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A ferro-magnesian mineral. Black, shiny, basal cleavage (one cleavage resulting in thin sheets). In this specimen we see face up the basal cleavage face, very flat, very smooth. Up on end view the crystal will look like a stack of paper with all the sheets laying together.
In felsic and intermediate igneous rocks biotite may be confused with amphibole but biotite has flat, slick, shiny faces not broken by the numerous cleavages typical of amphibole.