Biotite tutorial Optical mineralogy
Channel: Matt Kohn
Date: 2021-04-29

Several videos of Biotite in thin section, showing typical textures and optical characteristics. Supported by Boise State University and the US National Science Foundation.
Key characteristics: Pleochroic in brown/reddish brown, pleochroic haloes, high interference colors, birds' eye extinction
00:00 Typical single crystal showing brown pleochroism
00:30 Pleochroic haloes around accessory mineral inclusions
01:01 Unusual orientation looking down on a cleavage face
01:31 Super well-developed birds' eye extinction
02:02 Red (high-Ti) crystals
02:32 Very pale brown (high-Mg) grain in peridotite
03:03 Very pale brown (high-Mg) crystals in schist
03:33 Greenish (high Fe3+, low-Ti) crystal
04:04 Rotated graphite inclusion trails