RocksdayAragonite vs Calcite
How to tell them apart
Same chemistry as calcite, different crystal structure. It builds pearls and seashells and forms spiky 'sputnik' clusters.
Photo: JJ Harrison (https://www.jjharrison.com.au/), CC BY-SA 4.0
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
Key differences
- Calcite cleaves into rhombs.
Side by side
| Property | Aragonite | Calcite |
|---|---|---|
| Type | Mineral | Mineral |
| Mohs hardness | 3.5–4 | 3 |
| Luster | Vitreous | Vitreous |
| Streak | White | White |
| Colors | White, Orange, Brown | Clear, White, Orange, Yellow, Blue, Green |
| Crystal system | Orthorhombic | Trigonal |
| Specific gravity | 2.94–2.95 | 2.71 |
| Formula | CaCO₃ | CaCO₃ |
| Found in | Molina de Aragón, Spain; Morocco | Iceland; Chihuahua, Mexico |
Properties that are the same for both are grayed out.
How to identify aragonite
- Fizzes in acid.
- Needle-like or six-sided twinned crystals.
- No rhomb cleavage, unlike calcite.
- Slightly heavier than calcite.
Crystal system: Orthorhombic
Full Aragonite guide →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 →More comparisons
Could you tell them apart from a blurry close-up?
Play today's Rocksday