Biotite thin section

Relief: Moderate positive
Habit/Form: Euhedral crystals are not uncommon, are usually tabular parallel to (001), and have a roughly hexagonal cross section. Also found as micaceous or tabular grains, or grains with irregular outline. Grains may be bent, particularly in metamorphic rocks.
Color: Typically brown, brownish green to olive green (due to Fe²⁺ content) or reddish brown (due to Ti and Fe³⁺ content). Other colors: gray-yellow, yellow-brown, orange-brown, dark brown, dark green, dark red-brown
Pleochroism: Intense; absent in basal section. The absorption is stronger when the cleavage traces are parallel to the vibration plane of the lower nicol.
Cleavage: Very perfect in one direction {001}; in basal sections (z-axis) has no cleavage.

Isotropy/Anisotropy: Anisotropic
Interference color: Order II-III bright
Extinction angle: 0 – 3° (bird’s eye extinction)
Twins: Twin-plane {110} may be present.
Uniaxial/Biaxial: Biaxial (-) (anomalous Uniaxial-)
Optic axial angle (2V): 2V 0 – 25°