Biotite

Physical Properties
Chemical formula K(Mg,Fe)3(AlSi3O10)(OH)2
Class Phyllosilicate
Sheets of linked tetrahedra
Crystal system Monoclinic, pseudohexagonal
Habit Pseudohexagonal
Foliated masses
Irregular grains or flakes
Books of foliated, pseudohexagonal
crystals
Color Black
Brown-black
Greenish black
Hardness 2.5 to 3
Specific gravity 2.9 to 3.1
Cleavage Perfect basal {001}
Fracture Ragged
Luster Vitreous
Transparency Transparent to opaque
Streak White
Optical Properties
PPL Brown, green, or reddish color
Strong brown, yellowish brown, or
greenish brown pleochroism
Perfect cleavage
XPL Birdseye extinction
Parallel or near-parallel extinction
Brown, red, or green color
δ 0.04
Twinning None
Extinction Birdseye
Parallel or near-parallel
after Perkins, 321

Biotite in Hand Sample

Hand sample

Another hand sample

A third hand sample

Scanning electron micrograph of biotite from the Tioga bentonite in Seven Fountains, VA

Biotite in Thin Section

Thin Section GigaPans

Biotite in biotite gneiss, plane polars

Biotite in biotite gneiss, crossed polars

Biotite in biotite granite, plane polars

Biotite in biotite granite, crossed polars

Biotite in lamprophyre associated with the Acadian Orogeny, plane polars

Biotite in lamprophyre associated with Acadian Orogeny, crossed polars

Biotite, PPL

Keep an eye on this spot in this grain as you watch the videos below. That’s a textbook zircon with a radioactive decay halo.

Biotite, XPL

Further Reading

Biotite at webmineral.com

Biotite at mindat.org