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
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
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.
