Anhydrite Identification Guide
How to identify anhydrite, anhydrous calcium sulfate, by its three right-angle cleavages and hardness, and tell it from gypsum and calcite.
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What Anhydrite Looks Like
Anhydrite is anhydrous calcium sulfate (CaSO4) — gypsum without the water. It is usually colorless, white, or gray, but can be pale blue, lavender (the 'angelite' variety), or pink-tinged. Luster is vitreous to pearly, and it ranges from transparent to translucent. It most often occurs as massive, granular, or fibrous aggregates in evaporite beds; well-formed crystals are blocky and orthorhombic. A diagnostic feature is its three cleavage directions meeting at right angles, which can produce cube-like or rectangular cleavage fragments — earning the cleavable variety the name 'cube spar.'
Step-by-Step Field-ID Checklist
- Test hardness — Mohs 3–3.5; a knife scratches it, but it is harder than gypsum (gypsum is Mohs 2, scratched by a fingernail).
- Try the fingernail test — anhydrite resists a fingernail; gypsum does not. This is the quickest gypsum/anhydrite separator.
- Look for three right-angle cleavages producing blocky rectangular fragments.
- Check the setting — bedded white/gray rock with halite or gypsum suggests an evaporite, favoring anhydrite or gypsum.
- Note any blue color — pale blue massive material may be angelite (anhydrite).
Key Diagnostic Tests
- Mohs hardness: 3–3.5 (harder than gypsum's 2).
- Streak: white.
- Cleavage: three directions at ~90° (a key identifier).
- Specific gravity: ~2.9–3.0 (denser than gypsum's ~2.3).
- No effervescence in acid (distinguishes it from calcite/aragonite).
- Slowly hydrates to gypsum in contact with water over time.
Common Look-Alikes
- Gypsum: softer (Mohs 2, scratched by fingernail), lower density, and has one perfect cleavage that yields flexible flakes; anhydrite is harder, denser, and has three right-angle cleavages. The fingernail test is decisive.
- Calcite: similar hardness but fizzes vigorously in dilute HCl; anhydrite does not react. Calcite also has rhombohedral (non-right-angle) cleavage.
- Halite (rock salt): has cubic cleavage too but tastes salty and is softer (Mohs 2.5) and water-soluble.
- Marble/limestone: carbonate, fizz in acid.
Where It Is Found
Anhydrite forms chiefly in marine evaporite deposits, where it precipitates from concentrated seawater alongside gypsum and halite, and also in salt-dome cap rock and some hydrothermal veins. Major occurrences are in evaporite basins of Germany, Poland, Italy, Mexico, and the U.S. (Texas, New Mexico, the Gulf Coast). Near the surface it commonly hydrates back to gypsum.
Frequently asked questions
How can you tell if it's anhydrite?
Anhydrite is a calcium sulfate with Mohs hardness 3–3.5 (resists a fingernail, unlike gypsum), three cleavages meeting at right angles, density ~2.9, and no fizz in acid. The fingernail and acid tests separate it from gypsum and calcite.
What is the difference between anhydrite and gypsum?
Gypsum is hydrated calcium sulfate (water in its structure), soft (Mohs 2, scratched by a fingernail), and lighter; anhydrite is the water-free form, harder (Mohs 3–3.5), denser, and has three right-angle cleavages. Anhydrite slowly hydrates to gypsum.
Does anhydrite react with acid?
No. Anhydrite is a sulfate and does not effervesce in dilute hydrochloric acid, which distinguishes it from carbonate minerals like calcite that fizz strongly.
What does anhydrite look like?
It is usually a colorless, white, or gray massive-to-granular mineral with a glassy-to-pearly luster, sometimes pale blue (angelite), often forming blocky rectangular cleavage fragments.
Anhydrite identified by the community
Recent Anhydrite specimens identified with Rock Identifier.
