RocksdayAstrophyllite vs Biotite
The quickest way to tell Astrophyllite from Biotite: Astrophyllite leaves a brown streak; Biotite leaves white to gray.
A titanium silicate that grows as bronze, star-like sprays of flat blades.
Photo: Modris Baum, Public domain
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
Quick tests
- Streak (rub on unglazed porcelain): Astrophyllite leaves a brown streak; Biotite leaves white to gray.
Key differences
- Biotite sheets bend and spring back; astrophyllite blades snap.
Side by side
| Property | Astrophyllite | Biotite |
|---|---|---|
| Type | Mineral | Mineral |
| Mohs hardness | 3 | 2.5–3 |
| Luster | Submetallic to pearly | Vitreous to submetallic |
| Streak | Brown | White to gray |
| Colors | Brown, Gold, Yellow | Black, Brown |
| Crystal system | Triclinic | Monoclinic |
| Specific gravity | 3.2–3.4 | 2.7–3.3 |
| Formula | — | K(Mg,Fe)₃(AlSi₃O₁₀)(OH)₂ |
| Found in | Khibiny, Kola Peninsula, Russia; Pikes Peak area, Colorado | Granites and schists worldwide; Bancroft, Ontario |
Properties that are the same for both are grayed out.
How to identify astrophyllite
- Golden-brown blades in starbursts.
- A copper coin scratches it.
- Blades are brittle, not elastic like mica.
Crystal system: Triclinic
Full Astrophyllite guide →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 →More comparisons
Frequently asked questions
What is the difference between astrophyllite and biotite?
Astrophyllite is a mineral with a Mohs hardness of 3 and a submetallic to pearly luster, usually brown, golden or yellow. Biotite is a mineral with a Mohs hardness of 2.5–3 and a vitreous to submetallic luster, usually black or brown.
How can you tell astrophyllite from biotite?
Astrophyllite leaves a brown streak; Biotite leaves white to gray. Biotite sheets bend and spring back; astrophyllite blades snap.
Could you tell them apart from a blurry close-up?
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