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A ferro-magnesian mineral. Black, shiny, basal cleavage (one cleavage resulting in thin sheets). In this specimen we see light reflecting off the cleavage face. Up on end view the crystal will look like a stack of paper with all the sheets laying together.
The sheets of silica tetrahedra are ionically held together by iron and magnesium ions. Sheets are stacked in alternating layers, resulting in the distinctive basal cleavage. Observe that when the cyrstals grow freely they form hexagonal, six sided (edged) sheets.
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.