and as a special casting plaster to make dental molds. Alabaster, which is fine-grained white gypsum, is used as an ornamental stone and for sculpture, but its softness restricts its utility.

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