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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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