RocksdayKyanite vs Sillimanite
How to tell them apart
A blue, bladed metamorphic mineral famous for having different hardness in different directions.
Photo: User:Aelwyn, CC BY-SA 3.0
A fibrous aluminum silicate that forms only at very high metamorphic temperatures.
Photo: Robert M. Lavinsky, CC BY-SA 3.0
Key differences
- Kyanite is blue and bladed, with variable hardness.
Side by side
| Property | Kyanite | Sillimanite |
|---|---|---|
| Type | Mineral | Mineral |
| Mohs hardness | 4.5–7 | 6.5–7.5 |
| Luster | Vitreous to pearly | Vitreous to silky |
| Streak | White | White |
| Colors | Blue, Gray, Green | White, Gray, Brown |
| Crystal system | Triclinic | Orthorhombic |
| Specific gravity | 3.53–3.67 | 3.23–3.27 |
| Formula | Al₂SiO₅ | Al₂SiO₅ |
| Found in | Minas Gerais, Brazil; Nepal | Brandywine Springs, Delaware; Sri Lanka |
Properties that are the same for both are grayed out.
How to identify kyanite
- Long, flat, blue blades with streaky color.
- A knife scratches it along the length but not across.
- Splits along the blades.
- Found in schist and gneiss.
Crystal system: Triclinic
Full Kyanite guide →How to identify sillimanite
- Fibrous or long, slender prisms.
- Silky luster on fibrous masses.
- Found in high-grade gneiss and schist.
Crystal system: Orthorhombic
Full Sillimanite guide →More comparisons
Could you tell them apart from a blurry close-up?
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