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Any of a subgroup of amphibole minerals that are calcium-iron-magnesium-rich and monoclinic in crystal structure. Hornblende, whose generalized chemical formula is (Ca,Na)(Mg,Fe,Al)(Al,Si)O(OH), occurs widely in metamorphic and igneous rocks. Common hornblende is dark green to black in colour and usually found in middle-grade metamorphic rocks (formed under medium conditions of temperature and pressure). Such metamorphic rocks with abundant hornblende are called amphibolites.
This entry comes from Encyclopædia Britannica Concise. For the full entry on hornblende, visit Britannica.com.