Anhydrite

Anhydrite

Chickenwire Gypsum anhydrite

Anhydrite Kiln by Ophelia Gordon Bell

Anhydrite is a mineral—anhydrous calcium sulfate, with the chemical formula CaSO4. It is closely related to gypsum, which has a chemical formula of CaSO4·2H2O. The difference between anhydrite and gypsum is the presence of water in gypsum's structure, which is absent in anhydrite. Anhydrite takes its name from the Greek word "anhydrous," meaning "without water."

Formation and Occurrence

Anhydrite forms in evaporite deposits when seawater evaporates, and it can also be found in volcanic rocks. It occurs in extensive beds associated with other evaporite minerals such as halite, sylvite, and magnesite. Anhydrite can also form as a secondary mineral in other deposits through the dehydration of gypsum when exposed to high temperatures. This process can occur during the burial of sediments which can lead to the transformation of gypsum to anhydrite.

Properties and Uses

Anhydrite is a relatively common mineral, but it is not as widely used as gypsum. Its hardness on the Mohs scale is 3 to 3.5, and it has a white, gray, or blue color, often with a pearly or vitreous luster. Anhydrite is soluble in water, which can lead to the rehydration of the mineral back to gypsum under certain conditions.

In the industry, anhydrite is used as a drying agent in plaster and paint. It is also used in the production of cement, where it acts as a source of sulfate, controlling the setting time of the cement. Anhydrite can also be used as a filler in the production of paper and as a soil conditioner and fertilizer in agriculture.

Environmental and Geological Significance

The presence of anhydrite in geological formations can be an indicator of past environmental conditions, particularly in the study of evaporite deposits. These deposits can provide valuable information about past climates and the history of Earth's oceans. Anhydrite's transformation to gypsum and vice versa can also impact the porosity and permeability of rock formations, influencing the flow of oil, gas, and water in subsurface reservoirs.

See Also

• Evaporite
• Sedimentary rock
• Mineralogy
• Geology of salt