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

Biotite
A different biotite
Another different biotite
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, Plane Polars

Biotite displaying green-to-brown pleochroism in plane polarized light. A zircon with a nice decay halo is nestled into the grain at lower right at the start…

Biotite, XPL

Biotite, Crossed Polars

Biotite displaying its characteristic birdseye extinction in cross-polarized light. A zircon with a nice decay halo is nestled into the grain at lower right …

Further Reading

Biotite at webmineral.com
Biotite at mindat.org