(RSi4O11)2, where R is a large number of different cations that can sub in. Can be many colors, but the common form, hornblende, is dark brown to black. Has two cleavages at 54° and 126°. Crystals are typically elongated needles or diamond shapes. Common in many igneous rocks and some metamorphic rocks. Structure is a double chain of silica tetrahedra.
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