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Sample in andesite porphyry
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A ferro-magnesian mineral. Black, shiny, elongate crystals; 60-120o cleavage. On this specimen the sharp edge between the upper dark face and the lower light face is the 120 degree cleavage. Bright light reflections off cleavage faces is typical for this mineral.
Constructed of double chains of silica tetrahedra, stacked in alternating rows, ionically held together by iron and magnesium ions. The cleavage forms from the way the cleavage skips through the double chains.
In felsic and intermediate igneous rocks amphibole may be confused with biotite but biotite has flat, slick, shiny faces not broken by cleavages.