Anhydrite has a chemical composition of calcium sulfate giving its formula; CaSO4, and naturally forms orthorhombic crystals ( grows in three directions at right angles, forming rectangular prisms). Anhydrite can come in a variety of colors such as; white, grayish, bluish (Angelite), or purple.
Anhydrite is an anhydrous mineral, which means it contains no water. Anhydrite is formed from dehydrated Gypsum deposits. Gypsum is transformed to Anhydrite when it is heated to around 400 degrees and all of the water has been evaporated from the mineral, in turn if the Anhydrite is exposed to water it can be transformed back to Gypsum. Do to this fact, many specimens are found at great depths below the surface. The reason for this is that there is less circulating surface water at greater depths which would alter the Anhydrite to Gypsum. Anhydrite can sometimes form on shorelines and tidal flats from the complete evaporation of sea water. Anhydrite also forms atop of large salt domes once the gypsum has evaporated all water.
If you have an Anhydrite specimen in your collection and live in a very humid area, it may transform to Gypsum over a long period of time if it is not kept in a dry climate controlled area of your home.
First discovered in 1794 in a salt mine near ‘Hall in Tirol’, which is a town in Austria. Named in 1804 by a german geologist, A.G Werner. The name Anhydrite comes from the Greek word “anhydrous’, meaning “without water”. Anhydrite is sourced in many localities within Canada, Austria, Germany, Italy, Peru, and some states within the U.S. One of the the most noteworthy locations is the New Mexico’s Naica mine where some specimens have been known to reach 34 feet in length.
Ancient Egyptians would commonly use Anhydrite to carve drinking vessels and other objects. When crushed into power form, it was used as a filler in many early Italian paintings and sculptures. Today Anhydrite has many uses; it makes for an excellent soil treatment additive due to its high calcium composition and solubility, this also makes it a great drying agent in plaster, paint and varnishes. It is extensively used in the construction industry as a main component in wall board and joint compound.