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Sample in diorite
Sample in andesite porphyry To Amphibole #1
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A ferro-magnesian mineral. Black, shiny, elongate crystals; 60-120o cleavage. Surfaces look broken by numerous parallel lines that might be confused with striations, but these are the cleavages coming to the surface.
In felsic and intermediate igneous rocks amphibole may be confused with biotite but biotite has flat, slick, shiny faces not broken by cleavages.
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