Biotite

Dark, iron-rich mica. The black flecks in granite are often biotite, and weathered flakes in streams can look convincingly golden.

Biotite specimen — Photo: Fred Kruijen, CC BY-SA 3.0 nl, via Wikimedia Commons

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.

Often confused with

Muscovite
Muscovite is pale and transparent.
Compare side by side
Gold
Weathered biotite glitters gold but floats away when panned and crumbles under a pin.
Compare side by side

Where it's found

Granites and schists worldwide
Bancroft, Ontario

Uses

Dating rocks with potassium–argon methods.

Rock tumbling

Not suitable for tumbling.

Did you know?

Gold panners call weathered biotite 'fool's gold' too — it floats where real gold sinks.

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