Anhydrite
Anhydrous calcium sulfate mineral found in evaporite deposits.
Anhydrite is a mineral composed of anhydrous calcium sulfate (CaSO4). It is notable for its occurrence in evaporite deposits and its transformation to gypsum upon exposure to water, making it a key mineral in geological and industrial contexts.
- chemical_formula
- CaSO4
- crystal_system
- Orthorhombic
- mohs_hardness
- 3.5
- specific_gravity
- 2.9
- color
- White, greyish, bluish, or purple
Lore & Background
Anhydrite has been found in some igneous rocks, including the intrusive dioritic pluton of El Teniente, Chile, and in trachyandesite pumice from El Chichón volcano, Mexico. It has several obsolete names, including muriacite and karstenite, and varieties such as tripe-stone and vulpinite. A semi-transparent light blue-grey variety from Peru is known by the trade name angelite.
Reader's Guide
Anhydrite is significant primarily as a mineral in evaporite deposits and salt dome cap rocks, where it plays a role in the geological cycle of calcium sulfate. Its transformation to gypsum upon water absorption is a key process in subsurface environments, as demonstrated by the extensive structural damage in Staufen im Breisgau, Germany, where geothermal drilling allowed water to invade an anhydrite layer, causing ground swelling. The mineral also participates in thermochemical sulfate reduction in oil fields, reducing sulfate to hydrogen sulfide and precipitating calcite. Industrially, anhydrite has been used in ornamental carving, as seen in a relief carving of an anhydrite kiln at the Catalyst Science Discovery Centre in Widnes, England. Its naming history reflects early confusion with chlorides, and its varieties like angelite are used for decorative purposes.
Did You Know?
- When exposed to water, anhydrite readily transforms to gypsum by absorption of water.
- Anhydrite occurs in a tidal flat environment in the Persian Gulf sabkhas as massive diagenetic replacement nodules called chicken-wire anhydrite.
- A semi-transparent light blue-grey variety from Peru is referred to by the trade name angelite.
Frequently Asked Questions
What does Anhydrite look like and how hard is it?
Specimens typically show up in shades of white, grey, blue, or purple, and the mineral sits at 3.5 on the Mohs hardness scale. With a specific gravity around 2.9, it feels noticeably dense in the hand for a sulfate mineral.
What happens when Anhydrite comes into contact with water?
The mineral readily absorbs water and converts into gypsum, its hydrated counterpart. This hydration reaction is so characteristic that Anhydrite rarely persists in wet or groundwater-rich settings.
Where is Anhydrite typically found in nature?
It is most commonly encountered in evaporite deposits, where ancient seawater or lake water evaporated and concentrated sulfate and salt minerals. These settings often place Anhydrite alongside rock salt and other evaporite companions.
Why is Anhydrite considered important in geology and industry?
Its easy conversion to gypsum makes it a key link in the sulfate-mineral cycle and in the buildup of major sedimentary rock sequences. In industry it serves as a precursor for plaster, cement additives, and other calcium-sulfate products.
More in Minerals & Crystals 1-24
Elsewhere in the Minerals & Crystals universe
Spotted an error? Know more?
This is a living reference — every entry is fact-audited, and reader corrections feed straight into our audit queue. Suggest an edit · See this site's audit record