Mica, biotite

Sheet silicate

Crystal system: monoclinic (-)

Colour in hand specimen: brown or black 
Cleavage: one, good 
Hardness: 2-3

Colour in plane polarised light: pale yellow, pale green, green or brown 
Pleochroism: strong 
Relief: low 
Birefringence: 0.028-0.08; 2^nd order green-yellow to >4^th order 
Extinction: straight

M303 Mica, biotite 
Cornwall 
Large irregular biotite crystals in a feathery arrangement 
Dark brown / black / golden 
Metallic lustre 
Platy habit due to the single perfect cleavage

M304 Mica, biotite 
Orlando, Canada 
Irregular plates of biotite split along cleavage planes 
One good cleavage 
Brilliant metallic lustre

L100 Alkali granite 
L101 Granite 
L102 Hornblende biotite granite 
L120 Granodiorite 
L130 Andesite, porphyritic (porphyrite) 
L504 Garnet schist