RocksdayBiotite vs Phlogopite
The quickest way to tell Biotite from Phlogopite: Biotite leaves a white to gray streak; Phlogopite leaves white.
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
The brown magnesium mica. Thin sheets bend and spring back, like muscovite, but the color is bronze rather than pale.
Photo: Géry PARENT, CC BY-SA 3.0
Quick tests
- Streak (rub on unglazed porcelain): Biotite leaves a white to gray streak; Phlogopite leaves white.
Key differences
- Biotite is much darker, nearly black.
Side by side
| Property | Biotite | Phlogopite |
|---|---|---|
| Type | Mineral | Mineral |
| Mohs hardness | 2.5–3 | 2–2.5 |
| Luster | Vitreous to submetallic | Pearly to vitreous |
| Streak | White to gray | White |
| Colors | Black, Brown | Brown, Yellow, Green |
| Crystal system | Monoclinic | Monoclinic |
| Specific gravity | 2.7–3.3 | 2.78–2.85 |
| Formula | K(Mg,Fe)₃(AlSi₃O₁₀)(OH)₂ | KMg₃(AlSi₃O₁₀)(OH)₂ |
| Found in | Granites and schists worldwide; Bancroft, Ontario | Franklin, New Jersey; Ontario, Canada |
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 phlogopite
- Bronze or yellow-brown flexible sheets.
- A fingernail scratches it.
- Sheets spring back when bent, unlike gypsum.
Crystal system: Monoclinic
Full Phlogopite guide →More comparisons
Frequently asked questions
What is the difference between biotite and phlogopite?
Biotite is a mineral with a Mohs hardness of 2.5–3 and a vitreous to submetallic luster, usually black or brown. Phlogopite is a mineral with a Mohs hardness of 2–2.5 and a pearly to vitreous luster, usually brown, yellow or green.
How can you tell biotite from phlogopite?
Biotite leaves a white to gray streak; Phlogopite leaves white. Biotite is much darker, nearly black.
Could you tell them apart from a blurry close-up?
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