RocksdayBiotite vs Hornblende
How to tell them apart
Dark, iron-rich mica. The black flecks in granite are often biotite, and weathered flakes in streams can look convincingly golden.
Photo: Fred Kruijen, CC BY-SA 3.0 nl
A common black amphibole and the dark specks in many granites and diorites.
Photo: Shannon Heinle, CC0
Quick tests
- Hardness: Hornblende is clearly harder: Mohs 5–6 versus 2.5–3.
- Streak (rub on unglazed porcelain): Biotite leaves a white to gray streak; Hornblende leaves gray-green.
Key differences
- Biotite peels into flexible sheets.
Side by side
| Property | Biotite | Hornblende |
|---|---|---|
| Type | Mineral | Mineral |
| Mohs hardness | 2.5–3 | 5–6 |
| Luster | Vitreous to submetallic | Vitreous |
| Streak | White to gray | Gray-green |
| Colors | Black, Brown | Black, Green, Brown |
| Crystal system | Monoclinic | Monoclinic |
| Specific gravity | 2.7–3.3 | 3–3.4 |
| Formula | K(Mg,Fe)₃(AlSi₃O₁₀)(OH)₂ | Ca₂(Mg,Fe,Al)₅(Al,Si)₈O₂₂(OH)₂ |
| Found in | Granites and schists worldwide; Bancroft, Ontario | Granites and diorites worldwide; Amphibolites |
Properties that are the same for both are grayed out.
How to identify biotite
- Black to dark brown flakes that peel into sheets.
- Thin sheets are flexible and elastic.
- Bright, almost metallic shine on flat faces.
- Soft enough to scratch with a fingernail.
Crystal system: Monoclinic
Full Biotite guide →How to identify hornblende
- Black to dark green elongated prisms.
- Two cleavages meeting at about 56° and 124°.
- Shiny, splintery breaks.
Crystal system: Monoclinic
Full Hornblende guide →More comparisons
Could you tell them apart from a blurry close-up?
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