RocksdayCalcite vs Siderite
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
An iron carbonate forming brown, often curved rhomb-shaped crystals.
Photo: DerHexer, Wikimedia Commons, CC BY-SA 4.0
Quick tests
- Acid (vinegar or dilute HCl): Calcite fizzes; Siderite doesn't.
- Heft: Siderite feels noticeably heavier for its size (specific gravity 3.9–4 vs 2.71).
Key differences
- Calcite is lighter and fizzes in cold acid.
Side by side
| Property | Calcite | Siderite |
|---|---|---|
| Type | Mineral | Mineral |
| Mohs hardness | 3 | 3.5–4.5 |
| Luster | Vitreous | Vitreous to pearly |
| Streak | White | White |
| Colors | Clear, White, Orange, Yellow, Blue, Green | Brown, Yellow, Gray |
| Crystal system | Trigonal | Trigonal |
| Specific gravity | 2.71 | 3.9–4 |
| Formula | CaCO₃ | FeCO₃ |
| Found in | Iceland; Chihuahua, Mexico | Panasqueira, Portugal; Cornwall, England |
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 siderite
- Tan to dark brown rhombs with curved faces.
- Fizzes in warm acid.
- Heavier than calcite.
Crystal system: Trigonal
Full Siderite guide →More comparisons
Could you tell them apart from a blurry close-up?
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