RocksdayAnhydrite vs Gypsum
How to tell them apart
Calcium sulfate without water — gypsum's dry cousin, forming in salt deposits.
Photo: Alcinoe, Public domain
A soft, water-bearing calcium sulfate formed as seawater and lakes evaporate. Clear crystals are called selenite; sand-filled rosettes are 'desert roses'.
Photo: Didier Descouens, CC BY-SA 4.0
Quick tests
- Fingernail: A fingernail scratches Gypsum (Mohs 2) but not Anhydrite (Mohs 3–3.5).
Key differences
- Gypsum is softer — a fingernail scratches it.
Side by side
| Property | Anhydrite | Gypsum |
|---|---|---|
| Type | Mineral | Mineral |
| Mohs hardness | 3–3.5 | 2 |
| Luster | Vitreous to pearly | Vitreous to pearly |
| Streak | White | White |
| Colors | White, Blue, Purple, Gray | Clear, White, Brown |
| Crystal system | Orthorhombic | Monoclinic |
| Specific gravity | 2.9–3 | 2.31–2.33 |
| Formula | CaSO₄ | CaSO₄·2H₂O |
| Found in | Naica, Mexico; Peru | Naica, Mexico; White Sands, New Mexico |
Properties that are the same for both are grayed out.
How to identify anhydrite
- Breaks into blocks along three cleavages at right angles.
- Harder than gypsum.
- Doesn't fizz in acid.
Crystal system: Orthorhombic
Full Anhydrite guide →How to identify gypsum
- Scratches with a fingernail (hardness 2).
- Clear blades that bend slightly before snapping.
- Doesn't fizz in acid.
- Feels warm, not cold like glass.
Crystal system: Monoclinic
Full Gypsum guide →More comparisons
Could you tell them apart from a blurry close-up?
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