Anhydrite
Anhydrite is calcium sulfate, CaSO4, chemically identical to gypsum but lacking the water molecules bound into gypsum's crystal structure. It forms in evaporite basins where seawater or saline lake water concentrates through evaporation, and it readily absorbs water to convert into gypsum, which is why anhydrite outcrops are less common at the surface than at depth. Its name, from the Greek for "without water," directly references this dehydrated composition.
Anhydrite properties at a glance
| Type | Minerals |
|---|---|
| Chemical formula | CaSO4 |
| Mohs hardness | 3–3.5 |
| Crystal system | Orthorhombic |
| Luster | Vitreous, pearly on cleavage surfaces |
| Streak | White |
| Cleavage | Perfect in three directions at right angles |
| Fracture | Uneven to splintery |
| Specific gravity | 2.9–3.0 |
| Transparency | Transparent to translucent |
| Common colors | Colorless, White, Gray, Blue, Pink |
| Rarity | common |
Geology & background
Anhydrite crystallizes when brines become so concentrated that calcium sulfate precipitates directly without incorporating water, a condition more common in deep, warm evaporite basins than at shallow depths, where gypsum forms instead. Thick anhydrite beds occur within the Permian Basin of Texas and New Mexico, the Zechstein evaporites of Germany and Poland, and salt dome caprocks along the Gulf Coast, often interlayered with halite and gypsum. Because it slowly rehydrates to gypsum when exposed to groundwater near the surface, much anhydrite is discovered only in deep mines, quarries, or drill core.
Rockhounds should know that anhydrite specimens can be fragile and prone to alteration if kept in humid conditions, since the transformation to gypsum expands the mineral's volume and can crack or crumble a display piece over time. Well-formed tabular or prismatic crystals are less commonly seen than granular or fibrous massive material, and blue-gray to violet-tinted anhydrite from Naica, Mexico is particularly sought after for its color and association with that mine's giant selenite crystals.
How to identify Anhydrite
Anhydrite is usually white, gray, or bluish-gray, occasionally with violet or pink tints, and has a vitreous to pearly luster on cleavage surfaces. Its hardness of 3 to 3.5 is close to gypsum's, but anhydrite is harder and denser, and it shows three cleavage directions at right angles compared to gypsum's single perfect cleavage. The streak is white, and massive anhydrite can resemble marble or fine-grained gypsum, so a hardness test and cleavage check are needed to tell them apart reliably.
- Compare the specimen's color, luster, crystal habit, grain size, streak, and locality before confirming.
- Use multiple photos when possible, including a fresh surface and a close view of texture.
Where Anhydrite is found
Reported from multiple localities worldwide; verify locality against specimen provenance when available.
How much is Anhydrite worth?
| Entry-level specimen | $2 |
|---|---|
| Typical collector specimen | $20 |
| Premium specimen | $150+ |
Value depends on size, crystal quality, color, locality, rarity, damage, and provenance.
Common uses
- Collector specimens
- Education
- Field identification
- Industrial or lapidary use when suitable
Interesting facts
- Anhydrite absorbs water from the air or groundwater and expands roughly thirty percent in volume as it converts to gypsum.
- Thick anhydrite caprock over Gulf Coast salt domes helped geologists locate petroleum traps throughout the twentieth century.
- The giant selenite crystals of Naica, Mexico's Cave of the Crystals grew in a chamber where anhydrite dissolution helped regulate the mineral-saturated groundwater.
Care & safety
Handle as a collector specimen until hardness, cleavage, and fracture are verified.
Avoid inhaling dust. Do not cut, grind, or polish unknown specimens without eye protection and a proper respirator.
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