Quick Facts
Formation & Origin
Anhydrite precipitates from hypersaline brines in evaporite basins where seawater or lake water evaporates under arid conditions. It begins to crystallize when the brine reaches approximately 42°C and sufficient calcium and sulfate ions have concentrated, typically after about 90% of the original water volume has evaporated. In the evaporation sequence, anhydrite forms after carbonates but before halite, creating distinctive layered deposits that can extend hundreds of meters in thickness over geological time.
The relationship between anhydrite and gypsum is controlled by temperature, pressure, and water activity. Below approximately 42°C at surface pressure, gypsum (CaSO₄·2H₂O) is the stable phase. Above this temperature, or at burial depths where pressure drives off the water of crystallization, anhydrite is favored. This means that near-surface gypsum converts to anhydrite during burial, and deep anhydrite converts back to gypsum when exposed to groundwater during uplift. The gypsum-to-anhydrite transition involves a 38% volume decrease, while the reverse hydration expands the mineral by roughly 60%, a process that can buckle roads, lift foundations, and deform tunnel linings.
Blue anhydrite, sold commercially as angelite, gets its color from trace amounts of strontium substituting for calcium in the crystal lattice, combined with possible charge-transfer effects involving sulfur. This variety typically forms in specific evaporite horizons where strontium-enriched brines were present during crystallization. Hydrothermal anhydrite also occurs around volcanic vents and in the cap rocks of salt domes, where hot sulfate-rich fluids precipitate the mineral at temperatures between 100 and 350°C.
Identification Guide
Anhydrite is recognized by its three mutually perpendicular cleavage directions, which produce rectangular or blocky fragments. This pseudo-cubic cleavage pattern, combined with its moderate hardness of 3 to 3.5, is highly diagnostic. The mineral has a vitreous to pearly luster and can range from colorless through white, gray, blue, and lilac. Blue varieties show a distinctive soft, chalky blue color quite unlike the deeper blue of celestine or the greenish blue of apatite.
Distinguish anhydrite from gypsum by hardness. Anhydrite cannot be scratched by a fingernail (hardness 3-3.5) while gypsum can (hardness 2). Anhydrite also feels noticeably heavier due to its higher specific gravity (2.9-3.0 versus 2.3 for gypsum) and lacks the flexible cleavage sheets characteristic of selenite gypsum. Compared to calcite, anhydrite does not react with dilute hydrochloric acid. Compared to celestine (SrSO₄), anhydrite has a slightly lower specific gravity and different crystal habit.
Spotting Fakes
Blue anhydrite (angelite) is sometimes imitated with dyed howlite or dyed magnesite. Examine the surface with a 10x loupe. Dyed material often shows color concentrated in surface cracks and pits while the interior is white. Apply a drop of acetone or nail polish remover to an inconspicuous spot. If blue color transfers to a cotton swab, the piece is dyed. Test hardness. Genuine anhydrite is 3 to 3.5, scratchable by a steel knife but not by a fingernail. Howlite is softer at 3, and dyed versions often feel waxy rather than showing the vitreous to pearly luster of genuine anhydrite. Check for the characteristic three-directional cleavage. Genuine anhydrite breaks into blocky rectangular fragments. Be cautious of blue material sold as angelite that lacks visible cleavage planes, as it may be dyed calcite or magnesite. A simple specific gravity test (2.9-3.0 for anhydrite versus 2.5 for howlite) provides additional confirmation.
Cultural & Metaphysical Traditions
Presented as cultural traditions, not scientific evidence
Blue anhydrite, known as angelite in metaphysical communities, became popular in the 1980s and quickly developed associations with angelic communication and spiritual awareness. Practitioners use it in meditation focused on connecting with higher guidance, and it is commonly placed near the throat during energy work intended to enhance peaceful communication. The mineral's name (from the Greek anhydros, meaning without water) has been interpreted symbolically as representing transcendence of emotional turbulence. In crystal grid traditions, angelite is frequently paired with celestite and selenite to create what practitioners describe as an atmosphere of serenity. Some traditions associate its soft blue color with the energy of compassion and gentle truth-telling.
Where It's Found
Found alongside the famous giant selenite crystals in hydrothermal cave environments
Classic evaporite locality where massive anhydrite beds are interlayered with salt deposits
Source of blue anhydrite marketed as angelite, from sedimentary evaporite sequences
Thick evaporite sequences in the Windsor Group produce large anhydrite masses
Well-formed transparent crystals from alpine-type fissures in metamorphic terrain
Price Guide
Good to Know
Scratch test: At hardness 3.25, Anhydrite can be scratched with a copper coin. Handle gently and keep away from harder stones in your collection.
Sources: Found in 5 notable locations worldwide, from Naica to Swiss Alps.
Heft test: Anhydrite has average mineral density (2.9-3.0). It feels about as heavy as you'd expect from a stone its size.