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Composition
Gypsum is categorized as an evaporite mineral, a mineral formed by the evaporation of a solution (often sea water). When highly concentrated salt water evaporates, gypsum crystallizes and forms salt deposits. Gypsum can form when water is exposed to the mineral anhydrite. It can also form when limestone reacts with sulfuric gases emitted from volcanic hot springs. Gypsum is classified as a hydrated calcium sulfate. Calcium sulfate is a white salt that comes in both hydrous (with water) and anhydrous (without water) forms. Gypsum is associated with other minerals such as calcite, dolomite, salt, sulfur, quartz and pyrite. Gypsum is often white or clear, but it can also appear in shades of gray, brown, yellow, pink or red when it contains impurities. Gypsum can turn cloudy or opaque when exposed to flames and can be dissolved in hot water and hydrochloric acid.
Forms
One form of gypsum is selenite, which is characterized by long, transparent crystals that can grow together and create distinct forms, such as the fishtail or swallowtail forms. Alabaster is a massive granular form of gypsum with a waxy-looking appearance. Alabaster can be white, gray, brown or yellow. Satin spar is a form of gypsum composed of fibrous aggregates (or clusters) of long crystals with a satin or silky look. Satin spar is pink or white in color. Another form of gypsum is the desert rose. Desert roses appear in reddish-colored rosette-shaped aggregates. These aggregates often have grains of sand in them.
Occurrence
Gypsum is found in thick beds in Oklahoma, Tennessee, New York, Michigan, Virginia and Iowa. Gypsum can occur in veins or as crystals in isolated beds of clay, limestone or sand. Gypsum crystals can also be found around volcanic vents. Large-crystal gypsums are located in Mexico, Italy, Chile, the sulfur mines of Sicily and the state of Utah. Stratified deposits of gypsum are found in the United States (for example, in Texas and California), Nova Scotia, France, Italy, the northern Caucasus and the western Urals. Crystals can also be found in the clay beds of Maryland and Ohio. Gypsum appears in hills of wind-blown sand in New Mexico, and desert rose gypsum is found in Arizona, Mexico and Tunisia, Morocco.
Uses
Gypsum is central to building and construction. It is considered a natural plaster, along with earth and lime, that is used to protect and beautify interior walls. Millennia-old Arabic and European buildings have walls and ceilings decorated with gypsum. Gypsum is used to make plaster of Paris, deodorizer, cement, fertilizer, paint fillers and whitewash. It acts as a retarder, or a chemical agent that delays cement from hardening, in Portland cement. Satin spar and alabaster are used in ornamental works. Alabaster is specifically used for sculpture and interior decorating.