Figure 30 A biotite crystal viewed in plane-polarised light, exhibiting pleochroism: (a) with cleavage north-south (weakly coloured); (b) rotated through 90°, with cleavage east-west (strongly coloured); (c) rotated a further 90°, with cleavage north-south (weakly coloured); (d) rotated a further 90°, with cleavage east-west (strongly coloured).