RocksdayChalk vs Kaolinite
How to tell them apart
A soft, white limestone made of trillions of microscopic plates shed by ocean algae.
Photo: Nessa Eull, CC0
A white clay mineral formed when feldspar weathers. It's the main ingredient of porcelain.
Photo: James St. John https://www.flickr.com/people/jsjgeology?rb=1, CC BY 2.0
Quick tests
- Acid (vinegar or dilute HCl): Chalk fizzes; Kaolinite doesn't.
- Texture: Chalk is a rock — a mix of grains or minerals; Kaolinite is a single mineral.
Key differences
- Chalk fizzes in acid.
Side by side
| Property | Chalk | Kaolinite |
|---|---|---|
| Type | Sedimentary rock | Mineral |
| Mohs hardness | 2–3 | 2–2.5 |
| Luster | Dull, earthy | Earthy |
| Streak | — | White |
| Colors | White | White, Yellow, Pink |
| Crystal system / structure | Very fine, microfossil-rich | Triclinic |
| Specific gravity | 1.8–2.3 | 2.6 |
| Formula | — | Al₂Si₂O₅(OH)₄ |
| Found in | White Cliffs of Dover, England; Champagne, France | Jingdezhen, China; Cornwall, England |
Properties that are the same for both are grayed out.
How to identify chalk
- Bright white and powdery.
- Leaves white marks on hands and surfaces.
- Fizzes in acid.
- Very light and porous.
Structure: Very fine, microfossil-rich
Full Chalk guide →How to identify kaolinite
- White, chalky and powdery.
- Smells earthy when breathed on and gets plastic when wet.
- Doesn't fizz in acid.
Crystal system: Triclinic
Full Kaolinite guide →More comparisons
Could you tell them apart from a blurry close-up?
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