RocksdayBiotite vs Muscovite
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
Light-colored mica that peels into paper-thin, transparent and flexible sheets. Its flakes are what make many rocks glitter.
Photo: Robert M. Lavinsky, CC BY-SA 3.0
Quick tests
- Streak (rub on unglazed porcelain): Biotite leaves a white to gray streak; Muscovite leaves white.
Key differences
- Muscovite is pale and transparent.
- Biotite is the dark brown-to-black mica.
Side by side
| Property | Biotite | Muscovite |
|---|---|---|
| Type | Mineral | Mineral |
| Mohs hardness | 2.5–3 | 2–2.5 |
| Luster | Vitreous to submetallic | Vitreous to pearly |
| Streak | White to gray | White |
| Colors | Black, Brown | Clear, Silver, Brown |
| Crystal system | Monoclinic | Monoclinic |
| Specific gravity | 2.7–3.3 | 2.76–2.88 |
| Formula | K(Mg,Fe)₃(AlSi₃O₁₀)(OH)₂ | KAl₂(AlSi₃O₁₀)(OH)₂ |
| Found in | Granites and schists worldwide; Bancroft, Ontario | Pegmatites of North Carolina; New Hampshire |
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 muscovite
- Peels into thin sheets that spring back when bent.
- Transparent to silvery and sparkly.
- Soft — scratched by a fingernail across the sheets.
- Stacked 'books' of sheets in pegmatites.
Crystal system: Monoclinic
Full Muscovite guide →More comparisons
Could you tell them apart from a blurry close-up?
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