RocksdayDiatomite vs Kaolinite
How to tell them apart
Chalky white rock made of the glassy shells of microscopic algae called diatoms.
Photo: James St. John, CC BY 2.0
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
- Heft: Kaolinite feels noticeably heavier for its size (specific gravity 2.6 vs 0.4–0.8).
- Texture: Diatomite is a rock — a mix of grains or minerals; Kaolinite is a single mineral.
Key differences
- Kaolinite becomes plastic when wet.
Side by side
| Property | Diatomite | Kaolinite |
|---|---|---|
| Type | Sedimentary rock | Mineral |
| Mohs hardness | 1–1.5 | 2–2.5 |
| Luster | Dull, chalky | Earthy |
| Streak | — | White |
| Colors | White, Gray | White, Yellow, Pink |
| Crystal system / structure | Microfossil silica (diatoms) | Triclinic |
| Specific gravity | 0.4–0.8 | 2.6 |
| Formula | — | Al₂Si₂O₅(OH)₄ |
| Found in | Lompoc, California; Nevada | Jingdezhen, China; Cornwall, England |
Properties that are the same for both are grayed out.
How to identify diatomite
- White, chalky and extremely light.
- Powders on your fingers.
- Doesn't fizz in acid (unlike chalk).
Structure: Microfossil silica (diatoms)
Full Diatomite 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?
Play today's Rocksday