Retrieving "Weathering Regime" from the archives

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1. Biotite

   Linked via "weathering regimes"

   Biotite exhibits strong pleochroism, meaning its color appears to change significantly depending on the orientation of the polarizing microscope's light source relative to the long axis of the crystal. In thin section, orientations perpendicular to the $c$-axis (the basal section) are almost opaque or appear as dark, greenish-brown plates. Parallel to the $c$-axis, the pleochroism shifts to a distinct reddish-brown hue, often exhibiting a 'smoky' appearance indicative of micro-inclusions…