Anhydrite

Anhydrite is calcium sulfate without water (CaSO4), making it the anhydrous cousin of gypsum. It commonly occurs as massive beds in evaporite sequences and as a cap rock over salt domes.

Harder and denser than gypsum, anhydrite is often white to gray but can be tinted blue, violet, or pink. The well-known soft blue trade stone Angelite is a massive form of anhydrite valued in lapidary and metaphysical markets.

A notable property is its instability at the surface: when anhydrite absorbs water it converts to gypsum, expanding in volume by roughly 60 percent, a process important in both geology and engineering.

Anhydrite forms chiefly as a chemical sediment in marine evaporite environments, precipitating from concentrated seawater in hot, arid settings such as sabkhas and restricted basins. It also forms by dehydration of gypsum during deep burial, where heat and pressure drive off the structural water.

It is frequently associated with halite, gypsum, dolomite, and salt-dome cap rock. Hydrothermal veins and the alteration zones around volcanic systems can also produce anhydrite.

Major occurrences include the salt deposits of Germany, Poland, Mexico, and the Gulf Coast of the United States, as well as Peru, the source of much Angelite.

Anhydrite is harder than gypsum (3 to 3.5 versus 2), so it resists a fingernail scratch but yields to a copper coin or knife. It is also noticeably denser, with a higher specific gravity.

Color ranges from white and gray to distinctive pale blue or lavender. Luster is vitreous to pearly, and it shows three cleavage directions at right angles, sometimes giving cubic-looking fragments. Streak is white. It does not effervesce in dilute acid, separating it from carbonates.

The main look-alike is gypsum; the hardness test and greater density of anhydrite are the quickest discriminators.

Anhydrite is used as a drying agent (desiccant) because it readily absorbs water, and as a source material for sulfuric acid and sulfate fertilizers. Ground anhydrite serves as a soil conditioner and a cement additive.

The blue massive variety Angelite is carved into beads, cabochons, tumbled stones, and small figurines, though it is too soft and water-sensitive for hard-wearing jewelry. In metaphysical practice Angelite is associated with calm and communication, claims that are not scientifically established.

In engineering, anhydrite is significant because its swelling when wetted can damage tunnels and foundations built through sulfate-bearing rock.