Anhydrite is a sedimentary mineral composed of calcium, sulfur, and oxygen. It is also known as calcium sulfate. Good specimens of natural anhydrite are often sought by mineral and rock collectors.
Anhydrite is usually white, colorless, or gray. It can also be found with violet or blue tones. When it has a soft blue color it may be referred to as angelite. Scratching the mineral on a streak plate usually results in white residue. It is relatively soft, only rating 3.5 on the Mohs hardness scale.
This mineral can be found in many places in the world, including the United States, Mexico, Austria, and Germany. It is fairly common, but good specimens are not; it is not a mineral characterized by good crystal formation. Those specimens with good crystal formation are generally the ones sought after by collectors. The crystals generally range from transparent to translucent.
It is commonly believed that this mineral was discovered in 1794. It was not until 1804, however, that a German geologist named Abraham Gottlob Werner named it anhydrite. This name refers to the fact that this mineral does not contain water.
This mineral is part of a group known as evaporites. This means their formation is the result of evaporation. In this case, anhydrite often results from the dewatering of gypsum.
If the mineral absorbs water, it can be transformed back into gypsum. This alteration is not always complete. In some cases, part of the specimen can remain anhydrite while the other part becomes gypsum. There have been reports of improperly cared for gypsum that is sold as anhydrite to unsuspecting buyers.
The genuine mineral is commonly found underground and in salt basins. In many instances it is found with halite, which is also in the evaporite group. It may also be associated with dolomite, calcite, and sylvite.
Anhydrite can also be synthetically produced. One instance in which this happens is during the production of hydrofluoric acid. At one time, this synthetic material was considered a problematic waste product. It is now pulverized and used, for example to make flooring materials.
It is also used in the manufacture of certain types of cement. When it is used for this purpose, it has several positive characteristics. For example, it can help to retard the setting of the cement. It can also help ensure that the cement is of uniform quality.