RocksdayCalcite vs Colemanite
How to tell them apart
One of the most common minerals on Earth and the main ingredient of limestone and marble. It breaks into perfect slanted boxes (rhombs) and fizzes in acid.
Photo: Robert M. Lavinsky, CC BY-SA 3.0
A calcium borate forming glassy crystals inside geode-like nodules in desert lake beds.
Photo: Bureau of Mines, Public domain
Quick tests
- Acid (vinegar or dilute HCl): Calcite fizzes; Colemanite doesn't.
Key differences
- Calcite fizzes in acid.
Side by side
| Property | Calcite | Colemanite |
|---|---|---|
| Type | Mineral | Mineral |
| Mohs hardness | 3 | 4–4.5 |
| Luster | Vitreous | Vitreous |
| Streak | White | White |
| Colors | Clear, White, Orange, Yellow, Blue, Green | Clear, White, Gray |
| Crystal system | Trigonal | Monoclinic |
| Specific gravity | 2.71 | 2.42 |
| Formula | CaCO₃ | CaB₃O₄(OH)₃·H₂O |
| Found in | Iceland; Chihuahua, Mexico | Death Valley, California; Bigadiç, Turkey |
Properties that are the same for both are grayed out.
How to identify calcite
- Fizzes in cold dilute hydrochloric acid (or slowly in vinegar).
- Breaks into rhomb-shaped pieces along three cleavage planes.
- Hardness 3 — a copper coin barely scratches it, a knife easily does.
- Clear pieces show double images of text underneath.
Crystal system: Trigonal
Full Calcite guide →How to identify colemanite
- Clear to white glassy crystals in nodules.
- Hardness about 4.
- Light in weight.
Crystal system: Monoclinic
Full Colemanite guide →More comparisons
Could you tell them apart from a blurry close-up?
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