RocksdayBrucite vs Gypsum
How to tell them apart
Magnesium hydroxide forming pearly, flexible plates in serpentine and marble.
Photo: Robert M. Lavinsky, CC BY-SA 3.0
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
Key differences
- Gypsum is softer and doesn't dissolve in acid.
Side by side
| Property | Brucite | Gypsum |
|---|---|---|
| Type | Mineral | Mineral |
| Mohs hardness | 2.5–3 | 2 |
| Luster | Pearly to waxy | Vitreous to pearly |
| Streak | White | White |
| Colors | White, Green, Gray | Clear, White, Brown |
| Crystal system | Trigonal | Monoclinic |
| Specific gravity | 2.39 | 2.31–2.33 |
| Formula | Mg(OH)₂ | CaSO₄·2H₂O |
| Found in | Pakistan; Quebec, Canada | Naica, Mexico; White Sands, New Mexico |
Properties that are the same for both are grayed out.
How to identify brucite
- Pearly white to pale green plates.
- Flexible but not elastic flakes.
- Soft.
Crystal system: Trigonal
Full Brucite 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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