Anhydrite: Warning! Keep Dry!

The word “anhydrous” means “lacking water.” So, when gypsum looses the water molecules that help make up its chemical composition “anhydrite” is an apt name for the new mineral that results.

Anhydrite commonly forms when gypsum dehydrates, but it can form in other ways. It is often found between layers of gypsum and even between gypsum and halite. Because it usually forms when gypsum dries out the layers of anhydrite are often contorted.

Anhydrite crystallizes in the orthorhombic crystal system, but it is usually found in massive or fibrous habits, the latter looking a bit like satin-spar selenite (a variety of gypsum that you can read about in my March 17, 2011post Gypsum: The Mineral About the House) Anhydrite can be distinguished from gypsum by the fact that it is a little harder. Anhydrite is usually white or colorless, but it can also be lavender, reddish, bluish, or gray.

I’ve already stated that gypsum becomes anhydrite when it looses its water, but if the anhydrite absorbs water then it will become gypsum once again. Because of this, anhydrite can be ground into a powder and used to make concrete and plaster of Paris. This property also makes anhydrite difficult to find. Since it so easily reverts to gypsum, anhydrite tends only to be found deep underground where it won’t be exposed to much moisture.

Anhydrite can be found in such places as: Mexico, Cuba, Peru, Chile, Canada, Zaire, and the American states of: Alabama, Michigan, Louisiana, and Texas.

I hope that you’ve enjoyed this piece about anhydrite. I have one final note for collectors which concerns anhydrite’s relationship with gypsum: if you want to collect the stuff it is a good idea to keep it in an airtight case with some silica gel. Otherwise your anhydrite specimen might revert to gypsum!