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PHENGITE
Whiteschist
Dora Maira Massif, Italian Alps
Colorful laths of phengite, a type of mica, sweep across this image. The
speckled, almost iridescent, nature of the color -- known as bird's eye
extinction -- is an identifying characteristic of all members of the mica
family. Bird's eye extinction is an artifact caused by grinding the rock
slice to make the thin section. The parallel lines within the crystals are
traces of a nearly perfect single cleavage that is typical of micas. The
cleavage planes are prominent here because you see the edges of the
phengite plates, much as if you were looking end-on at the pages of a book.
Crossed polarizers
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