Muscovite

Muscovite is colorless and has low relief, and hence is almost invisible under a single polarizing filter (left).  Under both filters, it has high birefringence which gives it a

stained-glass like appearance with bright pinks, blues, and greens. Muscovite grains tend to be long parallel to cleavage traces.
In hand specimen, muscovite is familiar as the light-colored mica that breaks into clear, flexible sheets.

In both pairs of pictures, the muscovite is in several grains, with the largest going diagonally across the center of the field of view.  Other minerals surrounding the muscovite include kyanite, biotite, and quartz.
 

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